Oxalic acid for the control of varroosis in honey beecolonies – a review1
Eva RADEMACHER*, Marika HARZ
Free University of Berlin, Dept. of Biology/Chemistry/Pharmacy, Neurobiology, Königin-Luise-Str. 28–30,14195 Berlin, Germany
Received 17 February 2005 – revised 17 June 2005 – accepted 26 July 2005
Abstract – The review summarizes research results on the use of oxalic acid as an acaricide in honey beecolonies. Three different treatment techniques (i.e. trickling, evaporation and spraying) have beendeveloped for the application of oxalic acid. Detailed information is given on the efficacy against Varroadestructor, tolerability by Apis mellifera, protective procedures for the user, residue situation and consumersafety, as well as recommendations for use.
In the middle of the 20th century Varroadestructor moved onto the Western honey bee,Apis mellifera, as a new host and subsequentlyspread globally. Because there is no natural bal-ance between the new host and parasite; V.destructor destroys honey bee colonies withina few years. Since V. destructor appeared in A.mellifera colonies beekeepers have been forcedto use acaricides regularly to keep mite popu-lations under control. In Europe, many bee-keepers rely on coumaphos and syntheticpyrethroids to control varroosis. Over the last10 years the mite’s resistance to synthetic aca-ricides has increased tremendously, necessitat-ing a new strategy to combat it, including thedevelopment of new veterinary medications(Elzen et al., 1998; Lodesani et al., 1995;Milani, 1995; Moosbeckhofer and Trouiller,1996; European Group for Integrated VarroaControl, 1999).
Due to the biology of host and parasite, dif-ferent treatment methods are required during
the year (Concept of Integrated Varroa Con-trol). Biotechnical methods such as the removalof drone brood and the use of trapping combs(Arbeitsgemeinschaft der Institute für Bienen-forschung, 2001) and formic acid or thymol(Imdorf et al., 1999; Rademacher, 1995, 1996;Rademacher et al., 1999) can be used to reducemite populations during the summer months.During autumn and winter months the bee col-onies can be treated with lactic acid or oxalicacid (Charrière and Imdorf, 2002; Kraus et al.,1993; Nanetti et al., 2003a). Lactic acid is onlypractical for small apiaries. Within the Conceptof Integrated Varroa Control, autumn treatmentof large apiaries is based on the use of oxalicacid. This substance provides a wide range ofpossible application methods at the appropriatetime for the year.
Oxalic acid has been known to be effectiveagainst V. destructor since the end of the 20thcentury (Popov et al., 1989). Most investiga-tions into the development of oxalic acid as adrug in bee colonies have been carried out inEU countries and Switzerland using three
* Corresponding author: [email protected] Manuscript editor: Stefan Fuchs
Article published by EDP Sciences and available at http://www.edpsciences.org/apido or http://dx.doi.org/10.1051/apido:2005063
different application forms. The EuropeanGroup for Integrated Varroa Control testedoxalic acid in coordinated experiments. Theiraim was to provide beekeepers with a fullydeveloped treatment method (Nanetti et al.,2003a). To use oxalic acid legally it must beregistered as a drug for bees; to achieveapproval, each country’s legislation must beconsidered (Mutinelli and Rademacher, 2002).This review presents and evaluates currentknowledge about oxalic acid with regard to itsapplicability as a control for V. destructor. Inaddition to the scientific literature, we haveincluded articles from practical beekeepingjournals, because many studies were only pub-lished there.
2. EFFICACY OF OXALIC ACID AGAINST VARROA DESTRUCTOR AND TOLERABILITY BY APIS MELLIFERA
2.1. Trickling
Using a syringe or a similar applicator,oxalic acid dihydrate solution was trickleddirectly onto the bees in the spaces betweencombs, normally when colonies were in thebroodless phase. The application is quick(about 1 min per hive), cost-effective and easy.Trials were conducted in Southern, Central andNorthern Europe as well as North Americawhen outside temperature ranged from 3 to13 °C1. The investigations are listed in Table I,grouped by climatic area, season, number ofapplications and concentration of oxalic aciddihydrate solution.
2.1.1. Efficacy
The majority of trials were conducted inmoderate climates in Central Europe. Theinvestigators focused on the application ofoxalic acid in autumn when colonies are brood-less, as oxalic acid does not kill the mites insealed brood cells.
Single treatments during autumn were eval-uated by applying 5 mL per bee space (30–50 mL per hive) of various concentrations of
oxalic acid dihydrate. A concentration of 3%oxalic acid dihydrate provided an efficacy of> 90% in most trials (Charrière, 2001; Charrièreand Imdorf, 2002; Nanetti et al., 2003a); onlyin one investigation was an efficacy < 90%described (Nanetti et al., 2003a). A 3.5% solu-tion provided > 95% (Charrière and Imdorf,2002; Charrière et al., 2004). A lower dose of10–15 mL/hive (3.5%) showed a similarlygood acaricidal effect (98%) when smallercolonies were treated (2-Zanderflachzargen,Moosbeckhofer, 2001).
Büchler (2000) quantified a dose per beeafter estimating colony size. The applied dos-age was 3 µL per bee. A dose of 22 mL (smallcolonies, approx. 7400 bees) and 79 mL perhive (very strong colonies, approx. 26 500 bees)respectively, reached an efficiency of 91.3%using 3% oxalic acid dihydrate solution. Whena 4.6% oxalic acid dihydrate solution wasapplied, mite mortality increased to 98.4%. Theoverall dosage in the trials (22 mL with 4.6%and 79 mL with 3%) equalled 5 mL per beespace at 3.5% oxalic acid dihydrate solution.Trials using lower dosages (3.2 mL per beespace) and 3% oxalic acid dihydrate showed areduced efficacy of only 84.8% (Büchler,1999). Higher oxalic acid dihydrate concentra-tions of 4.6 and 6% (3.2 mL per bee way) killed92.2% and 94.3%, respectively, of the mitesparasitizing the colonies (Büchler, 1999).
Applying a similar overall dose, but dividingthe dose per bee in half in one group (3.0 and1.5 µL, respectively) and doubling the concen-tration from 2.3 to 4.6% oxalic acid dihydrateresulted in an obviously higher efficacy of 98%(2.3% solution) and 80% mite fall using thehigh oxalic acid dihydrate solution (4.6%;Büchler, 2000).
Trials in which more than 3.5% oxalic aciddihydrate solution were applied showed thathigher dosages do not clearly lead to anincreased efficacy (Charrière, 2001; Charrièreand Imdorf, 2002; Liebig, 1998; Nanetti et al.,2003a). There is a tendency that the highestconcentrations and doses of 6 or 8% (Liebig,1997, 1998) actually reduced the efficacy.Using these high dosages to combat varroosisis not feasible because the bees cannot toleratethem (see Sect. 1.1.2 Tolerability).
During summer in breeding colonies theefficacy was reduced to 36% (Schuster and1 Temperature not mentioned in all papers.
100 E. Rademacher, M. HarzTa
ble
I. T
rial
s on
tric
klin
g of
oxa
lic a
cid
dihy
drat
e.
Aut
hor
Yea
rN
o.C
onc.
Con
c.C
onc.
Dos
age
Eff
icac
y (%
) B
eeO
verw
inte
ring
No.
Col
ony
or s
war
mB
rood
ofO
A
dih.
O
A
anh.
su
gar
(mL
)m
ean
sd,
mor
talit
yof
(siz
e if
men
tion
ed)
(cel
ls)
appl
.(%
)#(%
)#(%
)#m
in–m
axco
l.
Cen
tral
Eur
ope
– T
reat
men
t in
aut
umn
Nan
etti
et a
l.20
03a
13
2.1
605
p.b.
w.
74.9
and
94.
3-
wea
keni
ng-
colo
ny (
CH
& D
B)
no
Cha
rriè
re a
nd20
021
32.
150
30–5
0 p.
h.93
+/–
1.6
–-
good
ove
rwin
teri
ng40
colo
ny (
CH
& D
B)
no
Imdo
rf98
+/–
0.7
* (
9000
–120
00)
Büc
hler
2000
13
2.1
5022
–79
p.h.
91
.3, 8
6.4–
-sl
ight
wea
keni
ng (
n.s.
)6
colo
ny-
(3 µ
L/b
ee)
96.6
(74
00–2
6500
)
Lie
big
1998
13
2.1
5060
p.h
.94
.7, 7
7–10
0-
-5
colo
ny
Büc
hler
1999
13
2.1
603.
2 p.
b.w
.84
.8, 5
1.1–
9371
8 b/
col
wea
keni
ng9
col
ony
(848
4)-
over
21
d
Cha
rriè
re20
011
32.
150
30–5
0 p.
h.56
.2 +
/– 4
.6–
95.9
+/–
1.5
*lo
wgo
od o
verw
inte
ring
37(1
0000
)no
Cha
rriè
re e
t al.
2004
13.
52.
550
30–5
0 p.
h.>
95*
***
-20
–30%
red.
col
ony
stre
ngth
(n.
s.)
18co
lony
(12
com
bs, D
B)
no
Büc
hler
2002
13.
52.
550
30–5
0 p.
h.-
good
ove
rwin
teri
ng11
col.
(141
84 +
/– 3
019)
(100
0)
Büc
hler
2002
13.
52.
550
30–5
0 p.
h.-
-go
od o
verw
inte
ring
7co
l. (2
1643
+/–
295
0)(1
000)
Spin
ks20
021
3.5
2.5
5030
–50
p.h.
--
good
ove
rwin
teri
ng6
5–6
o.c
.no
Moo
sbec
khof
er20
011
3.5
2.5
5010
–15
p.h.
98, 9
8–10
0-
2 co
l. lo
st4
colo
ny (
2-Z
.-no
Fla
chz.
)
Cha
rriè
re a
nd20
021
3.7
2.6
5030
–50
p.h.
98 +
/– 2
.3–
-go
od o
verw
inte
ring
51co
lony
(C
H &
DB
)no
Imdo
rf99
+/–
0.3
*(9
000–
1200
0)
Cha
rriè
re20
011
4.5
3.2
5030
–50
p.h.
97.8
+/–
1.4
–99
.2 +
/– 0
.1lo
wre
duce
d co
lony
str
engt
h14
(100
00)
no
Cha
rriè
re a
nd20
021
4.6
3.2
5030
–50
p.h.
92 +
/– 0
.9–
-go
od o
verw
inte
ring
68co
lony
(C
H &
DB
)no
Imdo
rf99
+/–
0.9
*(9
000–
1200
0)
Oxalic acid for the control of varroosis 101
Tabl
e I.
Con
tinu
ed.
Aut
hor
Yea
rN
o.C
onc.
Con
c.C
onc.
Dos
age
Eff
icac
y (%
) B
eeO
verw
inte
ring
No.
Col
ony
or s
war
mB
rood
ofO
A
dih.
O
A
anh.
su
gar
(mL
)m
ean
sd,
mor
talit
yof
(siz
e if
men
tion
ed)
(cel
ls)
appl
.(%
)#(%
)#(%
)#m
in–m
axco
l.
Cha
rriè
re a
nd20
021
4.6
3.2
3030
–50
p.h.
88 +
/– 3
.8–
-go
od o
verw
inte
ring
24co
lony
(C
H &
DB
)no
Imdo
rf97
+/–
*(9
000–
1200
0)
Büc
hler
2000
14.
63.
250
22–7
9 p.
h.98
.4, 9
5.5–
-w
eake
ning
(s.
)6
colo
ny-
(3 µ
L/b
ee)
99.3
(740
0–26
500)
Büc
hler
1999
14.
63.
260
3.2
p.b.
w.
92.2
, 82.
6–64
8 b/
col
wea
keni
ng10
colo
ny (
8744
)-
97.3
over
21
d
Nan
etti
et a
l.20
03a
14.
63.
260
5 p.
b.w
.99
.1-
redu
ced
colo
ny s
tren
gth
-co
lony
in D
B
no
Lie
big
1998
15
3.5
050
p.h
.91
.8, 7
5–99
.4hi
ghw
eake
ning
7co
lony
(150
0–35
00)
Lie
big
1999
15
3.5
050
p.h
.>
90
high
wea
keni
ng
-co
lony
no
Nan
etti
et a
l.20
03a
16
4.2
605
p.b.
w.
95.6
and
99
-w
eake
ning
-co
lony
(C
H &
DB
)no
Büc
hler
1999
16
4.2
603.
2 p.
b.w
. 9
4.3,
89.
9–73
3 b/
col
wea
keni
ng
10co
lony
(10
273)
-
97.9
Ove
r 21
d
Cha
rriè
re20
011
64.
250
30–5
0 p.
h.85
.4 +
/– 3
.999
.9
+/–
0.2
*lo
wre
duce
d co
lony
str
engt
h47
(10
000)
no
Cha
rriè
re20
011
64.
20
30–5
0 p.
h.92
.2 +
/– 5
.8,
92.3
+/–
13.
6lo
w-
12co
lony
no
Cha
rriè
re e
t al.
1998
16
4.2
6030
–50
p.h.
98.5
low
-95
colo
nyno
Lie
big
1997
16
4.2
6030
–50
p.h.
87.2
, 72–
98.7
--
3 c
olon
y-
Lie
big
1998
18
5.7
050
p.h
.78
, 55–
97.9
high
-5
colo
ny(1
500–
3500
)
Büc
hler
2000
22.
31.
650
22–7
9 p.
h.98
**-
slig
ht w
eake
ning
(n.
s.)
6co
lony
-
(3 µ
L/b
ee)
(740
0–26
500)
Büc
hler
2000
24.
63.
250
11–3
9 p.
h.
80.2
-sl
ight
wea
keni
ng (
n.s.
)6
colo
ny
-
(1.5
µL
/bee
) (
7400
–26
500)
102 E. Rademacher, M. HarzTa
ble
I. C
onti
nued
.
Aut
hor
Yea
rN
o.C
onc.
Con
c.C
onc.
Dos
age
Eff
icac
y (%
) B
eeO
verw
inte
ring
No.
Col
ony
or s
war
mB
rood
ofO
A
dih.
O
A
anh.
su
gar
(mL
)m
ean
sd,
mor
talit
yof
(siz
e if
men
tion
ed)
(cel
ls)
appl
.(%
)#(%
)#(%
)#m
in–m
axco
l.
Cen
tral
Eur
ope
– T
reat
men
t in
sum
mer
Büc
hler
1998
14.
13
6040
per
sw
arm
6316
.8%
-
5sw
arm
(23
333)
-
Büc
hler
1998
18.
36
6040
per
sw
arm
8135
.2%
-5
swar
m (
2311
1)-
Büc
hler
1998
111
8.5
6040
per
sw
arm
100
46.1
%-
5sw
arm
(22
756)
-
Lie
big
1999
2 or
33
2.1
0-
< 9
0hi
gh-
-co
lony
yes
Cha
rriè
re20
013#
#6
4.2
6030
–50
p.h.
Low
high
redu
ced
colo
ny s
tren
gth
-(u
p to
20
000)
yes
“Bie
nenw
ohl”
and
“Ip
erea
t” (
read
y-m
ixed
sol
utio
ns c
onta
inin
g ot
her
subs
tanc
es)
– T
reat
men
t in
aut
umn
Lie
big
1999
13.
52.
5-
50 p
.h.
92.
8, 8
0–99
high
--
colo
nyno
Moo
sbec
khof
er20
011
3.5
2.5
-10
–40
g 96
, 82–
100
-5
colo
nies
dea
d19
colo
ny (
2-Z
.-no
p.h.
**F
lach
z.)
Mut
inel
li a
nd20
021
4.2
3-
5 p.
b.w
.92
+/–
7.8
-go
od o
verw
inte
ring
10co
lony
no
Bag
gio
Mut
inel
li a
nd20
021
4.2
3-
5 p.
b.w
.93
.9 +
/– 6
.2-
good
ove
rwin
teri
ng7
colo
nyno
Bag
gio
“Bie
nenw
ohl”
– T
reat
men
t in
sum
mer
Lie
big
1999
2 or
33.
52.
5-
-<
90
high
--
colo
nyye
s
Schu
ster
and
2003
33.
52.
5-
20 p
.h.
max
. 36%
-8
colo
nies
dea
d36
colo
ny
yes
Sch
ürzi
nger
4 qu
eenl
ess
(11.
4–13
.9 o
.c.)
Sout
hern
Eur
ope
– T
reat
men
t in
aut
umn
Nan
etti
and
Str
adi
1997
13.
12.
260
5 p.
b.w
.89
.636
.9 b
/col
.no
que
en lo
ss30
colo
ny (
10 c
ombs
, DB
)(2
00–2
000)
Ove
r 14
d
Mut
inel
li a
nd20
021
53.
6-
5 p.
b.w
.89
.7 +
/– 9
.6-
good
ove
rwin
teri
ng5
colo
nyno
Bag
gio
Oxalic acid for the control of varroosis 103
Tabl
e I.
Con
tinue
d.
Aut
hor
Yea
rN
o.C
onc.
Con
c.C
onc.
Dos
age
Eff
icac
y (%
) B
eeO
verw
inte
ring
No.
Col
ony
or s
war
mB
rood
ofO
A
dih.
O
A
anh.
su
gar
(mL
)m
ean
sd,
mor
talit
yof
(siz
e if
men
tion
ed)
(cel
ls)
appl
.(%
)#(%
)#(%
)#m
in–m
axco
l.
Ferr
ero
et a
l.20
041
64.
260
5 p.
b.w
.94
.3 +
/– 6
.5,
78.4
–98.
6-
good
ove
rwin
teri
ng9
colo
nyno
Ferr
ero
et a
l.20
041
64.
230
5 p.
b.w
.93
.8 +
/– 4
.8,
84.2
–97.
9-
good
ove
rwin
teri
ng7
colo
nyno
Ferr
ero
et a
l.20
041
64.
215
5 p.
b.w
.92
.9 +
/– 4
.1,
85.2
–98.
6-
good
ove
rwin
teri
ng9
colo
nyno
Mut
inel
li an
d20
021
64.
2-
5 p.
b.w
.96
.9 +
/– 4
.4-
good
ove
rwin
teri
ng7
colo
nyno
Bag
gio
Bag
gio
and
2003
a1
64.
2-
5 p.
b.w
.91
+/–
12.
3,
low
good
ove
rwin
teri
ng7
colo
nyno
Mut
inel
li70
.3–1
00
Nan
etti
et a
l.20
03a
16
4.2
605
p.b.
w.
90.3
-w
eake
ning
-
colo
ny-
Nan
etti
and
St
radi
1997
16
4.2
605
p.b.
w.
96.8
47.6
b/c
ol.
no q
ueen
loss
30co
lony
(10
com
bs, D
B)
(200
–200
0)
Ove
r 14
d
Bag
gio
and
2003
b3
64.
250
5 p.
b.w
.56
.2 +
/– 3
1.6,
-
-5
colo
ny (
10 c
ombs
)ye
s
Mut
inel
li48
.5–7
6.7
Mut
inel
li et
al.
1997
37
520
5 p.
b.w
.
95 +
/– 7
.39
12.5
9 b/
d(n
.s.)
no q
ueen
loss
10(5
–6 o
.c.,
DB
)(3
400
p.h.
)
Gre
gorc
and
Pl
anin
c20
011
4.1–
5.2
w/w
2.9–
3.7
w/w
31.9
–26.
1 w
/w50
p. h
.99
.4 +
/– 0
.54
1.63
+/–
1.3
2b/
dno
rmal
ove
rwin
teri
ng24
colo
nyno
Sout
hern
Eur
ope
– T
reat
men
t in
sum
mer
Gre
gorc
and
Pl
anin
c20
013
4.1
w/w
2.9
w/w
31.9
w/w
50 p
. h.
39.2
3.25
+/–
1.2
5b/
d (n
.s.)
norm
al o
verw
inte
ring
7co
lony
5–7
com
bs
Gre
gorc
and
Pl
anin
c20
013
4.8
w/w
3.4
w/w
47.6
w/w
50 p
. h.
52.3
3.25
+/–
1.2
5b/
d (n
.s.)
norm
al o
verw
inte
ring
6co
lony
5–7
com
bs
Gre
gorc
and
Pl
anin
c20
013
5.2
w/w
3.7
w/w
26.1
w/w
50 p
. h.
40.7
3.25
+/–
1.2
5b/
d (n
.s.)
norm
al o
verw
inte
ring
7co
lony
5–7
com
bs
104 E. Rademacher, M. HarzTa
ble
I. C
onti
nued
.
Aut
hor
Yea
rN
o.C
onc.
Con
c.C
onc.
Dos
age
Eff
icac
y (%
) B
eeO
verw
inte
ring
No.
Col
ony
or s
war
mB
rood
ofO
A
dih.
O
A
anh.
su
gar
(mL
)m
ean
sd,
mor
talit
yof
(siz
e if
men
tion
ed)
(cel
ls)
appl
.(%
)#(%
)#(%
)#m
in–m
axco
l.
Nor
ther
n E
urop
e –
Tre
atm
ent
in a
utum
n
Frie
s20
011
2.2
1.6
5060
p.h
.68
.3-
3 co
loni
es d
ead
(n.s
.)30
colo
ny (
10 c
ombs
)no
Nan
etti
et a
l.20
03a
13
2.1
305
p.b.
w.
57.6
, 24.
7–75
.3-
bett
er th
an 6
% O
A d
ih.
-co
lony
no
Nan
etti
et a
l.20
03a
13
2.1
605
p.b.
w.
61.5
, 26.
3–78
.2-
bett
er th
an 6
% O
A d
ih.
-co
lony
no
Frie
s20
011
4.5
3.2
5030
p.h
.92
.2-
1 co
lony
dea
d (n
.s.)
28co
lony
(10
com
bs)
no
Nan
etti
et a
l.20
03a
16
4.2
305
p.b.
w.
90.4
, 88.
5–94
.6-
wor
se th
an 3
% O
A d
ih.
-co
lony
no
Nan
etti
et a
l.20
03a
16
4.2
605
p.b.
w.
92.6
, 85–
96.1
-w
orse
than
3%
OA
dih
.-
colo
nyno
Nor
th A
mer
ica
– T
reat
men
t in
aut
umn
Nas
r et
al.
2001
12.
82
5040
–50
p.h.
55.5
+/–
6.4
8-
25%
of
colo
nies
sur
vivi
ng**
*8
4–9
o.c.
yes
Nas
r et
al.
2001
13.
52.
550
40–5
0 p.
h.89
.4 +
/– 2
.39
-71
.4%
of
colo
nies
sur
vivi
ng**
*7
4–9
o.c.
yes
Con
cent
rati
ons
refe
r to
oxa
lic
acid
dih
ydra
te: M
ol. w
eigh
t: 1
26.0
4 g;
* m
ean
effi
cacy
of
apia
ries
;
o.c.
: occ
upie
d co
mbs
;**
dos
age/
effi
cacy
in g
ram
;n.
s.: n
ot s
igni
fica
nt;
p.h.
: per
hiv
e;**
* co
ntro
l wit
h A
pist
an 6
2.5%
of
colo
nies
sur
vivi
ng;
s.: s
igni
fica
nt;
p.b.
w.:
per
bee
way
;**
** e
ffic
acy
take
n fr
om f
igur
e;co
l.: c
olon
y;
red.
: red
uced
;A
ppl.:
app
licat
ion;
DB
: Dad
ant-
Bla
tt;
b: b
ees;
OA
dih
: oxa
lic
acid
dih
ydra
te;
CH
: Sch
wei
zerk
aste
n (a
Sw
iss
type
hiv
e);
d: d
ay;
Con
c.: c
once
ntra
tion
;#:
con
cent
ratio
ns a
re g
iven
as
w/v
unl
ess
mar
-ke
d as
w/w
.
hyph
en: n
ot m
entio
ned
in o
rigi
nal a
rtic
le;
2-Z
.-Fl
achz
.: 2-
Zan
der-
Flac
hzar
gen
(= n
arro
w s
uper
s).
##: T
reat
men
t fro
m A
ugus
t to
Nov
embe
r.
Oxalic acid for the control of varroosis 105
Schürzinger, 2003). The presence of broodseemed to be the main reason for low mite mor-tality in summer treatments. An efficiency ofup to 100% was reached in broodless artificialswarms (Büchler, 1998).
Investigations from Canada in a climaticregion comparable to Central Europe con-firmed the results of autumn treatments: 2.8%oxalic acid dihydrate solution (40–50 mL/hive)killed about 55% of the mites, 3.5% about 90%in colonies with remaining brood (Nasr et al.,2001).
Different concentrations of oxalic acid werefound to be effective depending upon the Euro-pean climatic region. A higher oxalic acid dihy-drate concentration appeared more suitable ina southern climate. Treatments with 6% oxalicacid dihydrate, 5 mL per bee way, resulted inan efficiency of > 90% and > 95%, respec-tively, while treatments with 3.1 and 5% oxalicacid dihydrate resulted in < 90% (Ferrero et al.,2004; Mutinelli et al., 1997; Mutinelli andBaggio, 2002; Nanetti and Stradi, 1997;Nanetti et al., 2003a).
Only one investigation resulted in high mitemortality of 99%; low concentrations of 4.1–5.2% oxalic acid dihydrate were applied invarying sugar solutions calculated as w/w(Gregorc and Planinc, 2001). For comparabil-ity, this corresponds to approximately 4.5–5.9% oxalic acid dihydrate w/v as described inother studies.
Repeated treatments with a high concentra-tion (6 and 7%) did not increase the efficacy(Mutinelli et al., 1997); one study evendescribed a reduced mite mortality of 56.2%(Baggio and Mutinelli, 2003b).
Mutinelli and Baggio (2002) appliedIpereat, a ready mixed solution containingabout 4.2% oxalic acid dihydrate, ethereal oils,propolis and sugar and reported efficacies up to93.9%.
Gregorc and Planinc (2001) treated colonieswith brood three times in the summer by apply-ing 4.1–5.2% oxalic acid dihydrate in varyingsugar solutions w/w corresponding to approx-imately 4.5–5.9% oxalic acid dihydrate w/v.An efficacy of 39–52% was achieved.
In a northern climate, a concentration of 4.5and 6% oxalic acid dihydrate was described aseffective with > 90% mite fall, while lower con-centrations (2.2 and 3%) led only to approxi-
mately 60% efficacy (Fries, 2001; Nanetti et al.,2003a).
Most authors applied oxalic acid dihydratein sugar syrup. By adding sugar to oxalic aciddihydrate solutions in a concentration of 50–60% the efficacy could be increased comparedto 0 or 30% sugar (Charrière, 2001; Charrièreand Imdorf, 2002; Nanetti et al., 2003a). Theaddition of higher concentrations of sugarincreased the efficacy by about 5% (Charrière,2001).
2.1.2. Tolerability
In Central Europe, a single treatment withoxalic acid dihydrate solutions in autumn inmost cases is well-tolerated by the bees in con-centrations up to 4.6% (Büchler, 1999, 2000,2002; Charrière, 2001; Charrière and Imdorf,2002; Charrière et al., 2004; Nanetti et al.,2003a; Spinks, 2002). A normal weakening ofthe colonies during winter was described, sim-ilar to controls (Büchler, 1999, 2000; Charrièreet al., 2004; Nanetti et al., 2003a). Büchler andCharrière et al. observed in their repeated stud-ies a slight, but not significant, tendencytowards better overwintering of the controls(ANOVA; Duncan-test, P > 0.05); the devel-opment of the colonies in spring of both groupswas equal (ANOVA, Charrière et al., 2004).Relatively high bee mortality (8%) was observedduring 3 weeks after treatment with 3, 4.6, 6%oxalic acid dihydrate and in the control group(Büchler, 1999). The author suggested that badweather conditions during the time bees wereflying could have caused the high mortality.Büchler (2002) found equivalent bee tolerabil-ity when comparing the treatments of 3.5%oxalic acid dihydrate, lactic acid and Perizin.
At a concentration of 4.6% oxalic acid dihy-drate (3 µL per bee), the treated coloniesshowed a significant weakening during wintercompared to controls (Duncan-test, P < 0.05;Büchler, 2000). Studies with a higher concen-tration of 5% described doubled bee mortalityin autumn, bad overwintering (reduced colonystrength) of treated colonies and impairedspring development (Charrière, 2001; Liebig,1998, 1999).
Multiple autumn or summer treatments (2–3) by trickling were poorly tolerated by the beesin this Central European region (Büchler, 1998;Charrière, 2001; Liebig, 1999).
106 E. Rademacher, M. Harz
The ready-mixed solution “Bienenwohl”(oxalic acid, citric acid, alcohol, ethereal oilsand propolis) applied repeatedly in late summerand autumn (20 mL per hive) can causecolony losses during the winter (Schuster andSchürzinger, 2003). A single application of50 mL per hive in autumn induced highbee mortality over the winter period (Liebig,1999). The losses of colonies in studies byMoosbeckhofer (2001) cannot be definitelyattributed to the application of “Bienenwohl”or oxalic acid dihydrate, as the colonies werealready weak at the beginning of the treatment.Mutinelli and Baggio (2002) reported good beetolerability applying Ipereat (composed simi-larly to Bienenwohl).
Investigations in comparable climatic regionsin Canada described bad overwintering withcolony break down in both the oxalic acid andthe Apistan groups. The damages could not beattributed to the oxalic acid treatment (Nasret al., 2001).
It is noticeable that there are differences intolerability in varying climatic areas. In South-ern Europe, oxalic acid dihydrate concentra-tions up to 7% are tolerated (Gregorc andPlaninc, 2001; Mutinelli et al., 1997; Mutinelliand Baggio, 2002; Nanetti and Stradi, 1997;Nanetti et al., 2003a) even if the bee coloniesare treated 3 times. No losses of queens werereported and bee mortality did not increase.After winter, all colonies showed reduced sizebut the treated colonies did not differ from con-trols. In Northern Europe, bee tolerance of upto 6% oxalic acid dihydrate is described, butcolony size after overwintering following treat-ment with 6% was slightly poorer than in col-onies treated with 3% oxalic acid dihydrate(Nanetti et al., 2003a).
2.2. Evaporation
Oxalic acid dihydrate in the form of crystals,gelatine capsules or tablets was heat-evapo-rated (the correct chemical term is “subli-mated”) with different types of evaporators,predominantly during the broodless period.The application took about 4 minutes per hiveand required complex equipment. The trialswere mainly conducted in Central Europe,except for two studies from Southern Europeand Asia. Oxalic acid was applied at an outside
temperature ranging from 2 to 16 °C2. Theinvestigations are listed in Table II, grouped bytype of evaporator, geography and oxalic acidformulation.
2.2.1. Efficacy
A Varrox evaporator was used in most trials.This is a small electrical heating device whichcan be inserted into the hive in the entrance andallows the evaporation of oxalic acid in theclosed hive (Radetzki, 2000). This applicationdevice, with a dose of 1–2 g oxalic acid dihy-drate (crystals, capsules or tablets), providedhigh acaricidal effectiveness of > 90%, even95% or more in most studies (Charrière et al.,2004; Imdorf et al., 2002; Radetzki et al., 2000;Radetzki and Bärmann, 2001). One exceptionwas a study by Moosbeckhofer and Baumgartner(2002) with low efficacy (78%), but the authorssuggested that a very high mite populationmight have been the cause. In another study,carried out in Southern Europe, less than 90%mite mortality was reached (Baggio andMutinelli, 2003a), but it is known from Nanettiand Stradi (1997, using the trickling method)that in southern climates higher oxalic aciddihydrate concentrations are needed to achievehigh mite mortality. Ferrero et al. (2004) showeddifferences in efficacies over three years (64%,93.9%, 97%) after double administration of 2 goxalic acid dihydrate. The author suggestedthat high humidity during treatment could havecaused the low efficacy in one of the years.
A higher dosage of 2.8 g did not lead to anincreased efficacy (Ferrero et al., 2004;Radetzki et al., 2000). Application of 3 g or 5 g(more than doubled dosage when compared to1–2 g) oxalic acid dihydrate resulted in an effi-ciency of 99%. Using a dose of 0.5 g oxalic aciddihydrate crystals per hive did not ensure suf-ficient effectiveness: Radetzki et al. (2000)achieved 82.8%. The threshold for high effi-ciency seems to be 1 g per colony. This dosageshould be used in single-story hives; largerhives should be treated with 2 g oxalic aciddihydrate (Radetzki (2004) – personal commu-nication).
In colonies with brood a comparativelylower mite fall of 91.4% was achieved by
2 Temperature not mentioned in all papers.
Oxalic acid for the control of varroosis 107
Tabl
e II
. Tri
als
on e
vapo
ratio
n of
oxa
lic a
cid
dihy
drat
e.
Aut
hor
Yea
rSe
ason
No.
Eva
pora
tor
Dos
age
Eff
icac
y (%
)B
eeO
verw
inte
ring
No.
Col
ony
or s
war
mB
rood
ofO
A d
ih.
mea
n sd
,m
orta
lity
of
(siz
e (c
ells
)
appl
.(g
)m
in–m
axco
l.if
men
tion
ed)
Cen
tral
Eur
ope
Rad
etzk
i et a
l.20
00
autu
mn
1V
arro
x0.
5 (c
r.)82
.8, 5
1.7–
96.0
20
0–25
0*no
que
ens
loss
12
colo
ny
no
Rad
etzk
i et a
l.20
00au
tum
n1
Var
rox
1 (c
r.)96
.0, 9
3.2–
99.2
100–
150*
no q
ueen
s lo
ss10
colo
ny
no
Rad
etzk
i et a
l.20
00au
tum
n1
Var
rox
2 (c
r.)97
.2, 9
1.7–
99.6
150–
200*
no q
ueen
s lo
ss13
colo
ny
no
Imdo
rf e
t al.
2000
autu
mn
1V
arro
x2
(cr.)
96.8
--
5co
lony
no
Cha
rriè
re e
t al.
2004
aut
umn
1V
arro
x2
(cr.)
> 9
5***
*-
15–3
0% r
educ
ed12
(130
00, D
B)
no
col.
stre
ngth
(n.
s.)
Rad
etzk
i20
00au
tum
n1
-3
(cr.)
-13
4 +
/– 3
1.3
good
ove
rwin
teri
ng19
6484
no
Rad
etzk
i et a
l.20
00au
tum
n1
Var
rox
3 (c
r.)99
.0, 9
8.1–
99.7
200*
no q
ueen
s lo
ss11
colo
ny
no
Rad
etzk
i et a
l.20
00au
tum
n1
Var
rox
5 (c
r.) 9
9.2,
99.
0–99
.820
0–25
0*no
que
ens
loss
5co
lony
no
Rad
etzk
i & B
ärm
ann
2001
autu
mn
1V
arro
x1.
4 (c
ap.)
95.7
150–
200*
-20
8co
lony
no
Rad
etzk
i & B
ärm
ann
2001
autu
mn
1V
arro
x1.
4 (c
ap.)
92.9
150–
200*
-63
colo
ny ye
s
Rad
etzk
i & B
ärm
ann
2001
autu
mn
1V
arro
x2.
8 (c
ap.)
96.0
150–
200*
-32
7co
lony
no
Rad
etzk
i & B
ärm
ann
2001
autu
mn
1V
arro
x2.
8 (c
ap.)
91.4
150–
200*
-88
colo
ny ye
s
Moo
sbec
khof
er
2002
win
ter
1V
arro
x1
78.1
+/–
13.
3,
--
6co
l. (2
-Z.-
Flac
hz.)
no
and
Bau
mga
rtne
r(t
able
t***
)65
.2–9
4.3
Moo
sbec
khof
er
2002
win
ter
1V
arro
x1–
2 86
.6 +
/– 1
0.1,
-
-6
colo
ny
no
and
Bau
mga
rtne
r(t
able
ts)
67.4
–96.
8
Büc
hler
2002
autu
mn
1V
arro
x2
tabl
ets
-lo
wsl
ight
8
colo
ny
(100
0)
wea
keni
ng(2
1350
+/–
418
4)
Moo
sbec
khof
er e
t al.
2002
sp,s
u,au
1V
arro
x1
(tab
let)
/box
-in
crea
sed
-
7220
bee
s /c
age*
*no
108 E. Rademacher, M. HarzTa
ble
II. C
ontin
ued.
Aut
hor
Yea
rSe
ason
No.
Eva
pora
tor
Dos
age
Eff
icac
y (%
)B
eeO
verw
inte
ring
No.
Col
ony
or
swar
mB
rood
ofO
A d
ih.
mea
n sd
,m
orta
lity
of
(siz
e (c
ells
)
App
l.(g
)m
in–m
axco
l.if
men
tion
ed)
Imdo
rf e
t al.
2002
autu
mn
1V
arre
x2
(cr.)
90.6
--
12co
lony
no
Lie
big
& H
ampe
l20
02la
te
1V
arre
x(c
r.)45
.0hi
gh-
6co
lony
(2
000–
sum
mer
7000
)
Imdo
rf e
t al.
2004
autu
mn
1V
arro
gaz
2.4
(cr.)
92.0
--
7co
lony
no
Imdo
rf e
t al.
2004
autu
mn
1K
rüso
2.4
(cr.)
29.0
--
7co
lony
no
Imdo
rf e
t al.
2004
autu
mn
1Is
enri
ng3
(cr.)
88.6
--
5co
lony
no
Sout
hern
Eur
ope
Bag
gio
and
2003
aau
tum
n1
Var
rox
2 (c
r.)83
.4 +
/– 1
2.4,
lo
w (
n.s.
)-
7co
lony
no
Mut
inel
li71
.7–9
5.7
Ferr
ero
et a
l.20
04au
tum
n1
Var
rox
2.8
95.8
+/–
2.3
,92
-98
-go
od o
verw
inte
ring
5co
lony
no
Ferr
ero
et a
l.20
04au
tum
n2
Var
rox
264
.0 +
/– 5
.64,
56.2
–73.
3-
good
ove
rwin
teri
ng8
colo
nyno
Ferr
ero
et a
l.20
04au
tum
n2
Var
rox
293
.9 +
/– 5
.5,
83.6
–99.
1-
good
ove
rwin
teri
ng8
colo
nyno
Fer
rero
et a
l.20
04au
tum
n2
Var
rox
297
.0 +
/– 1
.9,
94.8
–99.
0-
good
ove
rwin
teri
ng6
colo
nyno
Asi
a
Rom
mel
1999
a.l.h
.3
self
mad
e1
(cr.)
> 9
0%lo
wno
que
ens
loss
60C
olon
y -
Con
cent
rati
ons
refe
r to
oxa
lic a
cid
dihy
drat
e: M
ol. w
eigh
t: 1
26.0
4 g;
hyph
en: n
ot m
enti
oned
in o
rigi
nal a
rtic
le;
a.l.h
.: af
ter
last
har
vest
;
* nu
mbe
r of
dea
d be
es f
rom
gra
phic
al p
rese
ntat
ion
(4–5
wee
ks a
fter
trea
tmen
t);
cap.
: oxa
lic
acid
cap
sule
s;
** 2
0 be
es p
er c
age/
1box
with
cag
es;
cr.:
oxal
ic a
cid
cris
tals
;
***
1 ta
blet
con
sist
s of
1g
Oxa
lic
acid
dih
ydra
te;
App
l.: a
ppli
cati
on;
****
eff
icac
y ta
ken
from
fig
ure;
col.:
col
onie
s;
n.s.
: not
sig
nifi
cant
;
sp: s
prin
g;D
B: D
adan
t-B
latt
;
su: s
umm
er;
OA
dih
.: ox
alic
aci
d di
hydr
ate;
au: a
utum
n.2-
Z.-
Flac
hz.:
2-Z
ande
r-F
lach
zarg
en(=
nar
row
sup
ers)
.
Oxalic acid for the control of varroosis 109
applying the high dosage of 2.8 g (Radetzki andBärmann, 2001).
Another electric evaporator (Varrex), con-structed in a similar way to the Varrox device,was described by Imdorf et al. (2004). When2 g oxalic acid dihydrate was sublimated, anefficiency of 90.6% was reached in broodlesscolonies (Imdorf et al., 2002) and 45% in col-onies with brood (Liebig and Hampel, 2002).
In testing three different gas evaporators(Varrogaz, Isenring, Krüso described byImdorf et al., 2004), which heat with gas burn-ers, only Varrogaz reached mite mortality com-parable to the Varrox electrical evaporator.Isenring and Krüso evaporators showed lowerefficiencies, although high oxalic acid dihy-drate dosages were sublimated (2.4 resp. 3 g);Krüso (2.4 g) was least effective with 29% mitefall (Imdorf et al., 2004).
2.2.2. Tolerability
Most authors reported that the evaporationof oxalic acid dihydrate (0.5 to 5 g crystals)with the Varrox evaporator had no impact onbee mortality and colony overwintering; nolosses of queens were observed (Baggio andMutinelli, 2003a; Ferrero et al., 2004; Radetzki,2001; Radetzki et al., 2000; Radetzki andBärmann, 2001). Charrière et al. (2004) foundthat after a single treatment with 2 g oxalic aciddihydrate higher bee mortality was observedduring the winter compared to the controlgroup (15 and 30% compared to 12 and 16%),but the differences between the groups were notsignificant (ANOVA). Observations concern-ing repeated treatments using the Varrox evap-orator did not result in colony damage(Radetzki, 2002).
Moosbeckhofer et al. (2002) tested the tol-erability of oxalic acid dihydrate (1 g) on cagedbees, placed in a one-story hive, using the Var-rox evaporator. Mortality among treated beeswas significantly higher than in control groups,irrespective of the season when the beesemerged.
2.3. Spraying
Solutions of oxalic acid dihydrate weresprayed onto the bees on both sides of eachcomb and the bees resting on the hive walls;
spraying was normally carried out during thebroodless period. The application took approx-imately 4–5 minutes per hive. Most trials wereconducted in Central Europe at outside temper-atures ranging from 5 to 12.3 °C. The investi-gations are listed in Table III grouped byclimatic area, season and dose.
2.3.1. Efficacy
The investigations in Central Europe weremostly conducted on broodless colonies. Aconcentration of 3% oxalic acid dihydrate anddoses of 2.5–4 mL or 3–4 mL per comb sidereached efficacies of 97.3 to 98.8% (Charrièreet al., 1998; Charrière et al., 2004; Imdorf et al.,1995; Radetzki, 1994). In colonies with brood,only 61% of the mites were killed (Charrièreet al., 1998). In the warm climate of SouthernEurope, the broodless period is very shortand mite reproduction high, therefore severaltreatments may be necessary. Two treatments(7.3 and 6.4 g per comb side; one gram corre-sponds here to approximately to one millilitre)conducted by Nanetti et al. (1995) showed highefficacy of 99.5% in broodless colonies. Trialswith repeated treatments (every seven days forfour weeks) were carried out when brood waspresent in the colonies and led to a reduced mitemortality to 73% in spring and 94% in autumn,respectively (Higes et al., 1999).
2.3.2. Tolerability
Single treatments with 3% oxalic acid dihy-drate solution in a dosage of 2.5–4 mL or 3–4 mL per comb side were well tolerated(Charrière et al., 1998; Radetzki, 1994, 2001).None of the colonies lost their queen; bee mor-tality was not increased. Charrière et al. (2004)described losses during winter (11 to 26% ofthe bees per hive) that were slightly, but not sig-nificantly, higher compared to the controls (12to 16%, ANOVA). Even the highest concentra-tion evaluated (5% oxalic acid dihydrate)caused no further problems; mortality did notincrease during the five days after treatment(Radetzki, 1994).
Colonies treated twice with 3% oxalic aciddihydrate solution (7.3 and 6.4 g per comb side;one gram corresponds here approximately toone millilitre) resulted in high bee mortality aver-aging 170 dead bees after each administration
110 E. Rademacher, M. HarzTa
ble
III.
Tri
als
on s
pray
ing
of o
xali
c ac
id d
ihyd
rate
.
Aut
hor
Yea
rN
o.C
onc.
C
onc.
D
osag
eE
ffic
acy
(%)
Bee
Ove
rwin
teri
ngN
o.C
olon
y or
sw
arm
Bro
od
ofO
A d
ih.
OA
anh
. (m
L)
Mea
n sd
, m
orta
lity
of(s
ize
(cel
ls)
appl
.(%
, w/v
)(%
, w/v
)m
in–m
axco
l.if
men
tion
ed)
Cen
tral
Eur
ope
– T
reat
men
t in
aut
umn
Rad
etzk
i19
941
32.
12.
5–4
p.c.
s.97
.3, 9
2.7–
99.3
low
-
9co
lony
no
Rad
etzk
i20
01.
13
2.1
--
246
b/co
l.w
eake
ning
19(6
200)
no
over
5 w
eeks
Imdo
rf e
t al.
1995
13
2.1
3–4
p.c.
s.97
.3–9
8.8
not i
ncre
ased
-40
colo
nyno
Cha
rriè
re e
t al.
1998
13
2.1
3–4
p.c.
s.97
.6, 8
9.5–
99.8
not i
ncre
ased
good
ove
rwin
teri
ng11
2co
lony
no
Cha
rriè
re e
t al.
1998
13
2.1
3–4
p.c.
s.61
, 42–
87no
t inc
reas
edgo
od o
verw
inte
ring
10co
lony
c.c.
Cha
rriè
re e
t al.
2004
13
2.1
3–4
p.c.
s.>
95*
11–2
6% r
educ
ed c
ol.
18co
lony
no
stre
ngth
(n.
s.)
(10–
1300
0, D
B)
Rad
etzk
i19
941
53.
520
/com
b-
29–8
3 b/
col.
-5
colo
ny
no
over
5 d
Sout
hern
Eur
ope
– T
reat
men
t in
aut
umn
Nan
etti
et a
l.19
952
32.
17.
3 an
d 6.
4g99
.517
0 b.
aft
er e
ach
1 qu
eenl
ess
10co
lony
no
p.c.
s.**
adm
inis
trat
ion
Hig
es e
t al.
1999
43
2.1
4 p.
c.s.
94, 8
3.3–
99.6
1 co
l. de
ad, 1
que
enle
ss,
5co
lony
(1
557
+/–
657
)
less
bro
od in
sp.
Sout
hern
Eur
ope
– T
reat
men
t in
spr
ing
Hig
es e
t al.
1999
43
2.1
4 p.
c.s.
73, 6
5–79
.81
col.
dead
5co
lony
(5
144
+/–
169
0)
less
bro
od p
rodu
ctio
n (s
.)
Con
cent
rati
ons
refe
r to
oxa
lic
acid
dih
ydra
te: M
ol. w
eigh
t: 12
6.04
g;
c.c.
: cap
ped
cell
s;sp
.: sp
ring
;s.
: sig
nifi
cant
;O
A d
ih.:
oxal
ic a
cid
dihy
drat
e;D
B: D
adan
t-B
latt
;
col.:
col
onie
s;b:
bee
s;n.
s.: n
ot s
igni
fica
nt.
App
l.: a
ppli
cati
on;
* ef
fica
cy ta
ken
from
fig
ure;
p.c.
s.: p
er c
omb
side
.d:
day
.C
onc.
: con
cent
rati
on.
** d
osag
e in
gra
mm
.
Oxalic acid for the control of varroosis 111
and queen loss in one colony (Nanetti et al.,1995). When oxalic acid dihydrate solution(3%, 4 mL per comb side) was applied multipletimes during autumn or spring (every sevendays for four consecutive weeks) queen andcolony losses occurred. Significantly fewercells containing sealed brood were reported intreated colonies compared to the control groups(Wilcoxon–Mann-Whitney-test, P < 0.05; Higeset al., 1999).
The effect of oxalic acid on bee larvae wasinvestigated by Gregorc et al. (2004). A solu-tion of 6.5 g oxalic acid dihydrate / 50 g sugar/100 mL water sprayed on honey bee larvae(0.121 mg / larvae) affected the columnar cellsof the midgut, leading to necrosis.
3. HEALTH AND SAFETY
Gumpp (2002) studied health and safetyissues of the beekeeper with two differentapplication methods: evaporation (Varrox, 1–2 g oxalic acid dihydrate per hive) and spraying(3% oxalic acid dihydrate solution, 2.5 mL percomb side). Air samples were gathered fromthe beekeeper’s working area. Membrane fil-ters were inserted into a standard air samplerconnected to an air-collection pump. Two par-ticle fractions were taken: fraction E (breatha-ble intake of particles) and fraction A (alveolicintake of particles). Ten beekeepers used theevaporation method; ten used the sprayingtechnique. The beekeepers were mostly work-ing with free-standing beehives, but occasion-ally in bee-houses. When the spraying methodwas used, air samplers were positioned close tothe beekeeper’s working area within one meterof the colony. For oxalic acid evaporation thesamplers were placed within one meter of thehive entrance. To gain personalized samples afilter was placed on the beekeeper’s clothes(fraction A only). After air sampling, oxalicacid-carrying membrane filters were extractedwith distilled water and analyzed using HPLC.
Fraction E: The mean value (n = 10) for thespraying method was 0.22 mg/m3 and 0.23 mg/m3 for evaporation. There was no significantdifference between the two methods (Shapiro-Wilk-test, P = 0.05). Both results stayed sig-nificantly under the maximum exposure limitof 1 mg/m3 (MAK-value for Germany, BMA,2000; TLV-TWA for USA, Department of
Health and Human Services, 2005; MEL forUK, University of Bristol, 2005). A significantdifference was found comparing treatments infree-standing bee hives (n = 15) and in bee-houses (n = 5) (Shapiro-Wilk-test, P = 0.05).The oxalic acid concentration surrounding thefree-standing bee hives was lower than in bee-houses: the average was 0.15 mg/m3 and0.30 mg/m3, respectively. Both results wereunder the maximum exposure limit of 1 mg /m3.
Fraction A: The mean value (n = 10) for thespraying method was 0.15 mg/m3 and 0.07 mg/m3 for evaporation. There was no significantdifference between the two methods (Shapiro-Wilk-test, P = 0.05). No significant differencewas found when comparing treatments in free-standing bee hives (n = 15) and in bee-houses(n = 5) (Shapiro-Wilk-test, P = 0.05). The aver-age was 0.06 mg/m3 and 0.18 mg/m3, respec-tively. The data from personalized samplers(n = 9) showed a lower concentration than fromthe samplers in the beekeepers’ working area(n = 11): 0.06 mg/m3 resp. 0.18 mg/m3 (Shapiro-Wilk-test, P = 0.05). The results were under themaximum exposure limit of 1 mg/m3.
Gumpp’s study provides evidence thatoxalic acid dihydrate, when applied correctly,poses no inhalation risk to the beekeepers’health. The existing exposure limit for oxalicacid was not exceeded in any test case. Thereis no known risk in terms of systemic effects ofthe compound.
This information is based on the applicationof oxalic acid with one evaporator. When usingmultiple systems, as described by Gotti (2004),the oxalic acid concentration in the air sur-rounding the apiary may increase beyond thevalues mentioned above. Furthermore, the bee-keeper risks contamination when handlingoxalic acid crystals. Apart from that, the colo-nies should not be inspected directly after treat-ment, because no data is available about oxalicacid concentration in the air of the hive over aperiod of time. However, to avoid local effectsthe beekeeper should wear the recommendedsafety equipment. A FFP2 SL-type face masksufficiently protects the user (European Regu-lation EN 149). The apiarist should also wearprotective glasses, acid-proof protective glovesand long sleeves to avoid direct contact withoxalic acid dihydrate solution and crystals. Abasin for washing hands and an eyewash shouldbe placed in the apiarist’s working area. When
112 E. Rademacher, M. Harz
working with oxalic acid dihydrate, bee-housesshould be adequately ventilated.
On the basis of the investigation into usersafety during the application of oxalic aciddihydrate by either evaporation or sprayingtechniques, we suggest that the risks from usingthe trickling method are even lower, becauseinhalation of oxalic acid is not likely.
4. RESIDUES AFTER TREATMENT WITH OXALIC ACID
It is not expected that oxalic acid will accu-mulate in beeswax and propolis due to itshydrophilic properties. Oxalic acid is a naturalcomponent of honey. Concentrations in honeyvary between 3.3–761.4 mg/kg (Nanetti et al.,2003c) depending on the botanical origin of thenectar. Natural values for oxalic acid in honeyare, for example: honeydew 38–119 mg/kg;wildflower honey 8–51 mg/kg (Bogdanovet al., 2002); heather 48–151 mg/kg and 85.5–168 mg/kg (Nozal et al., 2000, 2003); honey-dew 59–158 mg/kg, oilseed rape honey 13–53 mg/kg (Pechhacker et al., 2004). Most hon-eys contain < 200 mg oxalic acid/kg honey(Wibbertmann, 2003). A review of residuesafter oxalic acid dihydrate treatment of bee col-onies are listed in Table IV, grouped by appli-cation method, number of treatments and timeof year.
4.1. Trickling
Single autumn treatments using oxalic aciddihydrate (3.1 to 6%, 5 mL/bee space) wereconducted in several studies. Nanetti and Stradi(1997) reported that the content of oxalic acidin the remaining winter food was not increased.Another investigation by Nanetti et al. (2002)compared the oxalic acid content of springhoney after autumn treatment: higher concen-trations up to 76.3 mg/kg (+/– 18.3) werefound, but were still within the natural contentlevels of honey from various botanical origins.Application of 4.7% and 10–15 mL per hive(Moosbeckhofer et al., 2003) did not increasethe oxalic acid content of spring honey.
Repeated treatments with oxalic acid dihy-drate solutions from 3.5 and 6% and 50–55 mLper hive slightly increased the oxalic acid con-tent in honey after treatment by 22 mg/kg
(Floris et al., 1998) and in spring honey by 0.3 mgand 7 mg/kg, respectively (Moosbeckhofer et al.,2003; Bogdanov et al., 2002). In one case, eventhe highest concentration (7% oxalic acid dihy-drate, 25–30 mL per hive) did not raise theoxalic acid content of the honey directly aftertreatment (Mutinelli et al., 1997).
Treatments in spring or summer with vary-ing dosages led to somewhat higher oxalic acidcontents in the honey after application (Liebig,1999; Brødsgaard et al., 1999), but the differ-ence was not significant compared to controls(Kruskal-Wallis-test, P < 0.05; Brødsgaardet al., 1999).
4.2. Evaporation
After treating colonies during autumn withthe Varrox evaporator (1–5 g oxalic acid dihy-drate crystals/tablets) the oxalic acid content ofspring honey was within control limits (Radetzkiand Bärmann, 2001; Moosbeckhofer andBaumgartner, 2002).
4.3. Spraying
Oxalic acid treatments in autumn using 3%oxalic acid dihydrate solution (3–4 mL percomb side) did not influence the oxalic acidcontent in spring honey (Bogdanov et al.,2002); however the oxalic acid content ofhoney after the treatment did increase slightly(Aumeier, 1998).
Repeated application (4 times) in autumn orwinter (3% oxalic acid dihydrate, 50–80 mLdepending on colony size) increased the oxalicacid content in honey after the treatment by 13and 18 mg/kg, respectively (Floris et al., 1998;Nozal et al., 2000).
In spring (March), a single oxalic acid treat-ment (3%, 3–4 mL per comb side) caused a sig-nificant increase in the oxalic acid content inhoney up to 62.8 mg/kg eight days after thetreatment (Kruskal-Wallis-test, P < 0.05). ByJune, the levels in honey were back within con-trol limits (Brødsgaard et al., 1999).
5. CONSUMER SAFETY
Oxalic acid is an ubiquitous substance inplants and can be found in high concentrations
Oxalic acid for the control of varroosis 113Ta
ble
IV. T
rial
s on
the
resi
dues
of
oxal
ic a
cid
in h
oney
.
Aut
hor
Yea
rSe
ason
No.
Con
c.C
onc.
Con
c.D
osag
eO
A c
onte
nt
OA
con
tent
O
A c
onte
nt
Val
idat
ion
No.
ofO
A
dih.
OA
suga
r(m
L)
(con
trol
s)*
afte
r in
spr
ing*
**m
etho
dof
appl
.(%
)# (
%)#
(%)#
trea
tmen
t*co
l.
Tri
cklin
g
Nan
etti
and
Str
adi
1997
autu
mn
13.
12.
260
5 p.
b.w
.-
no in
crea
se-
-30
Nan
etti
et a
l.20
02au
tum
n1
3.1
2.2
605
p.b.
w.
45.9
+/–
10.
5 m
g/kg
-76
.3 +
/– 1
8.3
mg/
kg-
-
Moo
sbec
khof
er e
t al.
2003
autu
mn
14.
73.
350
10–1
5 p.
h.18
.1m
g/kg
-16
.4 m
g/kg
GC
-MS
-
Nan
etti
et a
l.20
02.
autu
mn
16
4.2
605
p.b.
w.
45.9
+/–
10.
5 m
g/kg
-56
.1 +
/– 2
3.5
mg/
kg-
-
Nan
etti
& S
trad
i19
97au
tum
n1
64.
260
5 p
.b.w
.-
no in
crea
se-
-30
Moo
sbec
khof
er e
t al.
2003
autu
mn
33.
52.
550
55 p
.h.
25.9
mg/
kg-
26.2
mg/
kgG
C-M
S-
Flor
is e
t al.
1998
win
ter
46
4.2
2050
p.h
.67
.4 +
/– 1
2 m
g/kg
89.4
+/–
26.
2 m
g/kg
-SI
GM
A D
iagn
. kit
-
Enz
ymat
ic a
ssay
Mut
inel
li e
t al.
1997
autu
mn
37
520
25–3
0 p.
h.23
9.79
mg/
kgno
incr
ease
-S
IGM
A D
iagn
. kit
10
Enz
ymat
ic a
ssay
Bog
dano
v et
al.
2002
autu
mn
26
4.2
5050
p.h
.19
mg/
kg-
26 m
g/kg
Boe
hrin
ger
OA
kit
12
Enz
ymat
ic a
ssay
Nan
etti
et a
l.20
03b
sum
mer
16
4.2
6050
p.h
.-
max
. 54.
2 m
g/kg
--
-
Brø
dsga
ard
et a
l.19
99sp
ring
16
4.2
603
p.b.
w.
19.5
–35.
8 m
g/kg
-41
.56
mg/
kg-
15
June
(n.
s.)
Lie
big
1999
sum
mer
43
2.1
n.m
.20
p.h
.13
.4 m
g/kg
21.3
mg/
kg-
-5
Lie
big
1999
spri
ng4
3.5*
*2.
5-
20 p
.h.
13.4
mg/
kg20
.9 m
g/kg
5
Lie
big
1999
sum
mer
48
5.7
-20
p.h
.18
.4 m
g/kg
27.8
mg/
kg-
-4
114 E. Rademacher, M. HarzTa
ble
IV. C
onti
nued
.
Aut
hor
Yea
rSe
ason
No.
Con
c.C
onc.
Con
c.D
osag
eO
A c
onte
nt
OA
con
tent
O
A c
onte
nt
Val
idat
ion
No.
ofO
A
dih.
OA
suga
r(m
L)
(con
trol
s)*
afte
r in
spr
ing*
**m
etho
dof
appl
.(%
)# (
%)#
(%)#
trea
tmen
t*co
l.
Eva
pora
tion
Rad
etzk
i & B
ärm
ann
2001
autu
mn
12–
5 g
(cap
.)26
–34
mg/
kg-
22.8
–37.
7 m
g/kg
-20
8
Moo
sbec
khof
er
2002
autu
mn
11
g -
no in
crea
se-
-6
and
Bau
mga
rtne
r(t
able
t)
Spra
ying
Rad
etzk
i19
94au
tum
n1
32.
1-
2.5–
4 p.
c.s.
-<
25 m
g/kg
-H
PLC
12
Aum
eier
19
98au
tum
n1
32.
1-
-22
.2–3
3.4
mg/
kg18
.8–4
1.5
mg/
kg-
--
Bog
dano
v et
al.
2002
autu
mn
13
2.1
-3–
4 p.
c.s.
19 m
g/kg
-19
mg/
kgB
oehr
inge
r O
A k
it12
Enz
ymat
ic a
ssay
Noz
al e
t al.
2000
autu
mn
43
2.1
-m
ax 8
0 p.
h.36
.62
µg/g
55.0
7 µg
/g(1
4d a
fter
4th
tr.)
-C
olum
n li
quid
Chr
omat
ogra
phy
5
Flor
is e
t al.
1998
win
ter
43
2.1
-50
p.h
.67
.4 +
/– 1
2 m
g/kg
80 +
/– 2
3.4
mg/
kg-
SIG
MA
Dia
gn. k
it-
Enz
ymat
ic a
ssay
Brø
dsga
ard
et a
l.19
99sp
ring
13
2.1
-3–
4 p.
c.s.
19.5
–35.
8 m
g/kg
incr
ease
37
.78
ppm
-
15
62.8
mg/
kgJu
ne (
n.s.
)
(s.)
Con
cent
rati
ons
refe
r to
oxa
lic a
cid
dihy
drat
e: M
ol. w
eigh
t: 1
26.0
4 g;
p.h.
: per
hiv
e;d:
day
;O
A d
ih.:
oxal
ic a
cid
dihy
drat
e;D
iagn
.: di
agno
stic
;
p.b.
w.:
per
bee
way
;tr.
: tre
atm
ent;
OA
: oxa
lic
acid
;#:
con
cent
ratio
ns a
re g
iven
as
w/v
.
p.c.
s.: p
er c
omb
side
;co
l.: c
olon
y;A
ppl.:
app
lica
tion;
* ho
ney
or w
inte
r fo
od;
s.: s
igni
fica
nt;
Con
c.: c
once
ntra
tion
;
** “
Bie
nenw
ohl”
(o
xali
c ac
id, c
itri
c ac
id, a
lcoh
ol, e
ther
eal
oils
and
pro
poli
s);
n.s.
: not
sig
nifi
cant
;ca
p.: o
xali
c ac
id c
apsu
les.
***
hone
y or
rem
aini
ng f
ood.
n.m
.: no
t men
tion
ed.
Oxalic acid for the control of varroosis 115
in many vegetables; e.g. spinach, rhubarb, beet-root, tea and cocoa (Holmes and Kennedy,2000). The oxalic acid content in plants is muchhigher than in honey. According to Holmes andKennedy (2000) the oxalic acid content ofhoney from treated colonies is not or onlyslightly increased. Even the highest levelsfound after spring treatment did not exceed thenaturally occurring oxalic acid content ofhoney from various botanical origins. The dailyamount of oxalic acid ingested in a Europeandiet is 70–80 mg and can reach up to 400–600 mg/day in a vegetarian diet (Gay et al.,1984). Poul (2003) estimated the mean dailydietary intake of oxalic acid to be 80 mg/day.An ADI (acceptable daily intake) of 0.89 mg/kgwas suggested; this corresponds to a safe dailyintake of 53.4 mg/day for a 60 kg human. TheUS Environment Protection Agency concludedthat 0.14 mg oxalic acid or oxalate/kg/day overa 24-hour-period represents the allowablehuman exposure from all sources (US EPA,1992).
Assuming a daily intake of 20 g honey witha high content of 200 mg oxalic acid/kg honey,the additional consumption of oxalic acid willbe about 0.067 mg/kg b.w. for a 60 kg person(Wibbertman, 2003). The author concludedthat this would not cause a risk to human health.
The theoretical oxalic acid intake in honeyfrom either treated or non-treated bee hives isnegligible when compared to the daily intakefrom other sources (Committee for VeterinaryMedicinal Products, 2003).
6. SUMMARYAND RECOMMENDATIONS
Oxalic acid dihydrate is a suitable com-pound for the control of V. destructor in brood-free colonies during the autumn and winterperiod within the Concept of Integrated VarroaControl. Oxalic acid achieved very good effi-cacy against the parasite and tolerability in thetarget species A. mellifera. Considering thereviewed studies, all three application methodsfor oxalic acid can be recommended; however,the beekeeper will most likely choose the trick-ling method since it is the easiest to apply, hasthe lowest cost and offers the least contact withthe acid.
6.1. Application methods
6.1.1. Trickling
Trickling oxalic acid dihydrate seems to bethe most suitable application method, espe-cially for large apiaries. The beekeeper needsonly a syringe or a similar applicator, glovesand protective glasses to apply the substance.The application is quick, about 1 min per hive,cost-effective and easy to conduct.
The method can be used as a standard treat-ment due to its effectiveness, tolerability andsimplicity of application. The recommendedoxalic acid dihydrate concentration, which var-ies across the different European climaticregions, was determined by comparing efficacyand tolerability for Central-, Southern- andNorthern Europe. For Central Europe a concen-tration of 3.5% oxalic acid dihydrate in sugar-water solution (1:1) is recommended (= 35 goxalic acid dihydrate/litre). The dose of approx.5 mL solution should be used per bee space; i.e.,30 mL on a small colony, 40 mL on a medium-size and 50 mL on a large colony. Recommen-dations for Southern and Northern Europe aregiven as follows: for Southern Europe 6%oxalic acid dihydrate in sugar-water solution(1:1) (= 60 g oxalic acid dihydrate/litre) and thedose of 5 mL per occupied comb, for NorthernEurope 4.5% oxalic acid dihydrate in sugar-water solution (1:1) (= 45 g oxalic acid dihy-drate/litre) and the dose of 20–25 mL on a smallcolony, 25–30 mL on a medium-size and 30–35 mL on a large colony (European Group forIntegrated Varroa Control, 2000).
A single treatment should be conductedduring the broodless period at an outside tem-perature of > 3 °C. Repeated and summer treat-ments are not recommended as they result in ahigh level of bee mortality and low efficacy,respectively, due to large numbers of cappedbrood cells.
6.1.2. Evaporation
Beekeepers using the evaporation methodwill have additional expenses; e.g., safetyequipment and an evaporator. They need towear gloves, a face mask and protectiveglasses. The Varrox evaporator used in mosttrials (Central Europe) showed good efficacyand tolerability when 1–2 g oxalic acid dihydrate
116 E. Rademacher, M. Harz
was vaporized depending on colony size andbeehive type. One g is sufficient for treatingsingle-story hives, larger hives (e.g. Dadant ortwo-story hives) should be treated with 2 g. Agreater level of user protection can be achievedby using oxalic acid tablets (1 g oxalic aciddihydrate/tablet) in the Varrox evaporator, asthere is no direct contact with the crystals. Thesingle application should be conducted duringthe broodless period at an outside temperatureof > 2 °C, and takes about 4 min per hive.
6.1.3. Spraying
Spraying oxalic acid is more time-consum-ing than trickling, as every comb must beremoved from the hive; therefore it is mostlikely to be used in small apiaries. The recom-mended dose is based on trials conducted inCentral Europe: a water solution of 3% oxalicacid dihydrate (= 30 g oxalic acid dihydrate/litre water) is sprayed onto the bees on eitherside of the combs and onto bees resting on thehive walls. A dose of 3–4 mL solution shouldbe used per occupied comb side; i.e., approxi-mately 50 mL on a small colony, 65 mL on amedium-size colony, 80 mL on a large colony.The single treatment should be conducted dur-ing the broodless period at an outside temper-ature of > 5 °C, and takes approximately 4–5 minutes per hive.
6.2. Health and safety, residuesand consumer safety
Assuming that the beekeeper uses oxalicacid according to the recommended health andsafety instructions and wears protective cloth-ing, the treatment poses no risk to the apiarist’shealth. Autumn treatment with oxalic aciddihydrate, according to the methods and dos-ages described, causes no evident increase ofnatural oxalic acid content in honey, and thereis therefore no risk to human health from con-suming the honey.
6.3. Legalisation of the use of oxalic acid as a drug in bees
Research on oxalic acid has been conductedfor beekeepers, but its use as a drug in honeybee colonies is strongly regulated by legisla-
tion. Without formal approval the treatmentremains illegal.
In all EU countries, government approval isonly given to a new veterinary drug after theEuropean Union’s Agency for EvaluatingMedical Products (EMEA) has determined themaximum residue limit (MRL) of the activeingredient allowed in the final food productaccording to Council Regulation (EEC) 2377/90 (Mutinelli and Rademacher, 2002). Thisprocedure is intended to protect consumersfrom dangerous residues in food resulting frommedicines used on animals. Conducted as acommon European project by scientific insti-tutes and beekeeper organisations from mostEuropean countries, oxalic acid was listed inAnnex II of Council Regulation (EEC) 2377/90in December 2003 (Rademacher and Imdorf,2004). This means that the substance isdeclared not dangerous and that no residuelimit is needed to protect the consumer. On thisbasis every European country can apply forlegal approval of oxalic acid as a drug in beecolonies.
ACKNOWLEDGEMENT
Special acknowledgement is given to the Baye-risches Staatsministerium für Umwelt, Gesundheitund Verbraucherschutz for their financial support.
Résumé – L’acide oxalique comme moyen delutte contre la varroose dans les coloniesd’abeilles – une synthèse. Depuis l’arrivée de l’aca-rien ectoparasite Varroa destructor Anderson andTrueman dans les colonies d’abeilles domestiques(Apis mellifera L.), les apiculteurs doivent réguliè-rement traiter à l’acaricide pour maintenir le parasitesous le seuil de dégâts économiques. Au cours des10 dernières années les acariens ont développé unnombre croissant de résistances aux acaricides desynthèse si bien que de nouveaux produits vétérinai-res sont nécessaires pour combattre la varroose.Cet article donne une vue d’ensemble sur l’acideoxalique utilisé comme acaricide contre V. destruc-tor en goutte à goutte, par évaporation etpulvérisation. L’acide oxalique est un acide large-ment répandu dans le règne végétal ; il est présentdans de nombreux légumes, comme l’épinard, la bet-terave rouge ou la rhubarbe, et aussi dans le miel.Contre la varroose le dihydrate d’acide oxalique estappliqué (a) par dégouttement dans une solutionsucrée directement sur les abeilles dans les passagesentre les cadres, (b) sous forme de cristaux qui sesubliment dans la ruche ou (c) par pulvérisationd’une solution aqueuse sur les abeilles qui se tien-nent sur les rayons.
Oxalic acid for the control of varroosis 117
Pour évaluer l’acide oxalique comme acaricide on aprit en compte l’efficacité, la tolérance par lesabeilles, la protection de l’utilisateur, la teneur enrésidus et la sécurité pour le consommateur. Lestableaux I à III regroupent les données disponiblesconcernant l’efficacité et la tolérance par les abeillesdes divers modes d’application en fonction de ladose appliquée, de la concentration en sucre de lasolution utilisée, de la période et de la fréquence detraitement et de la zone climatique. Sous ses troismodes d’application l’acide oxalique est efficace à90 % et bien toléré par les abeilles. Par sa facilitéd’application le dégouttement paraît particulière-ment bien convenir à la pratique apicole.L’application d’acide oxalique n’occasionne aucunennui de santé chez l’utilisateur s’il l’utilise de façonappropriée et porte des vêtements de protectionadaptés. En ce qui concerne la protection du con-sommateur et les résidus éventuels, on part duprincipe que l’acide oxalique, par des propriétéshydrophiles, ne se concentre pas dans la cire ni dansla propolis. La teneur naturelle du miel en acide oxa-lique est très variable (3,3–761,4 mg/kg) et dépendde l’origine botanique du nectar. Après applicationde dihydrate d’acide oxalique sa concentration aug-mente dans le miel ou dans les réserves d’hiver, puisles valeurs redescendent aux niveaux naturels auprintemps suivant (Tab. IV). L’acide oxalique a étéinscrit en décembre 2003 à l’Annexe II du Règle-ment 2377/90 du Conseil de l’U.E. par l’EMEA(Agence Européenne pour l’Évaluation des Médica-ments) ; cela signifie que la substance est nondangereuse pour le consommateur et que toute limitesupérieure de résidu est inutile.L’acide oxalique convient pour traiter les colonies àl’automne et en hiver dans le cadre d’une lutte inté-grée. Il se caractérise par une efficacité élevée, unebonne tolérance par les abeilles, une facilité d’appli-cation pour l’apiculteur et par une situation desrésidus dans le miel favorable.
Zusammenfassung – Oxalsäure zur Bekämpfungder Varroose an Bienenvölkern. Seit Auftreten desParasiten Varroa destructor in Bienenvölkern vonApis mellifera müssen Imker regelmäßig Akarizideeinsetzen, um den Ektoparasiten unterhalb der Scha-densschwelle zu halten. In den letzten 10 Jahrenentwickelten sich zunehmend Resistenzen der Mil-ben gegenüber den synthetischen Akariziden, sodass weitere Tierarzneimittel zur Bekämpfung derVarroose nötig wurden.Der vorliegende Artikel gibt einen Überblick überOxalsäure als Akarizid zur Bekämpfung der Var-roose in den Applikationsformen Träufeln,Verdampfen und Sprühen. Oxalsäure ist eine weitverbreitete Pflanzensäure, die in vielen Gemüsenwie z.B. Spinat, Rharbarber oder Rote Beete und auchim Honig vorkommt. Zur Behandlung der Varroose
wird Oxalsäuredihydrat (a) in Zuckerwasserlösungauf die Bienen in den Wabengassen geträufelt, (b)als Kristalle im Volk sublimiert oder (c) als wässrigeLösung auf die bienenbesetzten Waben gesprüht.Bei der Bewertung der Oxalsäure als Akarizid wurdenWirksamkeit und Bienenverträglichkeit, Anwender-schutz sowie Rückstandsbelastung und Verbraucher-sicherheit berücksichtigt. Die verfügbaren Daten zuWirksamkeit und Bienenverträglichkeit der ver-schiedenen Behandlungsmethoden bezüglich appli-zierter Dosis, Zuckerkonzentration der verwendetenLösung, Behandlungszeitpunkt und -häufigkeitsowie der Klimazone sind in Tabelle I–III zusam-mengestellt. Bei allen drei Applikationsformen sindWirksamkeiten von über 90 % bei guter Bienenver-träglichkeit zu erzielen. Träufeln erscheint wegeneinfacher Anwendung in der imkerlichen Praxis alsbesonders geeignet.Die Applikation von Oxalsäure verursacht beisachgemäßer Handhabung und Tragen geeigneterSchutzkleidung kein Gesundheitsrisiko für denAnwender. Hinsichtlich des Verbraucherschutzesund der Rückstandssituation ist davon auszugehen,dass sich Oxalsäure aufgrund der hydrophilenEigenschaften nicht in Wachs und Kittharz anrei-chert. Der natürliche Gehalt an Oxalsäure im Honigist sehr unterschiedlich (3,3–761,4 mg/kg) undabhängig von der botanischen Herkunft des Nektars.Nach der Applikation von Oxalsäuredihydrat erhöhtsich die Konzentration im Honig oder Winterfutter.Bis zur nächsten Trachtperiode im Frühjahr sinkendie Werte wieder auf die natürlichen Gehalte ab(Tab. IV). Oxalsäure wurde im Dezember 2003 vonder EMEA (European Union’s Agency for Evalua-ting Medical Products) in Annex II der CouncilRegulation (EEC) 2377/90 aufgenommen, wasbedeutet, dass kein nomineller Wert für die Rück-standshöchstmenge zum Schutz des Verbrauchersfestgelegt werden muß.Oxalsäure ist eine geeignete Substanz für die Herbst– Winterbehandlung, um im Rahmen des integrier-ten Bekämpfungskonzeptes Varroa destructor unterder Schadensschwelle zu halten. Oxalsäure zeichnetsich durch hohe Wirksamkeit gegen den Parasiten,gute Verträglichkeit am Zieltier Biene, einfacheAnwendung für den Imker sowie eine günstig zubewertende Rückstandssituation im Honig aus.
Oxalsaüre / Varrose / Honigbiene / Rückstände
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