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LIBRARY LOREGONSTATE
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The Pear Psyllain Oregon
Technical Bulletin 122
AGRICULTURALEXPERIMENTSTATIONOregon StateUniversityCorvallis, Oregon
November 1972
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CONTENTS
Abstract ---------------------------
Introduction --------------------------------------- ---------------------------------------------- --------------- 3
Psylla Injury to Pear ------------------------------- ----- ------------------------- 5
Pear Decline ------------------------- --------------------------------------------- 5
Psylla Toxin --------------------------------------------------------------------------------------- 6
Psylla Honeydew ______________--_ 6-------------------------------------------------------_______-_-___
Psylla Densities and Economic Losses ____-__-____ _____-_--_____-__-__ 7
Biology ------------------------------------------------------------------------------------------------------------ 7
Life History --------------------------------- -
Number of Generations ----------------------------------------------------------------------- 9Host Range ---------------------------------------------------------------------------------------- 9
Control ------------------------------------------------------------------------ 10------------------------------------
Natural Control ------------------------------------------------------------------------------------- 10
Chemical Control ------------------------------------------ ------------------------------------ 14
Summary --------------------------------------------------------------------------------------------------------- 19
Literature Cited --------------------------------------------------------------------------------------------- 20
AUTHORS: P. H. Westigard is an associate professor of entomology at theSouthern Oregon Experiment Station, Medford, and R. W. Zwick is an associateprofessor of entomology at the Mid-Columbia Experiment Station, Hood River,Oregon State University.
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the southwest area in the Rogue Riverdrainage near Medford. About 10 per-cent of the acreage is located in theWillamette Valley. Of the 22.000 acresplanted to pear, the Bartlett varietyaccounts for approximately 50 percentwhile the remainder is planted to win-
3
The Pear Psylla in OregonP. H. WESTICARD and R. W. ZWICK
ABSTRACT
Since its discovery in Oregon in 1946, the pear psylla, Psylla pyricolaForster, has become the most serious insect pest of pear. Damage to peartrees include the transmission of pear decline disease which has caused lossesof trees, injection of a phytotoxic toxin resulting in tree shock and injury,and secretion of honeydew causing fruit marking. Aspects of pear psyllabiology are discussed in relation to the pest's control. Natural enemies areknown to exert some suppression of pear psylla populations late in the growingseason but the application of insecticides, many of which the pest hasbecome resistant to, is the only means presently available for reducing pearpsylla populations to subeconomic levels.
Key words: Pear decline, tree shock, honeydew, biology, control, naturalenemies, insecticides, resistance, subeconomic.
INTRODUCTION
Pears are the most valuable treefruit crop of Oregon. In 1967 the cropwas valued at about 43 million dollars,about one-half of which was returnedto the grower. The vast majority ofproduction is located in two widelyseparated areas of the state-the northcentral section around Hood River and
over a 25-year period which are perti-nent to the creation of a rational controlprogram. For the most part, the sectionon chemical control is taken from dataobtained at the Mid-Columbia Experi-ment Station (Hood River) or theSouthern Oregon Experiment Station(Medford). Pertinent information onbiology was gathered from numeroussources including Oregon, California,Washington, New York, British Co-lumbia, Nova Scotia, and a few Euro-pean reports.
ter varieties including D'Anjou, Bosc,and Cornice. All of the winter varietiesand about half of the Bartletts are soldin fresh markets and therefore thegrowers are especially sensitive to fruitquality.
The appearance of the pear psylla inthe state in 1946 presented a poten-tially serious threat to pear quality aswell as to production itself. Since thattime the potential destructiveness hasbeen more than realized, and at thepresent time the pear psylla must berated as the number one insect pest ofpear.
The purpose of this report is to sum-marize results of research conducted
SPREAD OF THE PEAR PSYLLAThe pear psylla, Psylla pyricola
Forster, was first reported in theUnited States from the state of Con-necticut in 1832 (51).11 It is thoughtthat the pest originated in southernEurope or Asia Minor (11), and recentdata (12) involving studies on the re-lationship between the pear psylla andPyrus species from several countriestend to support this theory. Thoughthe psylla spread rapidly through thepear-growing areas of the easternUnited States and Canada (74), it wasnot found west of the Rocky Moun-
' Numbers in italics refer to LiteratureCited, page 20.
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Spokane
1939
.11Milton - F,eeretet
1946
4
BRITISH COLUMBIA
Pont iCton
1942
WASHINGTON
CALIFORNIA
Lakeport1955
Figure 1. Spread of the pear psvlla on the Pacific Coast.
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5
tains until over 100 years later. In thesummer of 1939 specimens of P.pyricola were detected in Washingtonnear Spokane (58, 62), and by 1942they had spread northward into BritishColumbia (73) and southward toYakima, Washington (7).
In Oregon pear psylla were firstdiscovered in 1946 near Milton-Free-water. No additional infestations werefound until September 1949, when thepsylla was present in orchards fromMilton-Freewater to Hood River. By1950 all areas of Hood River wereinfested and the psylla had spread
south into the Willamette Valley. Inthe fall of 1950 two adult psylla weretrapped on sticky boards north of Med-ford, and a year later the pest wasfound in most Rogue Valley orchardsand south to the California border.
The pear psylla spread southwardinto California, being reported fromthe most northern counties in 1953and then from the important pear-growing counties of Lake in 1955,Sacramento in 1957, Santa Clara in1957, and El Dorado in 1958 (46).The spread of the pear psylla is il-lustrated in Figure 1.
PSYLLA INJURY TO PEARSeveral forms of pear damage are
attributable to pear psylla. These in-clude pear decline, injection of toxin,and secretion of honeydew.
PEAR DECLINE
Pear decline disease struck the Pa-cific Northwest in the late 1940's withaffected trees rapidly collapsing (quickdecline), as shown in Figure 2, orgradually losing vigor and produc-tivity (slow decline). In 1960 thedisease was shown to cause sieve-tubenecrosis below the graft union (1),resulting in blockage of the conductivetissue. Decline was noted as beingmore severe on cultivars grafted ontooriental rootstocks such as Pyrus us-suriensis and P. pyrifolia than on culti-vars grafted onto European stocks ofP. communis (1, 66, 76). The pearpsylla was first associated with declinein 1962, when a toxin secreted by thepsylla was identified as the responsibleagent (31). Further studies in Cali-fornia have confirmed the role of pearpsylla but have shown that the diseasewas graft-transmissible and thereforemost probably a virus in nature (3,20, 28, 29, 53, 54). Most recently,another report has incriminated myco-plasma-like bodies as the responsibleagent of decline (26).
Losses of trees and production dueto pear decline are rather difficult to
estimate, but some figures are avail-able. For instance, between 1956 and1959 the Bartlett pear crop in Wash-ington dropped nearly 30 percent, andthis was in large measure attributedto decline (1). In California it wasestimated that over one million treeshad been affected by 1962 (46), withproduction in some areas dropping asmuch as 50 to 60 percent (2).
The major pear-growing areas inOregon suffered quite differently from
Figure 2. Quick decline symptoms onpear tree in Medford, Oregon (photo
courtesy C. B. Cords)
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about 25,000 trees, or about 15 per-cent of the total (14, 50, 65). Verylittle decline loss was reported in theHood River area. This was probablydue to the preponderance of decline-
decline. One of the hardest hit areaswas southern Oregon, where pear de-cline appeared in 1957-1958 (67). Ithas been conservatively estimated that10 percent of the trees were com-pletely lost, with 10 percent of theremaining, trees left as "cripples" (17).The most severe losses occurred on theoriental rootstocks (P. ussuriensis, P.pyri f olia) , with 50 to 60 percent mor-tality, while 10 to 12 percent of thepears on French roots were lost (14,17, 65). Five to ten percent of theacreage was on these susceptible root-stocks (65).
Decline appeared in the WillametteValley in 1957, with actual loss of
tolerant or resistant rootstocks in use(65). Pear decline was positively iden-tified in Hood River in 1961 and prob-ably occurred earlier (50). Estimatesindicate that about 15 percent of treeson P. communis root were affectedwith slow decline (65).
Pear decline has now passed throughthe Pacific Coast, taking with it themost susceptible trees and leavingmany weakened "slow decliners," butcausing no damage to the majority oftrees. From a practical standpoint, thedevelopment of resistant rootstocks foruse in replantings essentially solvedthe decline threat (66).
PSYLLA TOXIN
Irrespective of its disputed role inproducing pear decline, a toxin issecreted by the pear psylla. In theeastern United States, where pear de-cline has not been reported, psyllafeeding has resulted in undersizedfruit, wilting of foliage, severe defolia-tion, reduction in tree productivity,and death to limbs or to entire treesfollowing several years of high infesta-tion levels (7, 24, 51, 74). In otherwork, suppression of pear root growthand general reduction in tree vigorhave followed psylla feeding (13, 34,
75). The effects of psylla toxin aregenerally apparent following highpsylla levels and have been referredto as psylla shock (32). The plantreactions described above are not gen-erally expected from plants fed on bypests which merely remove photo-synthate, and are, therefore attribu-table to a toxicogenic substance.
PSYLLA HONEYDEW
In the process of feeding, psyllanymphs secrete pools of a sticky sub-stance called honeydew (Figure 3).Under conditions of relatively highinfestations, especially close to harvest,this sticky material may drip from theleaves onto the fruit and cause ascalding of the surface (Figure 4). Inaddition, a sooty mold fungus maygrow in the honeydew (11, 58), lead-ing to further downgrading. Copiousamounts of honeydew on foliage atharvest time have resulted in pickercomplaints and increased harvestingcosts.
Figure 3. Pear leaf showing psyllanymphs with typical amounts of honey-
dew.
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for the pear psylla. This will eventuallyhave to he completed if rational con-trol of this pest is to be achieved. Pre-liminary data in Table 1 show a rela-tionship between psylla densities anddamage due to the secretion of honey-
Figure 4. Psylla honeydew marking onD'Anjou fruit.
be delayed until the pest densities ap-proach levels that will result in eco-nomic losses greater than the cost oftreatment (59). This density level hasbeen called the economic threshold. Itis quite apparent from the descriptionof the various types of psylla injurythat there are several economic thresh-olds for this pest. For example, agrower with pears planted on decline-susceptible rootstocks would expect aneconomic threshold much lower than ifthe trees were on resistant roots. Un-fortunately, there have been no criticalstudies to establish injury thresholds
PSYLLA DENSITIES ANDECONOMIC LOSSES
One of the principles of good pestmanagement states that controlmeasures by use of pesticides should dew.
Table 1. Discoloration of D'Anjou pears from honeydew at harvest due to variouslevels of psylla infestation (Hood River, Oregon)
Average no. nymphs per Percentage
12 spurs per month fruit
Block May June
1 ................ 0 1.32 -------------- -- 1.0 03 ................ 24.0 7.0
4 ................ 0 3.0
5 ................ 0 0.56 ................ 1.0 0
7 ................ 59.0 24.9
LIFE HISTORY
The general life history of the pearpsylla in Oregon is similar to thatdescribed from other areas (4, 5, 11,12, 16, 18, 19, 34, 35, 40, 45, 51, 52,56, 60, 66, 69, 73, 74). The insectoverwinters in the adult stage, some-what larger and darker than the sum-
discoloredJuly Aug. Sept. at harvest
1970
1.0 10.3 4.0 2.2
1.0 1.4 1.0 2.8
10.0 35.0 23.0 8.6
1971
1.2 16.3 4.0 2.3
3.4 7.3 4.3 7.5
9.5 33.6 69.6 10.477.0 25.3 18.0 29.6
BIOLOGY
mer adult (Figure 5). Both males andfemales overwinter, and mating appar-ently does not occur until prior to ovi-position in late January or early Febru-ary. The first eggs laid by the over-wintering females are deposited at thebase of the unopened fruit or leaf buds(Figure 6) but oviposition continues
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Figure 5. Comparison of overwinteringand summer psylla adults. Overwinter-ing (upper), summer (lower).
Figure 6. Eggs of overwintering psyllafemales laid at base of unopened leaf bud.
until after the fruit buds open. At thistime the eggs will be laid on leaf tissue(Figure 7). The interval between ovi-position and the appearance of thefirst instar nymphs (Figure 8) is de-pendent upon temperature but aver-ages about five weeks in southern Ore-gon (Table 2) and six weeks in theMid-Columbia area (Table 3).
The pear psylla passes through fivenymphal instars prior to reaching theadult stage. The female psylla at-taches the pale yellow egg to the barkby cementing the elongated peduncleso firmly into a crevice that it rupturesif attempts are made to dislodge it. Asembryonic development proceeds, thecolor changes to a deeper yellow-orange, and prior to eclosion two redeye spots of the nymph are visible
Figure 7. Psylla eggs laid along midvein of pear leaf.
Figure S. Eggs and first instar pealpsylla nymphs.
through the chorion. The first-stagenymphs move to green leaf tissue andinsert their stylet mouthparts to feedon the sap of the pear tree. The youngnymphs are soon immersed in a poolof honeydew consisting of the sap notutilized in their nutrition. After thefirst molt, wing pads are external andbecome more prominent in each suc-ceeding molt, with the body colorbecoming progressively darker brownor blue-green.
The fifth and last nymphal stage or"hardshell" is dark brown, has promi-nent wing pads, and is not attached to
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mer generations is given in Figure 9.Thus, there is a total of four,our psvllagenerations per year under Oregonconditions. Ontario, Canada, reportsas few as two generations (74) andCalifornia as many as five (34). Fonr
9
..............
Table 2. Approximate developmental time for first generation pear psylla(Medford, Oregon)
Date o Date of Eclosion Date of first Total daysYea: first egg first nymph time summer adults (egg to adult)
1961 Jan. 28 March 15days
47 May 3 961959 Feb. 12 March 30 46 April 28 751957 Feb. 18 March 20 31 April30 721955 March 3 April 4 32 May 9 671952 Feb. 14 March 25 40 May 5 81
AVERAGE 39 78
Table 3. Observations on first generation pear psylla egg deposition(Hood River, Oregon)
and hatching
971
YearDate offirst egg
Feb 11
+43° F degreedays until first
egg found
206
Date offirst nymph
April 5
Eclosion time
days53
1970 ...................... Feb. 17 101 March 24 351969 ---------------------- March 12 1331968 ...................... Feb. 16 148 March 19 321967 ...................... Feb. 9 211 April 3 531966 ...................... Feb. 24 124 April 6 411964 Feb. 20 204
AVERAGE 161 42.8
the leaves by its stylets in a pool ofhoneydew. This stage moves activelyabout, found most often at the baseof leaf petioles or in crevices of thebark of fruit spurs. The length of timerequired for completion of the firstgeneration nymphal developmentranges from 30 to 50 days, with thefirst summer adults appearing in lateApril or early May (Figure 9).
NUMBER OF GENERATIONS
In addition to the spring generationdescribed above, there are three sum-mer generations in Oregon, endingwith the formation of the overwinter-ing adults in October or November.The approximate duration of the sum-
generations per year have been re-ported in Washington (45) and partsof British Columbia (73) (Table 4).
The appearance of the overwinter-ing adult in the fall is brought aboutby exposure of fourth-generationnymphs to shortened day lengths (11,49, 77). These adults exhibit a sexualreproductive diapause which continuesuntil exposure to cold temperatures isfollowed by warmer temperatures (11,49, 77). The overwintering adults alsoexhibit a tendency to disperse (11,25), a phenomenon not noted in thesummer adults. The active dispersalphase accounts for the rapid spreadof the species throughout the PacificCoast states.
HOST RANGE
Although the adult psylla and oc-casionally the egg stage can be foundon other hosts, the insect can completeits development only on pear. ThoughCydonia is often listed as a host for
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1,11111 11111111111 1111111111ti11111
111111111111111111
I
111111111 111111
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11111111111111
I I
leryana (64, 76) (Table 5).Though pear is required for com-
pletion of the life cycle, adult psylla,especially of the overwintering gen-eration, can be found on many hosts
Area
Ist generation Eggs
Nymphs
Adults
2nd generation Eggs
Nymphs
Adults
3rd generation Eggs
Nymphs
Adults
4th generation Eggs
Nymphs
Adults
Jon Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Figure 9. Approximate time of occurrence of the four pear psylla generations inOregon.
pear psylla (34, 58), it has been as P. fauriei, P. pyrifolia, or P. cal-shown that development on this host isarrested in the early nymphal stages(30). Even within the genus Pyrus,to which pear belongs, there arespecies which will not support thecompletion of psylla development. (30, 70, 71, 72). One of these transi-Generally, pear species of European tory hosts may have served as theorigin, such as P. communis, are more original host for the pear decline virusfavorable than those from Asia, such or mycoplasma (30, 46).
Table 4. Number of generations of the pear psylla reported from various areas of theUnited States and Canada
CanadaOntario ---------- 2-3 74British Columbia 4-5 73
United StatesWashington ---------------------------------------------------- 4 45Oregon (Medford) ........................................ 4 63California ........................................................ 5 34
CONTROL
NATURAL CONTROL set limits to psylla numbers, severalnatural environmental factors favor or
In addition to the intrinsic factors, discourage increases in densities of thissuch as reproductive potential, which pest.
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--ic. ::....................._. .j.............
temperature and low humidity, thehoneydew may crystallize and entrapthe young nymphs (11, 35, 41, 73).Though this phenomenon has been ob-served under Oregon conditions, it
Table 5. Infestation level of pear psylla on Pyrus species (Medford, Oregon)
Test 1. Infestation on caged PyrusAverage no. psylla per 100 sq. in.
Geographic area Eggs
Nymphsreachingmaturity
Asia ....................Asia Minor ------------------------------------------------------------North Africa ----------------------------------------------------------Europe ------------------------------------------------------------------
1763
12899
4308161
Test 2. Natural infestation on Pyrus species collectionAverage no. psylla per 25 leaves
Geographic area Eggs Nymphs
Asia ------------------------------------------------------------------------ 27 8.8Asia Minor ------------------------------------------------------------ 86 48.0North Africa ---------------------------------------------------------- 44 9.0Europe -------------------------------------------------------------------- 95 49.0
Test 3. Infestation on individually caged PyrusNo. psylla x 10 per sq. in.
Geographic area Eggs Nymphs
Asia -------------------------------------------------------------------------- 17 4Asia Minor .............................................................. 23 10Europe -------------------------------------------------------------------- 19 8
HOST PLANT CONDMON
Young succulent foliage is preferredby female psylla for oviposition sites(5, 11, 19, 35). During the earlyspring there is generally an abundanceof these favorable sites, but the amountof tender forage decreases as stemsand leaves begin to harden off andoviposition may be restricted. Nymphson older leaves or on leaves injuredby previous infestations may be un-suited for development (11). Fecun-dity of adults reared from maturefoliage may be lower than that ofadults from succulent tissue (42).Cultural practices such as irrigationand fertilization which influence treegrowth pattern will influence psylladensities.
CLIMATIC FACTORS
Temperature. Though moderate in-creases in temperatures shorten thedevelopmental time for psylla and
favor the increase in the number ofgenerations, excessive summer tem-peratures cause severe mortality (5,11, 35, 41, 68, 73). Temperatures inexcess of 900 F cause reduction inoviposition, and temperatures over1000 F cause mortality to nymphs(35, 68). Under conditions of high
does not appear to play an importantrole in natural control in this state.
Precipitation. In areas of the coun-try that normally receive heavyamounts of summer rainfall, largenumbers of psylla nymphs may bewashed from the pear leaves (25, 73).However, the Pacific Coast states usu-ally receive little summer rain, andthe number of psylla deaths attribu-table to this factor are small. The in-
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stallation of overtree sprinklers to pro-vide spring frost protection and sum-mer irrigation recently has becomepopular in western states, and thismay result in increased psylla mor-tality.
NATURAL ENEMIES
Table 6 summarizes the reportswhich list the number and kinds ofpredators and parasites feeding on thepear psylla.
In Oregon's Hood River area the
Table 6. Predators and parasites of the pear psylla reported fromNorth America and Europe
Order Species Area Reference
Hemiptera ..........Anthocoris antevolens White California 35, 63, 47Oregon 63
Washington 11Anthocoris melanocerus Reuter British Columbia 33, 73Anthocoris musculus (Say) Nova Scotia 52Anthocoris nemoralis F. Europe 6, 44
British Columbia 44Anthocoris nemorum L. England 18Campylomma verbasci (Meyer) British Columbia 44Deraeocoris brevis (Uhler) British Columbia 44
Oregon 63Washington 11
Deraeocoris fasciolus Knight British Columbia 44Diaphnocoris provancheri (Burque) British Columbia 44Orius sp. California 35
Neuroptera .......... Chrysopa carnea Steph. Oregon 63Washington 11
Chrysopa pacificus Banks British Columbia 44Chrysopa plorobunda Fitch California 49, 39Chrysopa oculata Say British Columbia 61
New York 56Atractotomus mali (Meyer) Nova Scotia 52Hemerobius agustus (Banks) California 39
Coleoptera .......... Adalia bipunctata (L.) New York 56Nova Scotia 52
Adalia frigida Schn. British Columbia 44Anisoclavia quatuor
decimguttata (L.) Nova Scotia 52Calvia duodecemmaculata Gebl. British Columbia 44Coccinella transversoguttata Fald. British Columbia 44
Oregon 63Nova Scotia 52
Hippodamia quinquessignata Kirby British Columbia 44New York 56
Diptera ..............Platypalpus sp. British Columbia 44Hymenoptera ...... Asaphes sp. British Columbia 44
Eudopsylla agilis de Meijere Scotland 27Lygocerus sp. England 18Prionomitus mitratus (Dalm.) British Columbia 44
Europe 27Washington 11
Psyllaephagus sp. England 18Trechnites insidiosus Crawford Ontario, Canada 74
British Columbia 43Oregon 63California 39Washington 11
Trechnites psyllae Ruschka England 18
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psylla nymphs on unsprayed trees bymid-August. However, even at thishigh rate of parasitization, serious foli-age damage was evident and over95 percent of the D'Anjou fruit borevisible discoloration from honeydew
predators appears to play an importantpart in the natural control of P. pyri-colo. Figure 10 presents the popula-tion trends of the pear psylla froman orchard left unsprayed for severalwars but otherwise well cared for (in-eluding pruning, irrigation, and fer-tilization). Population levels of thelater generations were lower than thoseof the spring and early summer. Thepopulation trends for some of thepredators found in this orchard aregiven in Figure 11. There appears tobe a good correlation between thepeaks in predator densities and thoseof the psylla egg and nymphal stages.One of (lie most effective predators insouthern Oregon is the mirid bug,Deracocuris brecis (Uhler), which
role of natural enemies has not been minor significance and cannot be de-observed to account for substantial re- pended upon to reduce psylla infesta-duction in psylla damage. Heavy in- tions below economic injury levels.troductions of the hymenopterous in southern Oregon the role ofnymphal parasite, Trechnites insidi-osus (Crawford), may result in para-sitization of over 70 percent of the
secretion. As a result of the frequentinsecticide applications necessary tocontrol psylla early in the season inthe Hood River area, the rate of par-asitization by Trechnites and the oc-currence of natural psylla predatorsin commercial orchards have been of
can consume nearly 600 psylla eggs
z
Figure 10. Population trends of pear Figure 11. Population trends for psyllapsylla in an unsprayed Bartlett pear predators in an unsprayed Bartlett pear
orchard (Medford, Oregon). orchard (Medford, Oregon).
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only means of escaping tile- damagecaused by the pear psylla. The .chemi-cals used in control change rather'rapidly, primarily due to resistance,and the cost of obtaining economicsuppression has risen steadily over-the past 10 years. At the present timeit is not unusual for growers to apply=six to seven sprays aimed primarilyat pear psylla. The following sectionreports on the use pattern of pesticides:in Oregon and discusses the various
The dormant spray is directedagainst the adult psylla prior to egglaying but after psylla activity beginsin mid-to-late winter. Early workersrecommended the use of mineral oil
wide program utilizing synthetic pesti=cide applications aimed exclusively atdestruction of the overwintering adults.The need for this special spray wasbrought about by resistance to manyinsecticides used during the summermonths and the inability of the in-secticides to .lull all or even most
adult forms but tint on eggs. Re-sistance by the adult to Perthane nowhas been reported from Washingtonand northern Oregon.
The effectiveness of the, doi iTio
psylla have returned' to the pear or=chard by the time oviposition begins.The probability of this being correct'is strengthened by the past success ofthe Perthane spray, a material of short
II
and nymphs during its development.The predators belonging to the familyAnthocoridae, which have been re-ported as effective predators in BritishColumbia and California (33, 39), oc-cur only in low numbers in southernOregon.
CHEMICAL CONTROL
Chemical control still remains the
timings which have been found use-ful in obtaining control.
DORMANT APPLICATION
sprays at this timing because thesprays were effective on adults andalso inhibited egg laying (21). Morerecently (8) growers in the PacificNorthwest have resorted to an area-
psylla stages. The material that hasbeen most used in the dormant sprayis Perthane®, which is active on the
spray depends not only upon the avail-ability of effective materials but also
on critical timing of the application.The correct timing has its origins inthe behavior of the overwinteringadults and depends on several environ-mental factors as well. First, followingthe general dispersal of the fall brood,many psylla will winter outside thepear orchard. The list of transient hostsincludes a wide range of plants fromwhich the adults probably require onlywater in order to survive. Overwinter-ing adults have been found on suchdiverse plants as alfalfa, apple, andpeach.
The percentage of the adult psyllathat return to the pear from otherhosts is not presently known, butbecause successful development de-pends upon the presence of pear, it isprobable that many adults find theirway back to the pear orchard. Thus,the timing of the dormant spray mustbe delayed until the return of adultpsylla from sources outside the or-chard. Until work is done on thisaspect, it has to be assumed that most
residual activity.A second variable encountered in
timing the dormant spray is the avail-ability of the adult psylla within thepear orchard. During much of thewinter the adults are found in barkcrevices or in other places inaccessibleto sprays. They will emerge from theseareas when temperatures increase toabout 45° F (11). Application ofsprays should be made when tempera-tures are expected to reach or exceedthis range.
A third variable is the change insusceptibility of the adult to pesticides.In laboratory studies susceptibility toPerthane by the adult decreased in thefall, then increased in midwinter. Asecond drop in susceptibility was notedin late January (Figure 12).
Two techniques have been usedwith some success in guiding growersin correct timing of the dormant spray.
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75 -
11. '111
111
M
25.i
0
11
1
Jan 6
measurement of egg size has enabledaccurate prediction of when egg lay-ing will begin (Figure 13). Severalyears' dissections have indicated thatwhen 15 to 20 percent of the over-wintering female psyllids collected
Feb 17 Mar 2
correct timing involves accumulatingdaily maximum temperatures from thefirst of January and applying the dor-mant spray when a certain total isreached. This method is quite vari-able, being more useful in southernOregon than in the Hood River area(Table 3). In the former area themaximum daily temperatures over430 F, totaling about 250, usuallycoincide with the start of egg lay-ing (Table 7) . However, because ofthe variation, dissection of the females
............. IGal per 100
I Ot per 100
I Pt per 100
Oct 9 Nov 6 Dec 9 Jon 20 Feb 11
1970 1971
Figure 12. Variation in overwintering adult psylla susceptibility to Perthane overseveral months' time (Medford, Oregon).
First and best is the dissection of be found on fruit spurs in pear or-the overwintering females and exami- chards (79).nation of ovarian development. At The second method of estimatingHood River, dissection of females and
from field samples contain mature eggsin their oviducts, the first eggs can
Figure 13. Dissected overwinteringpsylla female showing egg development
is still needed for determining eggmaturity.
The future use of dormant spraydepends upon the continued avail-ability of materials which both con-trol overwintering adults and are ef-fective at more convenient timings.However, the dormant spray has theadvantage of being used at a timewhen it does not cause destruction ofpredators or parasites of the pearpsylla or of other orchard pests.
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---........................................ ...........................................................................---------------------------------------------------------------------------- ............................................... ----...............................................
....................... ....................................
The delayed dormant spray is ap-plied from the time. of pear bud swellto the opening of the bud and thedropping of the bud scales. Psyllastages at this time generally include.overwintering adults and eggs. Dur-ing this time several of the other im-
Table 7. Egg deposition by overwintering psylla in relation to accumulated degreedays over 43 ° F from January 1 (Medford, Oregon)
YearDate of firstpsylla eggs
Dateno. of degree daysover 43 ° F = 250
1970 January 271969 February 141968 February 101967 February 81966 February 101964 February 141963 February 111960 January 241959 February 51958 February 9]957 February 18
January 25February 26February 7February 15February 19February 18February 8February 6February 1February 7February 18
Table 8. Effectiveness of insecticides in control of pear pests at the delayeddormant timing
Target pest
San Jose EuropeanMaterial scale Rust mite red mite
Pear psylla
Eggs AdultsOil alone 1° 3 1 3 2Oil + lime sulfur .................... 1 1 1 3 2Oil + organophosphate .......... 1 2 1 3 2Perthane --------------------------- ------ 3 3 3 3 1Perthane + oil (6-8 GPA) .--- 1 3 1 3 1Thiodan ---------------------------------- 3 1 3 3 2Thiodan + oil (6-8 GPA) .... 1 1 1 3 2
°1 = good control expected, 2 = partial control, 3 = poor control.
The insecticides that have been usedat the dormant time, along with ex-amples of the degree of control ob-tained, are given in Tables 8 and 9.A gradual increase in Perthane toler-ance in Hood River orchards is shownin these results.
DELAYED DORMANT APPLICATION
portant pear pests become active andmore susceptible or exposed to chemi-cal treatment. Historically, the delayeddormant spray has been considered asan application directed against such
pests as San Jose scale, Quadraspidio-tus perniciosus Comstock; pear rustmite, Epitrimerus pyri Nalepa; and theEuropean red mite, Panonychus ulmiKoch. As a result, broad-spectrumchemicals usually are chosen for useat this time.
In Oregon the use of lime sulphurand oil was a standard recommenda-tion for the delayed dormant spraylong before the pear psylla was intro-duced. Use of this combination, aswell as the subsequent organophos-phate-oil combinations, has generallybeen less effective on psylla than onthe other pests. The lack of effective-ness is due not only to the emphasisgiven to control of pests other thanpsylla but to the lack of insecticideswhich are capable of killing the over-wintered adult psylla and their eggs.
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Table 9. Dormant sprays for control of overwintering pear psylla adults(Hood River, Oregon)
PDays after spray
re-trmt. 1-2 3-5 6-8 9+
Date Rate er Method ofapplied Material
pacre application No. adult psylla per tray
3-5-66 --------- Perthane EC 1.0 gal. Helicopter 1 .... 0 03-11-66 ------- Perthane EC 1.0 gal. Air carrier 2.9 .... .._. 0.1 02-18-67 ------- Perthane EC 1.0 gal. Air carrier 5.0 .... 0 02-20-67 ------- Perthane EC 1.0 gal. Helicopter 5.8 .... 0.2 0 02-24-68 ........ Perthane EC 1.0 gal. Air carrier 5.4 0 0 02-25-68 ________Perthane EC + 1.0 gal. Fixed wing 9.3 0.8 ____ 0.1 0
70 vis oil3-12-69 ........ Perthane EC +
1.0 gal.1.0 gal. Air carrier 02 0.1 0.4 0
70 vis oil2-20-70 ....... Perthane EC +
1.0 gal.1.0 gal. Air carrier 3.2 0.7 0.5
143 vis oil2-20-70 -------- Perthane EC +
3.0 gal.1.0 gal. Helicopter 5.8 5.2 7.3
70 vis oil2-7-71 ---------- Perthane EC +
1.0 gal.1.0 gal. Fixed wing 8.1 7.5 4.1 ....
70 ili l1 0v s o2-5-71 .......... Thiodan EC
.. ga1.0 gal. Air carrier 7.1 3.7 1.1 2.1
2-5-71 ..........Thiodan EC + 1.0 gal. Fixed wing 15.1 7.8 8.3 13.070 vis oil l1 0
2-3-71 ---------- Perthane EC +.. ga
0.5 gal. Air carrier 9.1 3.8 1.1
Thiodan EC 0.5 gal.
Psylla eggs are unusually resistant toinsecticides applied in the delayeddormant stage. If adults have not beeneliminated from the orchard by thedormant spray before they have ovi-posited significantly, effective ovicidaltoxicity cannot be achieved with pe-troleum oil concentrations which canbe used safely during the delayeddormant period (59, 79).
The problems encountered in se-lecting the most appropriate chemicalfor use at the delayed dormant timecan be seen by examination of Table 8,where it is shown that the availablematerials are not highly effectiveagainst all pests that should be con-trolled at this timing.
PINK Bun APPLICATION
Very few pink bud sprays were ap-plied for control of the pear psyllauntil the appearance of resistance toorganophosphates used during thesummer months. The advantage to thepink spray is that the psylla popula-tions are predominantly in the early
nymphal stage at this time and are themost susceptible to insecticides. Thedisadvantages to this timing includewet soil conditions which may existand the difficulty in covering largeacreages in the short period of thisstage. In addition, extension of thepink sprays into the bloom period maycause fruit injury or destruction ofpollinating insects. A review of theinsecticides commonly used in Oregonis given in Figure 14.
POSTBLOOM TO HARVEST APPLICATIONS
Because of the overlapping of psyllastages during the postbloom period,the materials used must be activeagainst all stages in order to achievecontrol with a single spray applica-tion. This high degree of control wasobtained during the first few yearsof use of the organophosphate insec-ticides in the late 1940's and early1950's. However, as the effectivenessof these materials lessened, it becamenecessary to decrease the interval be-tween sprays to obtain commercial
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E noosu if on
As the pear psylla continues to de-velop resistance to one material afteranother, the timing of the summersprays becomes more critical. Gen-erally, the younger nymphs remain the
1950
RIVER
I
64 65 66 67 69 71
MEDFORD
Figure 14. History of insecticide usage for pink bud applications against pear psyllain Oregon.
Figure 15. Summer use pattern of pesticides for pear psylla control.
control. The pattern of chemical useand of psylla resistance has been simi-lar in the Pacific Northwest (9, 10, 11,15, 22, 23, 45, 48) and is presentedin Figure 15.
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egg to adult in about 30 days. In re-sponse to decreasing day length uponfourth-generation nymphs, overwinter-ing adults develop from September toNovember and disperse widely byflight from the pear orchards in whichthey developed. Although the adultscan; derive moisture from and oviposit
most susceptible and therefore ma-terials should be applied when themajority of the population is in thesestages. This will often require a secondapplication 10 days to 2 weeks later,when the unaffected eggs are in theearly nymphal stage.
Another development in the chemi-cal control of psylla has been the useof petroleum oils either alone or incombination with other insecticides forincreased toxicity. There have been noreported instances of insects, includingthe pear psylla, developing a toleranceto petroleum oils. The toxicities ofseveral compounds which now givepoor psylla control alone due to thedevelopment of resistance are in-creased substantially by addition ofsuperior type oils (36, 37, 78). Oils ofhigher viscosities (> 100 S.S.U.) gave
the best control (36, 78) and resultedin less foliage injury than lower viscos-ity (< 100 S.S.U.) oils.
Lenticellular enlargement and pro-liferation have been observed follow-ing the summer use of light dosages ofoils (3 to 6 gallons per acre), but thesignificance of this symptom remainsunknown. After 24 dilute applicationsover seven years at 2 to 3 quarts per100 gallons of water to D'Anjou trees,no serious effects on tree vigor, growth,bloom, or fruit production are ap-parent. Under conditions of heavyreinfestation during the growing sea-son, oils applied alone have not beeneffective in preventing psylla popula-tions from causing defoliation andserious fruit marking from honeydewsecretion in an experimental plot.
SUMMARY
1. The pear psylla, Psylla pyricolaForster, is the most serious pest of22,000 acres of commercial pear inOregon. The pear is Oregon's mostvaluable deciduous tree fruit crop.The three principal areas of pear cul-ture which have suffered most fromthe depredations of this pest are: HoodRiver County, the Willamette Valley,and Jackson County.
2. Since its first discovery in north-eastern Oregon in 1946, the pearpsylla has successively invaded themajor pear-growing areas of the state:Hood River in 1949, the WillametteValley and Medford in 1950.
3. Pear psylla injure pear trees byserving as the vector for pear decline,a serious disease of indeterminateetiology affecting the graft union, es-pecially among cultivars on orientalrootstocks; by injecting a toxin whichcauses defoliation or "psylla shock";and by excreting honeydew whichburns foliage, discolors fruit, and in-terferes with harvest.
4. Pear psylla overwinter as matureadults in protected situations in or-chards and other vegetated areas.
Their winter reproductive diapause isterminated by the warmer tempera-tures of February when they mate andthe females lay their first eggs on thedormant pear buds. The youngnymphs hatch during blossom andinsert their sucking mouthparts intodeveloping leaves. The five nymphalstages in their life cycle are completedby May and the first adults of thethree summer generations emerge tolay their eggs on succulent pear foli-age. The summer adults are smallerand lighter-colored and develop from
their eggs on other vegetation,nymphal development to adult is pos-sible only on Pyrus species.
5. Although the pear psylla isknown to have a number of predatorsand parasites, populations of thesebeneficial insects develop too late in
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the season to prevent economic dam-age in commercial orchards. A pre-daceoui bug has been found to reducepsylla populations in unsprayed or-chards in Medford.
6. The pear psylla has become re-sistant to a number of insecticidesthat were formerly effective in its con-trol. As each new compound becameineffective for control, newer materialshave become available. Low rates ofsuperior-type oils in summer cover ap-
plications have extended the effective-ness of several insecticides to whichpsylla have become resistant.
7. A dormant application againstoverwintering adults before significantegg deposition, followed by pre- andpostboom sprays of effective materialsat critical stages of psylla development,is presently the only means availablefor limiting psylla damage to subeco-nomic levels.
LITERATURE CITED
1. Batjer, L. P., and Henry Schneider. 1960. Relation of pear decline to rootstocksand sieve-tube necrosis. Proc. Amer. Soc. Hort. Sci., 76:85-97.
2. Bethell, Richard. 1971. Personal communication.3. Blodgett, Earle C., Murit D. Aichele, and John L. Parsons. 1963. Evidence of a
transmissible factor in pear decline. USDA Plant Dis. Reptr., 47:89-93.4. Bollow, H. von. 1960. Die Blattsauger (Psylla) der Apfel and Birnbaume Auftreten,
Aussenhen, Lebensweise, Voraussage and Bekampfung. Pflangenschutz, 12:159-166.
5. Bonnemaison, L., and J. Missonnier. 1956. Le psylle du poirer (Psylla pyri L.).Ann. Epiphyt., 11:263-331.
6. Bronnimann, H. 1964. Rearing anthocorids on artificial medium. CommonwealthInstitute of Biological Control, Tech. Bull. 4, pp. 147-150.
7. Burts, Everett C. 1968. An area control program for the pear psylla. J. Econ.Entomol., 61(1) :261-263.
8. Burts, E. C. 1965. Dormant season control of the pear psylla. Proc. Wash. StateHort. Assoc., 61:139.
9. Burts, E. C. 1958. Field studies on the control of pear psylla resistant to dieldrin,toxaphene and related compounds. Proc. Wash. State Hort. Assoc., 53:22-24.
10. Burts, E. C. 1959. Insect control problems on pears. Proc. Wash. State Hort. Assoc.,55:161-164.
11. Burts, Everett C. 1970. Pear psylla in central Washington. Wash. Agric. Expt. Sta.Circ. 516.
12. Burts, E. C., and William R. Fischer. 1967. Mating behavior, egg production, andegg fertility in the pear psylla. J. Econ. Entomol., 60(5):1297-1300.
13. Burts, E. C., and Samuel G. Kelly. 1966. The effect of pear psylla, Psylla pyricola,control on growth and survival of Bartlett pear trees on two rootstocks. J. Econ.Entomol., 59(l):192-194.
14. Cameron, R. 1971. Personal communication.15. Carlson, F. W., and E. J. Newcomer. 1949. Control of pear psylla in the Pacific
Northwest. J. Econ. Entomol., 42(2):338-,342.16. Cook, P. P. 1963. Mating behaviour of Psylla pyricola Forster (Homoptera: Psyl-
lidae). Pan. Pacific Entomol., 39 (3) :175.17. Cordy, C. B. 1971. Personal communication.18. Georgala, M. B. 1957. A contribution to the biology of the pear sucker, Psylla
pyricola Forster. Rept. East Malling Res. Sta. for 1956.19. Glass, E. H. 1960. Pear psylla control studies. New York State Agric. Expt. Sta.
Res. Circ. 16.20. Gonzales, C. Q., W. H. Griggs, D. D. Jensen, and S. M. Gotan. 1963. Orchard
tests substantiate role of pear psylla in pear decline. Calif. Agr., 17(1) :4-6.21. Hamilton, Donald W. 1948. Pear psylla control with dormant sprays. J. Econ.
Entomol., 41(3) :443-445.22. Harries, F. H., and E. C. Burts. 1965. Insecticide resistance in the pear psylla.
J. Econ. Entomol., 58(l):712-713.
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23. Harries, F. H., and E. Burts. 1959. Laboratory studies of pear psylla resistance todieldrin and some related compounds. J. Econ. Entomol., 52(3):530-531.
24. Hartzell, F. Z. 1925. Susceptibility to dust and spray mixtures of the pear psylla
(Psylla pyricola Forster). New York State Agric. Expt. Sta. Bull. 527.25. Hartzell, F. Z., and F. L. Gambrell. 1931. Relation of environment to pear psylla
infestation. J. Econ. Entomol., 24:66-71.26. Hibino, H., and H. Schneider. 1970. Mycoplasma-like bodies in sieve tubes of pear
trees affected with pear decline. Phytopathology, 60:499-501.27. Jensen, D. D. 1957. Parasites of the Psyllidae. Hilgardia, 27(2) :71-99.28. Jensen, D. D., and W. R. Erwin. 1963. The relation of pear psylla to pear decline.
Calif. Agr., 17(l):2-3.29. Jensen, D. D,, W. H. Griggs, C. Q. Gonzales, and H. Schneider. 1964. Pear psylla
proven carrier of pear decline virus. Calif. Agr., 18(3):2-3.30. Kaloostian, G. H. 1970. Transitory hosts of the pear psylla. J. Econ. Entomol.,
63(6):1039-1041.31. Lindner, R. C., E. C. Burts, and N. R. Benson. 1962. A decline condition in pears
induced by pear psylla. USDA Plant Dis. Reptr., 46:59-60.32. Madsen, H. F., R. L. Sisson, and R. S. Bethell. 1962. The pear psylla in California.
Calif. Agric. Expt. Sta. Circ. 510.33. Madsen, Harold F. 1961. Notes on Anthocoris melanocerus Reuter (Hemiptera:
Anthocoridae) as a predator of the pear psylla in British Columbia. Can. Entomol.,93(8):660-662.
34. Madsen, Harold F., and Martin M. Barnes. 1959. Pests of pear in California. U.Calif. Circ. 478.
35. Madsen, H. F., P. H. Westigard, and R. L. Sisson. 1963. Observations on the naturalcontrol of the pear psylla, Psylla pyricola Forster, in California. Can. Entomol.,95(8):837-844.
36. Madsen, Harold F., and K. Williams. 1967. Control of the pear psylla with oilsand oil-insecticide combinations. J. Econ. Entomol., 60(l):121-124.
37. Madsen, Harold F., and K. Williams. 1969. The effect of petroleum oils on Anjoupears and on pear psylla, Psylla pyricola. Can. Entomol., 101(6) :983-989.
38. Madsen, Harold F., and K. Williams. 1967. The performance, phytotoxicity andpersistence of three petroleum oils for control of the pear psylla. J. Entomol. Soc.British Columbia, 64:3-8.
39. Madsen, Harold F., and Tim T. Y. Wong. 1964. Effects of predators on controlof pear psylla. Calif. Agr., 18(2):2-3.
40. Marlatt, C. L. 1895. The pear-tree psylla. USDA Circ., 7:1-8.41. Marshall, J. 1959. An unusual manifestation in the natural control of the pear
psylla, Psylla pyricola First. Proc. Entomol. Soc. British Columbia, 36:69-71.42. McMullen, R. D. 1970. Personal communication.43. McMullen, R. D. 1966. New records of chalcidoid parasites and hyperparasites of
Psylla pyricola Forster in British Columbia. Can. Entomol., 98(3):236-239.44. McMullen, R. D., and C. Jong. 1967. New records and discussion of predators of
the pear psylla, Psylla pyricola Forster, in British Columbia. J. Entomol. Soc.British Columbia, 64:35-40.
45. Newcomer, E. J. 1950. Orchard insects of the Pacific Northwest and their control.USDA Circ. 270.
46. Nichols, Carl, F. L. Blanc, A. A. Millecan, and G. Douglas Barbe. 1965. A newexplanation of the spread of pear psylla and pear decline virus in California. Calif.Dept. Agr. Bull., 54(3):133.
47. Nickel, John L., John T. Shimizu, and Tim T. Y. Wong. 1965. Studies on naturalcontrol of pear psylla in California. J. Econ. Entomol., 58(5):970-976.
48. O'Neill, W. J. 1949. Pear psylla control with parathion. J. Econ. Entomol.,42(4):636-639.
49. Oldfield, G. N. 1970. Diapause and polymorphism in California populations ofPsylla pyricola (Homoptera: Psyllidae). Ann. Entomol. Soc. Amer., 63(l):180-184.
50. Ostrowski, R. C. 1965. The relationship of phloem graft union responses to peardecline. Ph.D. thesis, Oregon State University.
51. Pettit, R. H., and Ray Hutson. 1931. Pests of apple and pear in Michigan. MichiganState Agric. Expt. Sta. Circ. 137.
9.1
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52. Ramsy, A. H., and A. W. MacPhee. 1970. Studies on pear psylla in Nova Scotia.Can. Entomol., 102(5):586-592.
53. Shalla, T. A., T. W. Carroll, and L. Chiarappa. 1964. Transmission of pear declineby grafting. Calif. Agr., 18(3):4-5.
54. Shalla, T. A., L. Chiarappa, and T. W. Carroll. 1963. A graft transmissible factorassociated with pear decline. Phytopathology, 53(3)!366-367.
55. Shalla, Thomas A., and Carl W. Nichols. 1961. Pear decline-1961. Calif. Dept.Agr. Bull., 50(4):217-220.
56. Slingerland, M. V. 1892. The pear-tree psylla. Cornell Univ. Agric. Expt. Sta. Bull.44, pp. 161-186.
57. Smith, E. H. 1965. The susceptibility of life history stages of the pear psylla to oiltreatment. J. Econ. Entomol., 58(3):456-464.
58. Smith, L. G. 1940. Pear psylla in Washington. State Col. Wash_ Ext. Bull. 225.59. Stern, V. M., R. F. Smith, R. van den Bosch, and K. S. Hagen. 1959. The integra-
tion of chemical and biological control of the spotted alfalfa aphid. The integratedcontrol concept. Hilgardia, 29:81-101.
60. Swirski, E. 1953. The bionomics of the pear psylla, Psylla pyricola Forster, in Israel.Ktavim, 4:61-68.
61. Watson, T. K., and W. H. A. Wilde. 1963. Laboratory and field observations ontwo predators of the pear psylla in British Columbia. Can. Entomol., 95:435.
62. Webster, R. L. 1939. Pear psylla survey. Proc. Wash. State Hort. Assoc., 35:36-40.63. Westigard, P. H., L. G. Gentner, and D. W. Berry. 1968. Present status of b ological
control of the pear psylla in southern Oregon. J. Econ. Entomol., 61(3) :740-743.64. Westigard, P. H., M. N. Westwood, and P. B. Lombard. 1970. Host preference and
resistance of Pyrus species to the pear psylla, P. pyricola Forster. J. Amer. Soc.Hort. Sci., 95(l):34-36.
65. Westwood, M. N. 1971. Personal communication.66. Westwood, M. N., and P. B. Lombard. 1966. Pear rootstocks. Proc. Ore. Hort. Soc.,
58:61-68.67. Westwood, M. N., H. R. Cameron, P. B. Lombard, and C. B. Cordy. 1971. Effects
of trunk and rootstock on decline, growth and performance of pear. J. Amer. Soc.Hort. Sci., 96(2):147-150.
68. Wilde, W. H. A., and T. K. Watson. 1963. Bionomics of the pear psylla, Psyllapyricola Forster, in the Okanagan Valley of British Columbia. Can. J. Zool.,41(6):953-961.
69. Wilde, W. H. A. 1962. Bionomics of the pear psylla, Psylla pyricola Forster, inpear orchards of the Kootenay Valley of British Columbia. Can. Entomol., 94(8):845-849.
70. Wilde, W. H. A. 1966. Climbing night-shade as a host of pear psylla. Can. Entomol.,98(5):558-559.
71. Wilde, W. H. A. 1970. Common plantain as a host of pear psylla (Homoptera:Psyllidae). Can. Entomol., 102(3):384.
72. Wilde, W. H. A. 1963. Downy chess grass as a host of the pear psylla. Can Entomol.,95(9):1005-1006.
73. Wilde, W. H. A. 1962. The pear psylla in British Columbia. Can. Dept. Agr., 12 pp.mimeo.
74. Wilde, W. H. A. 1965. The pear psylla, Psylla pyricola Forster, in Ontario (Homop-tera: Chermidae). Proc. Entomol. Soc. Ont., 1964.
75. Wilde, W. H. A., and D. L. McIntosh. 1964. Psylla pyricola Forster suppresses peartree root development. Can. Entomol., 96(8):1083.
76. Williams, M. W., L. P. Batjer, E. S. Degman, and E. C. Burts. 1963. Susceptibilityof some pear species to injury from pear psylla. Proc. Amer. Soc. Hort. Sci., 82:109-113.
77. Wong, T. T. Y., and H. F. Madsen. 1967. Laboratory and field studies on theseasonal forms of pear psylla in northern California. J. Econ. Entomol., 60(l):163-168.
78. Zwick, R. W., and F. W. Peifer. 1968. Oils for summer control of pear psylla andtheir effects on pear trees. J. Econ. Entomol., 61(4) :1075-1079.
79. Zwick, R. W. 1967. Unpublished data.
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