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Use of diatomaceous earth for the management ofstored-product pestsMohd Abas Shaha & Akhtar Ali Khana

a Division of Entomology, Sher-e-Kashmir University of Agricultural Sciences andTechnology of Kashmir, Shalimar, Srinagar, India-190 025Published online: 27 May 2014.

To cite this article: Mohd Abas Shah & Akhtar Ali Khan (2014): Use of diatomaceous earth for the management of stored-product pests, International Journal of Pest Management, DOI: 10.1080/09670874.2014.918674

To link to this article: http://dx.doi.org/10.1080/09670874.2014.918674

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Use of diatomaceous earth for the management of stored-product pests

Mohd Abas Shah* and Akhtar Ali Khan

Division of Entomology, Sher-e-Kashmir University of Agricultural Sciences and Technology of Kashmir,Shalimar, Srinagar, India-190 025

(Received 6 January 2014; final version received 27 March 2014)

Diatomaceous earth (DE) is almost pure amorphous silicon dioxide, made up of fossilised diatoms; it acts as an insecticideby absorption of epicuticular lipids and fatty acids, leading to desiccation in arthropods. Numerous DE formulations havebeen attempted for the management of stored-product pests with good results. DE is persistent in its action, poses few orno pest resistance problems, and it leaves no residue. The efficacy of DE is affected by factors such as: its provenance,temperature, humidity and characteristics of target pests and substrate. Application of DE at currently recommended dosescauses changes in bulk density of the grain; however, the use of enhanced DE formulations provides control at much lowerdoses. Therefore, new formulations of DEs usually include other low toxicity insecticides.

Keywords: diatoms; enhanced diatomaceous earth; epicuticular lipids; grain bulk density; resistance; storage losses;stored-product pests

1. Introduction

Diatomaceous earth (DE) is an inert dust derived from

amorphous sediment and comprised of the fossilised carapa-

ces of unicellular algae. It is a light material with low den-

sity and its colour ranges from white to dark grey. It

consists of approximately 80�93% silicon dioxide, the

remaining content comprising clay minerals, organic matter,

quartz, calcium and magnesium carbonate (Korunic 1998;

Stathers et al. 2004). DE insecticides are perceived as a

sound option for pest control in grain storage (Quarles

1992). They are both abrasive and slightly absorptive, and

the dust particles adhere to the insect’s cuticle and in doing

so remove the lipid monolayer, causing the insect to desic-

cate (Korunic 1998; Subramanyam and Roesli 2000).

About 10�30% of grains produced worldwide are lost

every year due to stored-grain pests (Singh et al. 2009).

Residual insecticides are the most commonly used protec-

tants in stored grain against stored-product pests. These

insecticides are applied directly to the product and provide

protection against stored-grain pests as long as the insecti-

cidal effect persists (Arthur 1996). However, the use of

these protectants poses several drawbacks, as they are

generally toxic to mammals and leave residues in the

product; furthermore, many insect species are resistant to

some residual protectants (Arthur 1996). These limitations

have led researchers to evaluate the potential use of alter-

native control methods, such as botanicals, insect growth

regulators, biological control, microbial control, and inert

dusts. One of the most promising alternatives to contact

insecticides is the application of diatomaceous earths

(DEs). These dusts are applied directly to the grain, with-

out specialized equipment, using much the same technol-

ogy as that employed for residual insecticides. DEs have

some incontestable advantages over residual protectants,

given that they are non-toxic to mammals, can be easily

removed from the product during processing, are very

effective for a wide range of species, and given that only a

physical method is involved, it has been postulated that

physiological resistance is unlikely to occur (Golob 1997;

Korunic 1998; Subramanyam and Roesli 2000). However,

recent studies indicate that resistance to DE can evolve

under certain circumstances (Vayias et al. 2008). Several

DE formulations are commercially available (Subrama-

nyam and Roesli 2000), and many studies document that

they are very effective against a wide range of stored-

product insect species. However, the main drawback in

the use of DEs is that they need to be applied at high-dose

rates, which affect the physical properties, chiefly bulk

density, of the stored grains (Athanassiou and Korunic

2007). This issue is being addressed by the combined use

of DEs with other low-risk control methods known as

enhanced DEs (EDEs), as proposed by Arthur (2003),

with promising results in most cases. Here, we provide a

brief overview of the nature and mode of action of DEs

followed by a summary of recent attempts at evaluating

the efficacy of DEs against stored-grain pests. The factors

affecting efficiency of DEs have been dealt with in detail

followed by an account on effects of DEs on grain quality.

Finally, the possibility of using EDEs for the management

of stored-product pests has been discussed.

2. Nature of diatomaceous earth

Diatomaceous earth (DE) is almost pure amorphous sili-

con dioxide, made up of fossilised diatoms. Diatoms are

unicellular algae and probably the most widespread group

*Corresponding author. Email: khubaib20@gmail.com

� 2014 Taylor & Francis

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of plants on the earth. There are more than 25,000 species

of diatoms with no two having the same morphology

(Round et al. 1992). They are abundant in all aquatic eco-

systems, but also occur in terrestrial environments. About

20 to 80 million years ago, mostly in the Eocene and Mio-

cene epochs, different species of diatoms extracted silicon

from water, producing a hydrated amorphous silica skele-

ton. When the diatoms died the tiny shells sunk, and over

the years these shells formed thick layers. Eventually

these deposits were fossilised and compressed into a soft,

chalky rock that is now called diatomaceous earth. A

well-documented fossil record of diatoms extends back to

the mid-Cretaceous period, but the vast majority of depos-

its are from Eocene/Miocene times. Deposits range in

thickness from a few centimetres to several hundred

metres. The deposit may be finely laminated or massive

(Karunic 1998). DE is used extensively and is prepared

for commercial use by quarrying, drying and milling

(Quarles and Winn 1996).

As mined, DE contains 50% or more moisture. Of the

solids, 86�94% is silica, the remainder being chiefly alu-

mina and alkalies from clay. The only change to DE dur-

ing processing is the reduction in the moisture content and

mean aggregate particle size. Moisture is reduced to

2�6% and milling reduces particle size to between 0.5

and 100 mm with the majority between 10 mm and

50 mm. The result of this process is a fine, talc-like pow-

der or dust considered to be non-toxic to mammals

(Quarles 1992). DE is extremely stable and does not pro-

duces toxic chemical residues or react with other substan-

ces in the environment. According to the Environmental

Protection Agency (EPA) in the United States, natural DE

is described as amorphous silicon dioxide which is classi-

fied as Generally Recognized as Safe (GRAS) as a food

additive (Anon. 1991). The US Food and Drug Adminis-

tration has exempted DE from requirements of fixed resi-

due levels when added to stored grain (Anon. 1961).

Different types of DE are commonly used for the purifica-

tion of water, the clarification of liquors and juices, filtra-

tion of commercial fluids, and the separation of various

oils and chemicals. Other uses include fillers in paints,

paper and rubber. Because DE can absorb two to three

times its own weight in liquids, yet remain free flowing, it

is also employed as a pesticide carrier, and for the safe

storage and transportation of hazardous liquids. DE and

diatomite are also used in numerous commercial products

such as detergents, deodorisers, cosmetic powders, in fil-

tration systems for swimming pools, as a mild abrasive, a

drilling-mud additive, anti-caking agent and animal feed

additives (Snetsinger 1988). Estimated world production

of diatomite in 2011 was 2.1 million metric tons (Mt).

World reserves are thought to be almost 1 billion metric

tons of which about 250 metric tons (25% of the whole) is

in the United States (Crangle 2012). The United States is

the leading producer of diatomite, accounting for 39% of

total world production, followed by China with 21%, Den-

mark with 11%, and Japan with 5%. Diatomite used for

filtration represents 61% of consumption, followed by its

use as a cement additive (13%), as an absorbent (12%),

and as a filler (12%). Other diatomite applications, includ-

ing abrasives, insecticides, insulation, and soil condi-

tioner, accounted for the remainder. Major diatomite

products were sold as various grades of calcined powders

(Crangle 2012).

3. Mode of action and use against stored-grain pests

DE is probably the most efficacious natural dust used as

an insecticide. The dust particles are trapped by the bodies

of the insects as they walk over it. The dust is most effec-

tive against insects with setaceous and rough surfaces.

The mode of action of DE is generally accepted to be a

desiccating effect on the insects (Ebeling 1971; Mewis

and Reichmuth 1998). This indicates that toxicity primar-

ily depends on its physical properties and not on its chem-

ical composition (Subramanyam and Roesli 2000).

Different insecticidal mechanisms have been proposed,

including abrasion of the cuticle (Fields 1998), absorption

of cuticular waxes from the epicuticle surface (Ebeling

1971; Mewis and Ulrichs 1999; Prasantha 2003), damage

to the digestive tract (Smith 1969), blockage of the spi-

racles and tracheae (Webb 1945), and surface enlargement

combined with dehydration (Zacher and Kunike 1931).

Recent research work has concluded that effective absorp-

tion of epicuticular lipids and fatty acids is the primary

mode of action of DE, leading to desiccation in arthropods

(Fields 1998; Mewis and Ulrichs 1999; Prasantha 2003).

The other mode of action of DE is repellence caused by

the physical presence of the dust (White et al. 1966).

“Bathing in sand” is a well known occurrence exhib-

ited by birds and poultry protecting themselves against

mites and other parasites. Four thousand years ago, obser-

vations of such natural phenomena probably led the Chi-

nese to use DE (diatomite) to control pests (Allen 1972).

In 1880 in the United States, it was noticed that road dust

killed caterpillars of the cotton moth (Stelle 1880). Until

the 1950s, clay dusts, sand or silica gels were used more

extensively in practice and in research than was DE. In

the early 1950s, DE was used to fight fruit moths, cucum-

ber beetles, Mexican bean beetle larvae, stored-products

pests and cockroaches (Bartlett 1951). Generally, the

dusts are insect repellents. The repellency depends on the

dosage applied. Increased dosage increases insect repel-

lency, and the negative influence of the dust on parasites

and predators (Flanders 1941; Bartlett 1951).

The use of DE for structural treatment in stored product

facilities was studied by Wright (1990), Desmarchelier

et al. (1993), McLaughlin (1994), Bridgeman (1994), and

Korunic and Fields (1995). Research has also been done

on the effect of DE on numerous other insects such as ants,

bedbugs, textile pests, various caterpillars in agriculture,

crickets, termites, earwigs, June beetles, potato beetles,

silverfish, fleas, as well as on poultry mites, ticks, centi-

pedes, pillbugs, snails, etc. (Wilbur et al. 1971; De Crosta

1979; Snetsinger 1982, 1988; Korunic 2013). Although

different and often completely opposing results were

obtained, there is a general conclusion that can be drawn

about the sensitivity of stored-product insects to DE.

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The most extensive research with DE has been under-

taken in the field of the protection of stored agricultural

products. Published results of studies with DE also imply

that there is little degradation or loss in efficacy of DE

with time (Arthur 2002), a very desirable quality for an

ideal grain protectant. The effect of DE on various stored-

product insects has been studied by numerous researchers.

Earlier work has been reviewed by Korunic (1998), recent

publications on relative efficacy of DEs against stored-

product pests are summarised in Table 1.

4. Factors affecting efficacy of diatomaceous earths

The insecticidal activity of DEs is affected by many fac-

tors such as source of DE, insect species, grain moisture

content, temperature, method of application, growth stage

of the pest species, and nature of the stored product.

Therefore, it is imperative to ascertain the effect of pre-

vailing conditions for effective use of DEs.

4.1. Relative humidity

The effect of the relative humidity (RH) on DE efficacy

has been reported by several researchers and the results

are consistent for all stored-product insect species tested

so far (Vayias and Athanassiou 2004). It is generally

accepted that, at high RH or moisture content values, des-

iccation through the abrading action of the DE particles

on the cuticle is reduced, because insects may moderate

their water loss (Subramanyam and Roesli 2000; Fields

and Korunic 2000; Mewis and Ulrichs 2001; Athanassiou

et al. 2011). Furthermore, DE particles absorb moisture

from the air, a fact which, when high RH values prevail,

causes an additional reduction in DE efficacy (Subrama-

nyam and Roesli 2000; Arthur 2001; Stathers et al. 2004).

Arthur (2002) reported that on wheat treated with DE, sur-

vival of Sitophilus oryzae increased with increasing RH,

and this effect was evident both in the initial survival of S.

oryzae and the number of F1 adults. According to Vayias

and Athanassiou (2004), a 10% increase in RH (from 55%

to 65%) is sufficient for a considerable reduction in DE

efficacy.

4.2. Temperature

Contradictory results have been reports regarding the

effect of temperature on the insecticidal efficacy of DE.

While using the DE formulation “Dryacide”, Aldryhim

(1990) noted that Tribolium confusum adults were more

susceptible at 20�C than 30�C. In addition, Fields and

Korunic (2000), using several DE formulations, reported

similar results for adults of the red flour beetle Tribolium

castaneum. On the other hand, Arthur (2000a), when

using the commercial formulation of DE “Protect-It”,

stated that the efficacy of DE against T. confusum and T.

castaneum adults increases progressively with increase of

Table 1. Recent� publications on the use of diatomaceous earth against stored-product pests.

Pest species References

Sitophilus oryzae (L.) Fields and Korunic 2000; Arthur 2002; Athanassiou et al. 2003; Athanassiou et al. 2005;Athanassiou et al. 2007; Saez and Mora 2007; Athanassiou et al. 2008; Kljajic et al. 2010;Rojht et al. 2010; Athanassiou et al. 2011; Sadeghi et al. 2012; Stadler et al. 2012

Tribolium confusum du Val Vayias and Athanassiou 2004; Athanassiou et al. 2005; Athanassiou et al. 2007; Mewis andUlrichs 2001; Athanassiou et al. 2011; Ziaee and Moharramipour 2012; Arthur andFontenot, 2013

Rhyzopertha dominica (F.) Fields and Korunic 2000; Athanassiou and Kavallieratos 2005; Athanassiou et al. 2007;Vardeman et al. 2007a, 2007b; Saez and Mora 2007; Kljajic et al. 2010; Athanassiou et al.2011; Beris et al. 2011; Sadeghi et al. 2012; Stadler et al. 2012; Arthur and Fontenot, 2013

Tribolium castaneum (Herbst) Fields and Korunic 2000; Collins and Cook 2006a, 2006b; Arnaud et al. 2005; Kljajic et al.2010; Arthur and Fontenot, 2013; Kabir 2013

Sitophilus granarius (L.) Collins and Cook 2006a; Mewis and Ulrichs 2001; Saez and Mora 2007

Oryzaephilus surinamensis (L.) Fields and Korunic 2000; Collins and Cook 2006a; Sadeghi et al. 2012; Arthur and Fontenot,2013

Ephestia kuehniella Zeller Collins and Cook 2006a, 2006b

Plodia interpunctella (Hubner) Mewis and Ulrichs 2001

Blattella germanica (L.) Faulde et al. 2006

Acarus siro (L.) Collins and Cook 2006a

Lepidoglyphus destructor (Schrank) Collins and Cook 2006a, 2006b

Tenebrio molitor L. Mewis and Ulrichs 2001

Callosobruchus chinensis L. Matti and Awaknavar 2009

Callosobruchus maculatus (F.) Prasantha 2003; Sadeghi et al. 2012

Cryptolestes ferrugineus (Stephens) Fields and Korunic 2000; Saez and Mora 2007; Sadeghi et al. 2012

Lasioderma serricorne (F.) Sadeghi et al. 2012

Sitophilus zeamaisMotsch Demissie et al. 2008; Sousa et al. 2013

Acanthoscelides obtectus (Say) Prasantha 2003; Bohinc et al. 2013

�From 2000 onwards.

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temperature. Athanassiou et al. (2004), when using

“SilicoSec” at temperatures between 22�C and 32�C,found that mortality of T. confusum adults was increased

as temperature increased from 22�C to 32�C. However, inthat study the mortality was reduced at 32�C, but this

reduction was evident only at short exposure intervals

(<48 h), while at longer exposures (>7 d), mortality at

32�C was significantly increased in comparison with that

at 30�C. In that study, the T. confusum adults used were of

mixed and not of standardized age, and this may partially

explain the different levels of mortality at 32�C. Gener-ally, for most of the stored-product beetle species tested

so far, such as Cryptolestes ferrugineus, Sitophilus oryzae,

Sitophilus granarius, Rhyzopertha dominica, Tribolium

confusum and Oryzaephilus surinamensis, there is a posi-

tive correlation between temperature and mortality

(Aldryhim 1990, 1993; Fields and Korunic 2000; Subra-

manyam and Roesli 2000; Arthur 2001; Athanassiou et al.

2004; Athanassiou et al. 2011). As such, Sousa et al.

(2013) have suggested that the combination of lower

doses with high temperatures and longer periods of expo-

sure can effectively control insect pests of stored products,

particularly in tropics.

Increased temperature would increase insect move-

ment, causing increased contact with the DE and greater

cuticular damage (Fields and Karunic 2000). Higher tem-

peratures and increased movement would also increase

water loss via the spiracles due to increased respiration.

Losses via the spiracles are estimated to be three times

greater than losses of water through the cuticle for a desert

tenebrionid (Zachariassen 1991). Also, the rate of cuticu-

lar transpiration rises only slightly with temperature until

the transition temperature, which for most insects is above

30�C (Wigglesworth 1972). However, increased tempera-

ture would also increase feeding and therefore moisture

replacement through the food and production of metabolic

water. The synthesis of cuticular waxes may be faster at

higher temperatures because of temperature effects on the

biochemical pathways. However, there may be other over-

riding factors such as hormones. Unlike the synthetic

insecticides, DE is inert and does not degrade in a temper-

ature-dependent fashion. Within a grain bulk, the RH will

change slightly with temperature. For stored wheat there

is about a 3% reduction in RH for each 10�C increase in

temperature (Pixton and Warburton 1971). However,

these changes would be too small to be responsible for the

observed effects.

4.3. Type of product

The efficacy of DE is determined by the type of storage

product the dust is applied to. Athanassiou et al. (2003),

using “SilicoSec” in dose�response tests against S. oryzae

adults in peeled rice, paddy rice, barley and maize, found

that mortality notably varied in different types of grain.

Survival of T. confusum adults and larvae was higher in

treated flour than in treated hard and soft wheat (Vayias

and Athanassiou 2004). Athanassiou et al. (2008) carried

out bioassays to assess whether the commodity from

which adults of the rice weevil Sitophilus oryzae had

emerged influenced the insecticidal efficacy of three DE

formulations. The S. oryzae populations tested were

reared on wheat, barley or maize. Barley-reared S. oryzae

were the most tolerant to all formulations and treated

commodities, whereas maize-reared beetles were the most

susceptible ones. DE effectiveness was always lower in

maize than in wheat or barley, irrespective of the com-

modity from which the populations were obtained. Thus,

the grain type, not only as the DE-treated substrate but

also as a rearing medium, affects the DE efficacy. Earlier,

Mewis and Ulrichs (2001) reported higher mortality when

T. confusum individuals were fed on kernels rather than

on flour. White and Loschiavo (1989) also noted that sur-

vival of T. confusum adults was higher in beetles that

were fed with flour after their exposure to DE. Similarly,

Arthur (2000b) recorded that mixing food with DE

increased adult survival of Tribolium spp. As a secondary

pest, the confused flour beetle cannot easily develop in

undamaged seeds, on the other hand, it can develop with

great ease in flour (Aitken 1975; Rees 1995). Thus, one

possible explanation for the increased survival in flour is

that exposed individuals may restore water due to food

consumption. Also, the presence of flour may help insects

to remove the DE particles from the cuticle. Thus, small

food quantities as a result of defective cleaning in a stor-

age facility may reduce DE efficacy (Arthur 2000b).

McGaughey (1972) reported that the dose rate of DE

required for 100% mortality of S. oryzae on rough rice did

not provide complete suppression on milled and brown

rice. The reduced effectiveness of DE on milled and

brown rice was attributed to the absorption of kernel lipids

by the DE particles. Vayias and Athanassiou (2004)

reported that a complete suppression could not be

obtained on rice treated with 1.5 g of SilicoSec/kg, even

after a 14-d exposure interval.

Apart from absorption, it is known that the physical

and chemical properties of the grain determine the amount

of dust that is retained on kernels. In general, the degree of

DE retention varies with the grain type, the grain class, the

dose rate and the dust used (Korunic and Ormesher 2000).

Hence, apart from the degree of adhesion of dust particles,

the structural and compositional properties of the grain are

also very important, due to their influence on insect sur-

vival, development and progeny production. However,

results from a specific DE formulation may not be appli-

cable to all situations: different DEs may adhere differ-

ently to different grain types or even different classes of a

given grain. High oil or lipid content of maize kernels

may cause an increased oil absorption by DE particles and

thus the DE becomes inactivated (Vayias et al. 2006).

According to Korunic (1997), DE adherence to kernels is

a fundamental issue that affects the insecticidal value of a

given DE. Also, Kavallieratos et al. (2005) reported that

the DE formulations “Insecto” and “SilicoSec” adhered

less to maize than to the other seven grains tested. More-

over, DE efficacy was generally lower on treated wheat

than on barley. This is in accordance with the data pre-

sented by Athanassiou et al. (2005), who found that the

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efficacy of DEs against S. oryzae, during a residual study,

was reduced on wheat more rapidly than on barley. Simi-

larly, Athanassiou et al. (2003) found that SilicoSec was

more effective against adults of S. oryzae on paddy rice

and barley than on maize.

When Baker (1988) evaluated the developmental rate

of various strains of S. oryzae in several grains, he noticed

that maize was the least suitable diet for this species.

Athanassiou et al. (2008) also noticed that the develop-

ment of this species was delayed in maize. Later, Athanas-

siou et al. (2011) reported that DEs were more effective in

barley or wheat than in maize or rice. It seems that the

high level of DE adherence to barley seeds, as has been

noted by Kavallieratos et al. (2005), produces high mor-

tality levels for S. oryzae although barley is the most suit-

able diet for this species.

From a practical point of view, these findings should

be taken into account by researchers when bioassay

response tests with DEs against stored-product pests are

planned, since the rearing medium may seriously affect

the mortality results. Variation in efficacy was also noted

by Chanbang et al. (2007) for combination treatments of

DE and methoprene against Rhyzopertha dominica on

long-, short-, and medium-grain rough rice. Ziaee et al.

(2007) compared the insecticidal efficacy of five DE for-

mulations, against adult T. castaneum on three oilseeds,

viz. safflower, sunflower and sesame, and reported signifi-

cant differences among the three oilseed types. Even at

the lowest treatment rate, adult mortality was high

(>90%) in safflower for all DE formulations. In contrast,

adult mortality was significantly lower in the case of ses-

ame. The lowest application rate enabled complete sup-

pression in progeny production on treated safflower and

the highest number of progeny was recorded for treated

sunflower seeds.

4.4. Growth stage

Larval instars differ in their susceptibility to DE: young

larvae are significantly more susceptible than older ones

(Vayias and Athanassiou 2004). Adults of the confused

flour beetle are much more tolerant to DE than the larvae

and can survive at application rates and exposure intervals

that are lethal to all larval stages. Vayias and Athanassiou

(2004) reported that 90% of larvae were killed by an

application rate of 1.5 g of SilicoSec/kg after only 48 h of

exposure, while the respective figure for adults was only

50%. This different reaction of T. confusum larvae and

adults to DE has also been reported by Mewis and Ulrichs

(2001).

Newly emerged beetles are also more susceptible to

SilicoSec than older ones. In fact, 1- and 2-day-old adults

were found to possess different levels of susceptibility to

DE, suggesting that periods of even 1 d are sufficient to

increase beetle survival. Older adults (7 d old) are consid-

erably more tolerant to DE than the newly emerged bee-

tles (Vayias and Athanassiou 2004). De Paula et al.

(2002), when using the DE formulation Insecto, reported

that adult mortality of T. castaneum in DE-treated wheat

varied significantly with adult age. Although the differen-

ces in DE-tolerance among species or among develop-

mental stages of the same species have not been

investigated in detail, the different level of cuticle thick-

ness and the insects’ behaviour may be responsible for

these variations. Hence, one possible explanation is that

adults and larvae, or even younger and older individuals

of the same developmental stage, may have different

thickness of cuticle or even a different composition of epi-

cuticular lipids, as this has been reported for other species

(Armold et al. 1969). In young larvae the cuticle may be

softer than in older larvae, and thus DE may cause more

rapid cuticle damage which may result in more rapid des-

iccation. Also, young larvae are particularly agile, a

behaviour which increases their contact with the dust par-

ticles as compared to older larval stages prior to pupation,

which are less active. Adult sensitivity to DE is also influ-

enced by these factors. Rigaux et al. (2001), comparing

different strains of T. castaneum, recorded increased DE-

tolerance in the less agile strains. Also, Fields and

Korunic (2000) reported noticeable variation among four

stored-product beetle species regarding the amount of DE

particles that were attached to the cuticle at the adult

stage. Thus, age/instar of the target pests has a conse-

quence for DE-based management.

4.5. Source of diatomaceous earth

DEs from different geological sources or even from the

same location have different physical properties (SiO2

content, tapped density, oil absorbency, particle size and

pH) that are correlated to their insecticidal efficacy against

stored-product insects (Korunic 1997, 1998; Athanassiou

et al. 2011). Also, Korunic (1997) studied DE formula-

tions collected from worldwide geographic locations and

reported large differences in efficacy against insects, in

their physical properties and in their influence on wheat

bulk density when applied at 50 parts per million (ppm).

Comparison among DEs is difficult because there is sig-

nificant variation in insecticidal activity from the same

geographical source, and only small quantities of DE are

used for laboratory testing. Testing a range of concentra-

tions and estimation of the LD50 or the LD95 values could

indicate the effect of DE source on efficacy. SiO2 content

is a critical factor that affects insecticidal activity of DEs;

however, the other substances such as major mineral

oxides, apart from SiO2, may also have a bearing on the

efficacy of DEs. Rojht et al. (2010) determined the effect

of geochemical composition of DE on insecticidal activity

of DE against adults of S. oryzae and reported that silica

was the DE ingredient that was significantly correlated

with efficacy in most of the bioassays. A weak positive

correlation was recorded between S. oryzae mortality and

MnO or CaO content. All significant correlations between

mortality and Al2O3, Fe2O3, K2O, TiO2, Cr2O3, P2O5, and

MgO content were negative, while correlation between

Na2O content and mortality was generally not significant.

Saez and Mora (2007) reported variable initial mortality

rate from two different types of DEs of freshwater and

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marine origin on four species of insects, Cryptolestes fer-

rugineus, Rhyzopertha dominica, Sitophilus oryzae and S.

granarius, applied either by spraying or dusting.

4.6. Species variation

Stored-product insects show a wide range of susceptibility

to DE (Aldryhim 1990, 1993). In general, the most sensi-

tive species are in the genus Cryptolestes, while Sitophilus

spp. are less susceptible, followed by Oryzaephilus, Rhy-

zopertha, and Tribolium spp. which appear to be the most

resistant (Korunic and Fields 1995; Fields and Muir

1996). The differences in tolerance among species is con-

siderable. Under certain conditions, and using DE with a

high efficacy against insects to achieve a 100% mortality

of C. ferrugineus, a dosage of 300 ppm (0.3 g kg�1) for

24 h was required. However, applying the same dosage

under the same conditions to T. castaneum, 100% mortal-

ity of the insects did not occur even after 21 days (Korunic

1994). Fields and Karunic (2000) reported the following

order of tolerance: C. ferrugineus >O. surinamensis ¼ S.

oryzae >R. dominica / T. castaneum.

A few studies have addressed why there are different

susceptibilities among species. Nair (1957) working with

magnesite dust and White and Loschiavo (1989) working

with silica aerogel found that susceptible insects had more

dust adhering to the cuticle. Fields and Karunic (2000)

observed that C. ferrugineus, the most susceptible insect,

had more DE attached to its cuticle than T. castaneum, the

least susceptible. General resistance to desiccation �either through better water retention, better water acquisi-

tion, or greater tolerance of dessication � could also be

responsible for these differences in susceptibility. How-

ever, Nair (1957) and Le Patourel (1986) stated that dessi-

cation tolerance did not strictly follow resistance to DE,

whereas Carlson and Ball (1962) found a good correla-

tion. Other factors that may account for differences

between the species are size (volume to surface area

ratio), quantitative or qualitative differences in cuticular

lipids, differences in rate of movement through grain,

behavioural reaction to DE, or desiccation.

4.7. Method of application

Spraying has several advantages over dusting in that

workers are exposed to less dust, the formulation is easier

to apply, and it does not affect grain bulk density as much

(Korunic et al. 1998). Sedimentation of the dust can be

overcome by using agitators in the spray tanks, though in

some facilities access to water is limited. As seen in other

studies (Maceljski and Korunic 1972; McLaughlin 1994),

slurry or spray application reduces efficacy. The sug-

gested reason for this drop in activity is that when DE is

applied as a slurry and than dries there is less contact

between it and the insect than when it is applied dry

(Maceljski and Korunic 1972). Another possibility is that

the particles in solution aggregate, reducing activity. Saez

and Mora (2007) reported a slightly increased rate of

achieving mortality of stored-product insects when

exposed to DEs applied by spraying. However, the overall

mortalities achieved at 8 days showed no significant dif-

ferences between application methods. The sum of indi-

vidual weights of insects indicated that dead insects for

each species treated by spraying were slightly heavier

than those treated by dusting. These differences could be

caused by an increased number of dust particles adhering

to the insects’ bodies (Subramanyam and Hagstrum 1995)

or else by a difference in the weight loss due to desicca-

tion. Saez and Mora (2007) concluded that the spraying

method increases adherence of dusts to an insect’s body.

Collins and Cook (2006b) assessed the efficacy of two DE

formulations applied as dry dust and slurry applications to

wooden surfaces against adult S. granarius, and Lepido-

glyphus destructor and against larvae of Ephestia kueh-

niella. Wood was used as the test substrate as it was

considered to present a realistic challenge to efficacy due

to its three-dimensional structure. Of the various treat-

ments, dry dust was the most effective against all the pest

species, producing 80�95%, 100% and 93�100% mean

mortalities of S. granarius, E. kuehniella and L. destruc-

tor, respectively, over the 12-week experimental period.

The slurry treatments were less effective than the dry

dusts.

In commercial practice, DEs are applied to structures

using the slurry (wet) and dry blown methods. In Aus-

tralia, Dryacides is applied as a dry dust to grain-handling

machinery, ducts and vertical silos, and slurries are

applied to horizontal grain stores (Golob 1997). Slurries

are useful where there is a need for personnel to avoid

exposure to the very dusty atmospheres created by the dry

blown method; however, some dusts tend to have reduced

efficacy when applied as slurries (McLaughlin 1994;

Gowers and Le Patourel 1984). As a result, recommended

rates of Dryacides are 2 g/m2 when applied as a dry dust

and 6 g/m2 when applied as a 10% aqueous slurry. It is

therefore important to evaluate both dry dust and slurry

applications. Generally, slurries are less effective than the

dry dusts, forming a smooth uniform coverage over the

surface of the wood allowing the pests to walk on the sur-

face without picking up much of the deposit. Therefore

higher doses and longer exposure times may be required

if using slurries, although they may be useful where there

is a need to avoid the dusty atmospheres created when

using dry dusts (Collins and Cook 2006).

5. Concerns about the use of diatomaceous earths

5.1. Effects on grain quality

Although the first commercial formulations became

widely available in the 1950s (Quarles 1992), there are

problems associated with the use of DE. When DE is

mixed with grain at the currently recommended dosages

of 500�3500 ppm, some physical and mechanical proper-

ties of the bulk commodity are adversely affected: flow-

ability and bulk density are reduced, visible residues are

evident on the grains, moisture readings taken when using

a dielectric moisture metre are affected, and an excessive

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amount of dust is produced during handling (Korunic

1997; Korunic et al. 1998). The addition of DE to grains

creates greater friction between kernels, which affects

their bulk density and flow properties. Bulk density or test

weight is an extensively used grading factor.

Freshwater DE has very little effect on end-use quality

of wheat (Korunic et al. 1996a). Bread-making properties,

flour yield, protein content of wheat, sedimentation, fat

acidity, flour ash, diastatic activity, and physical proper-

ties of the dough were unaffected by freshwater DE treat-

ments (3500 ppm) even after one year of storage (Fifield

1970). Twenty parts per million of “Dryacide” (modified

freshwater DE) in flour had no effect on the production of

carbon dioxide in dough, nor on the volume or texture of

sponge cakes (Desmarchelier and Dines 1987). Sponge

cakes made from flour containing 100 ppm Dryacide had

a coarser texture and had 7�10% less volume than cakes

made from untreated flour. However, because milling and

cleaning of wheat remove over 98% of DE (Desmarche-

lier and Dines 1987), grain would have to be treated at

5000 ppm, which is well above recommended application

rates, to leave a level of 100 ppm in the processed flour.

The milling industry is also reluctant to accept grain

treated with DE because of its abrasive nature and possi-

ble damage to milling machinery (Subramanyam et al.

1994). Korunic et al. (1996a) examined the effect of

“Protect-It” on quality, and physical and handling charac-

teristics of cereals. In field tests, wheat treated with Pro-

tect-It at 75 ppm and 100 ppm, concentrations that

controlled C. ferrugineus and reduced T. castaneum popu-

lations, did not cause a reduction in grain flow, nor did

this treatment cause an increase in air-borne dust when

grain was moved using a screw auger. Wheat treated with

300 ppm of Protect-It had reduced grain flow and caused

an increase in air-borne dust. In all tests where grain was

treated with Protect-It and processed, there was no effect

on the end-use quality. This is not surprising as DE is

made up mainly of silicon dioxide, which is chemically

inert (Quarles 1992). Also, processing removes much of

the DE, so that only a fraction of the DE applied is carried

over into the finished product (Desmarchelier and Dines

1987). Unlike synthetic insecticides which are fat-soluble

and migrate to inside the grain kernel (Rowlands 1975),

DE stays on the seed coat making it easier to remove dur-

ing milling or processing. Korunic et al. (1996a) also stud-

ied the effect of dockage in combination with DE on bulk

density, and concluded that regardless of the mechanism

of how dockage changes the bulk densities, the actual

impact of dockage on the importance of bulk density in a

field situation will probably be minor. The only significant

adverse effect demonstrated in this study was the reduc-

tion of grain bulk density: although Protect-It used at

75 ppm and 100 ppm does reduce bulk density, it does not

reduce end-use quality. Korunic et al. (1998) studied the

effect of the enhanced diatomaceous earth (EDE) insecti-

cide “Protect-It” at different concentrations on the bulk

density of wheat, corn, barley, rye and oats at different

moisture levels. The greatest changes in bulk density

occurred when the concentration of EDE ranged from

50 to 200 ppm. At concentrations greater than 500 ppm,

bulk density decreased little with increased EDE concen-

trations. The bulk density reductions in all five grains

tested were significantly higher for the grain at a dry basis

moisture content of 15% than at 12%. The dry application

caused a significantly greater reduction in wheat bulk den-

sity than did the wet application.

Moras et al. (2006) assessed the effects of two DE for-

mulations on technological, physical and cooking charac-

teristics of rice and reported that use of DE was efficient

in grain conservation and also showed no effect on tech-

nological quality and rice cooking parameters with respect

to parboiled and conventionally processed rice, even after

12 months of storage. Bodroza-Solarov et al. (2012)

investigated the changes in quality parameters of wheat

(mealy and vitreous) either non-infested or infested with

S. oryzae, caused by treatments with two types of DE, viz.

an enhanced DE and an inert dust. Non-infested, high-vit-

reous wheat treated with EDE showed the highest mois-

ture absorption. The percentage of test weight reduction

was greater in mealy wheat grain than in the high-vitreous

wheat grain. Significant improvement in dough rheology

was observed in the infested soft and hard wheat, particu-

larly through rise of dough energy.

The magnitude of the adverse effects of DE can be

reduced by lowering concentrations of DE. However,

lower concentrations of the current DE formulations can-

not achieve acceptable levels of control of stored-grain

insects. Use of the new enhanced diatomaceous earth for-

mulations (EDEs) is perceived as a viable option (Korunic

et al. 1996a, 1996b).

5.2. Health issues

There is no evidence of acute or chronic toxic effects of

natural DE. It has been shown to be non-toxic when con-

sumed by mammals. Rats receiving daily food containing

5% freshwater DE showed no signs of abnormality after

90 days (Bertke 1964). On farms, cattle have sometimes

been given food containing 1�2% DE to control worms

and other internal parasites (Allen 1972). Owing to its

demonstrable safety and non-toxicity, no permissible resi-

due levels have had to be prescribed for DE mixed with

grain in the United States. Also, the US Environmental

Protection Agency (EPA) allows the use of DE in product

storage and the food processing industry (Anon. 1961,

1981). DE has been registered as a grain protectant in the

United States, Canada, Australia, Japan, Indonesia, Saudi

Arabia and Croatia (Karunic 1998).

The known possible noxious and dangerous effects on

mammalian health can occur when workers are constantly

exposed to DE with prolonged inhalation of suspended

dust (Omura 1981). According to the International

Agency for the Research of Cancer (IARC), amorphous

silica belongs to group 3: it is classified as non-carcino-

genic. There is inadequate evidence for the carcinogenic-

ity of amorphous silica to humans and to experimental

animals (Anon. 1986). However, DE with a higher per-

centage of crystalline silica (e.g. the calcined DE) could

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be very noxious. As with all other dusts, DE can cause

mild irritation of the eyes, lungs and skin. Professional

workers dealing with DE should be protected with ade-

quate equipment, particularly respiratory masks (Miles

1990).

6. Resistance development against diatomaceous

earths

For DEs, only a physical mode of action is involved, and

according to Golob (1997) the development of physiologi-

cal resistance to DEs is unlikely to occur. However, newer

studies indicate that resistance may be developed at least

in the case of some species (Korunic and Ormesher 2000;

Rigaux et al. 2001; Fields 2003). Resistance development

against DE has been evaluated by a few workers; hence

this section is mainly based on the account put forth by

Vayias et al. (2008). Several studies document that T. con-

fusum adults can tolerate DE dose rates which are lethal to

other species (Aldryhim 1990; Arthur 2000a; Vayias and

Athanassiou 2004; Athanassiou et al. 2004, 2005). A

study by Athanassiou et al. (2005) using SilicoSec found

that T. confusum adults could survive at dose rates that

caused 100% mortality to S. oryzae adults. Among stored-

grain insect species, T. confusum is considered to be one

of the least susceptible to DEs (Korunic 1998; Arthur

2000a, 2000b; Vayias and Athanassiou 2004). Vayias et.

al. (2008) reported that after successive laboratory selec-

tion, T. confusum adults become more tolerant to Silico-

Sec. Karunic (1998) evaluated the possibility of the

development of DE resistance or tolerance in adults of the

T. castaneum, R. dominica, and C. ferrugineus under the

laboratory conditions. After exposing five generations of

the red flour beetle adults and rusty grain beetle adults and

seven generations of the lesser grain borer adults to DE,

and recording mortality, the ratios of the LD50 values of

the selected strains to the DE-susceptible laboratory

strains were 1.3, 2.2 and 1.9, respectively.

Due to the limited use of DEs in practice, most stored-

product insect populations have had no previous contact

with DEs, but the development of resistant strains is feasi-

ble. This fact should be taken into account when a DE-

based control strategy is planned, given that continuous

use of DEs in a storage facility may cause this effect.

Resistance is likely to be expressed more quickly when

DEs are used for treating parts of the grain mass since, in

this way, individuals may be exposed occasionally but

may also remove themselves from the treated material.

Several studies suggest that it is preferable to use DEs in

the entire grain mass (Athanassiou et al. 2004, 2005;

Vayias and Athanassiou 2004), or to combine DEs with

other substances, such as low doses of other insecticides

(Athanassiou et al. 2004; Vayias et al. 2006), entomopa-

thogenic fungi (Lord 2001; Akbar et al. 2004), or heat

(Dowdy 1999; Dowdy and Fields 2002). Under laboratory

selection of T. castaneum with Protect-It, Korunic and

Ormesher (2000) recorded a 1.3-fold increase in tolerance

after six generations. By using the same DE, Fields

(2003), after 3 years of laboratory selection, obtained a T.

castaneum population with a 2-fold increase in tolerance,

which was a considerably lower rate of increase compared

with S. oryzae, R. dominica and C. ferrugineus. Vayias

et al. (2008) produced a laboratory population of T. confu-

sum in which each generation was usually more tolerant

than the previous one to SilicoSec, especially at the low

dose rates, where adult survival was increased. This

accords with the finding of Fields (2003) that the develop-

ment of resistance is likely after occasional exposure to

DEs. Tolerance was not observed when adults were

exposed to 1500 ppm for either 7 days or 14 days, but it

was observed (1.3-fold decrease in mortality after 10 gen-

erations) with a 48 h exposure interval. Since all DEs are

based on the activity of SiO2 � which means that the

insecticidal effect is similar among DEs � the develop-

ment of resistance to one DE may be transferable to other

DEs as well.

However, the use of EDE formulations may provide a

possible solution to these potential problems (Fields

2003). Regarding the mechanism of resistance, DEs act

only in a physical way on the cuticle, so the reported resis-

tance to DEs is not a physiological response. Rigaux et al.

(2001) found that T. castaneum adults that were less agile

were more tolerant to Protect-It. Apparently, decreased

mobility decreases the contact with DE particles, with a

concomitant increase in survival in the DE-treated sub-

strate. Thus, the resistance reported by Vayias et. al.

(2008) and other workers can be considered as a

“behavioural” and not a “physiological” resistance, since

it is primarily based in the avoidance of increased contact

with DE particles.

7. Efficacy of enhanced diatomaceous earths

Several DE formulations are commercially available and

many studies document that they are very effective

against a wide range of stored-product insect species.

However, the main drawback in the use of DEs is that

they need to be applied at high-dose rates, and these affect

the physical properties, chiefly bulk density, of the stored

grains (Korunic et al. 1998). Also, the presence in the

atmosphere of high DE concentrations containing crystal-

line silica may cause respiration problems (silicosis) to

workers after long exposure. Many DE formulations are

only effective at dose rates of 1000 ppm or more (Atha-

nassiou and Korunic 2007), while most traditional grain

protectants should be applied at dose rates that usually do

not exceed 10 ppm. One of the possible solutions to the

problems caused by the use of DEs in high doses is the

combined use of other, reduced-risk methods, such as

extreme temperatures (Dowdy and Fields 2002), entomo-

pathogenic fungi (Lord 2001; Akbar et al. 2004; Micha-

laki et al. 2006) or low doses of insecticides (Vayias and

Stephou 2009). The combined use of DEs with other low-

risk control methods, as proposed by Arthur (2003), could

be a possible solution for the wider use of DEs as grain

protectants. For instance, Athanassiou and Steenberg

(2007) found that the simultaneous use of DEs with the

entomopathogenic fungus Beauveria bassiana (Balsamo)

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Vuillemin (Deuteromycotina: Hyphomycetes) was effec-

tive against adults of the granary weevil, Sitophilus gran-

arius. Similar combinations have also been examined for

other fungus species as well, with varied results (Micha-

laki et al. 2006, 2007). Another combination is the addi-

tion of low doses of insecticides, which could reduce the

effective dose rate of DEs. For instance, Athanassiou

(2006) noted that DEs could be combined with low doses

of the pyrethroid betacyfluthrin. Ceruti and Lazzari

(2005) evaluated the efficacy of different dosages of DE

mixed with powder deltamethrin for controlling Sitophilus

zeamais in stored corn. In the treatments mixing DE with

deltamethrin or using deltamethrin alone, the mortality

was registered since the first day. In the treatments using

only DE, the first dead insects were recorded after the

third day, especially at the highest dosages. It was con-

cluded that treatments using DE combined with low dos-

ages of powder deltamethrin represent an efficient control

measure against S. zeamais in stored corn, because insect

mortality is faster than with treatments using DE alone

and residues of active ingredients are much less compared

with using the insecticide at high dosages. Vayias et al.

(2006) attempted the combined use of DE formulations

and one natural pyrethrum formulation against pupae of

the confused flour beetle, T. confusum, and reported

encouraging results.

Some commercially available DE formulations con-

taining low doses of insecticides have been evaluated,

with promising results. Athanassiou et al. (2008) evalu-

ated a mixture of DE with the plant extract bitterbarkomy-

cin (BBM) against adults of R. dominica and the results of

their study were very promising, since the mixture was

very effective at dose rates of less than 150 ppm. Accord-

ing to Wang et al. (1991), BBM has strong antifeedant

and insecticidal properties at doses of 25�50 ppm and has

been tested against several crop pests. In another study,

Athanassiou et al. (2009) found that low doses of BBM

(0.0375�0.0875 ppm) proved very effective against sev-

eral stored-product insect species, namely S. zeamais, T.

castaneum and C. ferrugineus. Vayias and Stephou

(2009) evaluated an enhanced mixture of DE with BBM

against adults of S. oryzae, T. confusum and C. ferrugi-

neus, which are three major stored-product pest species.

This mixture (DEBBM) was applied at three dose levels

(50 ppm, 100 ppm and 150 ppm) and on four grain com-

modities, namely hard wheat, barley, rice, and maize.

DEBBM efficacy was increased with the increase of dose,

exposure and temperature, whereas it was decreased with

the increase of relative humidity. DEBBM was very effec-

tive against C. ferrugineus, as mortality of this species,

which was achieved with 150 ppm, was always >85%. Of

the remaining species, the least susceptible to DEBBM

was T. confusum. Although DEBBM caused significant

mortality to all three species, progeny production was not

totally avoided. However, progeny production was signifi-

cantly reduced in comparison with the untreated commod-

ities. Athanassiou and Korunic (2007) reported that two

natural DE formulations, enhanced with abamectin

(DEA-P/WP) or bitterbarkomycin (DEBBM-P/WP),

resulted in death of all adults of S. oryzae, even at the low-

est dose rate (100 ppm) of DEA-P, while 100% mortality

was noted at doses �125 ppm of DEBBM-P. For the other

species, mortality was 100% on wheat treated with

75 ppm of DEBBM-P, with the exception of T. castaneum

for which all adults were dead at doses �100 ppm. Prog-

eny production was low, and no progeny were produced

in the cases of R. dominica and C. ferrugineus, for both

DEs.

Riasat et al. (2013) evaluated the virulence of Isaria

fumosorosea alone and integrated with an enhanced for-

mulation of DE (DE þ bitterbarkomycin, DEBBM)

against R. dominica on stored wheat. The DE alone sup-

pressed the progeny emergence at higher dose rate as

compared I. fumosorosea alone, but their simultaneous

use further reduced the progeny production of R. domin-

ica. The integrated use of both tested biocontrol agents

synergized the effect of each other and caused the highest

mortality at 25�C and 56% relative humidity. The results

clearly demonstrated that I. fumosorosea and enhanced

DEBBM can be integrated to be an effective control mea-

sure for R. dominica in stored wheat.

Yang et al. (2010) carried out laboratory bioassays to

determine the efficacy of garlic, Allium sativum L. (Amar-

yllidaceae), essential oil applied alone or with DE against

adults of S. oryzae and T. castaneum. The results showed

that the combination treatment was significantly more

effective than either treatment alone. In addition, the

results also showed that the simultaneous application of

essential oil plus DE significantly reduced the concentra-

tion of essential oil alone that was required for an effec-

tive treatment, and the application rate of DE could be

reduced when combined with essential oil. Moreover, the

activity of the combination treatment lasted longer than

that of essential oil alone and the survival of eggs or lar-

vae to adult stage was significantly inhibited in the com-

bined treatments against both species, compared with the

use of essential oil alone.

Chintzoglou et al. (2008), Vayias et al. (2009) and

Kavallieratos et al. (2010) carried out laboratory tests to

evaluate the efficacy of DE formulations enhanced with

spinosad against various stored-product pests and reported

improved toxicity at lower doses. Kavallieratos et al.

(2010) found that even the lowest dose was highly effec-

tive (>90%) against R. dominica and S. oryzae. In the

case of T. confusum a combination of longer exposures

with higher doses was required for each formulation to be

effective. They also reported improved adherence ratios

for the tested DE and spinosad formulations. Vayias et al.

(2009) reported that it is possible to combine low doses of

DE (<600 ppm) with spinosad (<1 ppm) to control adults

and larvae of T. confusum, especially at temperatures

>25�C. Chintzoglou et al. (2008) concluded that the spi-

nosad dust and DE could be used in combination treat-

ments but efficacy varies with the target insect species

and commodity.

Chanbang et al. (2007) evaluated combination treat-

ments of DE “Protect-It” and the insect growth regulator

methoprene against R. dominica on stored rough rice.

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Application rates of DE and methoprene ranged from 0 to

500 ppm and 0 to 1 ppm, respectively. In the absence of

methoprene, mortality of exposed adults increased as the

concentration of DE increased, but even at the highest

rate of 500 ppm, mortality was not high. There was an

unexpected increase in adult mortality with the addition

of methoprene, so that at 1 ppm methoprene and 500 ppm

DE mortalities in long-, medium-, and short-grain rice

were 77.57%, 77.57% and 58.57%, respectively. With the

inclusion of methoprene there were few progeny produced

in any of the treatment combinations, and the overall aver-

age was 0.6 � 0.3. Thus, inclusion of insect growth regu-

lators with DE formulations seems to be a very promising

option.

Wakil et al. (2012) reported that DE in combination

with Beauveria bassiana or a neonicotinoid insecticide

thiamethoxam may provide safety for an extended period

against R. dominica. They reported that the mortality of

adults of R. dominica decreased over the 9 months of

storage period and the combined application of the test

materials enhanced the mortality rates compared with

alone treatments. The greatest mortality was observed

with the combination of DE and thiamethoxam. Progeny

suppression was decreased with the extended storage

period. However, the maximum rate of mycosis and

sporulation in the cadavers of R. dominica was observed

where B. bassiana was applied alone at the lowest dose

rate. In another study Wakil et al. (2013) evaluated the

effects of combining thiamethoxam at rates of 0.25, 0.5

and 0.75 mg/kg of active ingredient with the DE formu-

lation SilicoSec at the rate of 100 mg/kg against four

Pakistan populations of the lesser grain borer, R. domin-

ica. Mortality increased with increasing application rates

and exposure intervals for each population. Individually,

thiamethoxam alone was more effective at the highest

dose rate than DE alone, but after 14 days of exposure in

most cases, there was greater mortality with DE than

with the lowest dose of thiamethoxam. Populations dif-

fered in susceptibility to treatments and production of

progeny.

Arthur and Fontenot (2013) evaluated the efficacy of

Alpine�, a formulation containing dinotefuran and DE as

aerosol spray and a dust with DE at the rate of 5 g/m2 and

10 g/m2 against six adult stored product insect species: T.

castaneum, R. dominica, Oryzaephilus surinamensis, T.

confusum, Dermestes maculatus (DeGeer), and Mezium

affine Boieldieu. Mortality of T. castaneum, R. dominica,

and O. surinamensis generally increased with exposure

interval, and was 90% or more after 3 days of exposure at

both dust rates. Mortality of D. maculatus and T. confu-

sum after 3 days ranged between 50% and 70%. Mortality

of all species except M. affine was generally lower when

exposed to the spray rather than the dust. No late-stage

larvae of T. castaneum, T. confusum and O. surinamensis,

exposed to either the spray or the dusts, emerged as adults.

Based on the results obtained, Arthur and Fontenot (2013)

concluded that the enhanced DE formulation could be

incorporated into management plans for control of stored-

product insects.

8. Conclusion

DEs are the most efficient among all inert dusts for the

management of stored-product pests. The major bottle-

neck in their wider adoption is the decrease in grain bulk

density caused by addition of high doses of DE dusts.

However, inclusion of different classes of low toxicity

insecticides with DE formulations enables control at

lower doses, although results vary with target species.

Thus, the best combinations need to be worked out for

each situation and only then will enhanced DE formula-

tions find a place in the market in competition with cur-

rently used synthetic insecticides.

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