Action of amorphous diatomaceous earth against different stages of the stored product pests Tribolium confusum, Tenebrio molitor, Sitophilus granarius and Plodia interpunctella

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Abstract

Environmental and human health problems associated with the use of synthetic pesticides have prompted the demand for non-polluting, biologically specific insecticides. The current study tested the use and action of diatomaceous earth against several stored product pests. Fossil Shield® applied to wooden plates was lethal to adult Tenebrio molitor and Tribolium confusum, but larvae of the mealworm were unaffected. Beetles died within 14 days exposure in the absence of food to a dose of 2 and 4 g/m2, but mortality was reduced in those fed grain bran. Fossil Shield® was lethal to first instar larvae of Plodia interpunctella, but not lethal to older larval stages. Two-week old larvae of T. confusum were more sensitive to diatomaceous earth than P. interpunctella at the same age. Contact with diatomaceous earth caused adult Sitophilus granarius, T. molitor and T. confusum to lose weight and reduced their water content, suggesting disruption of “the water barrier”. Death of stored product insects treated with diatomaceous earth decreased with increased r.h., due to reduced transpiration through the cuticle. High r.h. delays, or above 60% can prevent, the drying action of diatomaceous earth.

Introduction

Use of amorphous diatomaceous earth for pest control in stored products and in the field has increased during recent years. Diatomaceous earth and artificial silica dusts have been reported to be effective alternatives to chemical pesticides for control of insect pests in stored products, e.g. when applied to grain or to the walls of storage facilities (McLaughlin, 1994, Golob, 1997). Furthermore, silicate dust is used to control household pests (Melichar and Willomitzer, 1967), structural pests (Ebeling and Wagner, 1959) as well as to protect plants in the field (Ebeling, 1971).

Use of dusts to control arthropod pests in stored products is not new. Sand, clay and ash have been used for centuries (Ebeling, 1971). Zacher was the first since 1900 to study this topic. He described the mode of action of inert dust and metal oxides on insects (Zacher and Kunike, 1931). At first the use of SiO2-dusts was regarded as problematic because of reports that linked crystalline particles to silicosis (black-lung diseases; Zaidi, 1969). Diatomaceous earths are amorphous dusts and are no longer considered hazardous to human health in contrast to crystalline dusts, if they are used correctly and exposure time is short (Ferch et al., 1987). Several modified diatomaceous earth products (e.g. Dryacide®) and other amorphous silica dusts are commercially available in Australia (McLaughlin, 1994). The product Silico-Sec® was first approved for use in Germany in 1997. Natural and artificial preparations are applied as dusts, spray powder, suspensions or granules.

Silica dusts gave variable results when used against different target organisms (Korunic, 1997a, Korunic, 1997b). To understand the basis for the variability it was necessary to determine the mode of action of the dusts. Different theories were proposed: (1) surface enlargement of the integument following dehydration (Zacher and Kunicke, 1931); (2) impairment of the digestive tract (Smith, 1969); (3) blockage of spiracles and tracheae (Webb, 1945); (4) ad- or absorption of cuticle lipids (Ebeling, 1964) and/or (5) damage of the protective wax layer (Wigglesworth, 1942, Beament, 1945). Furthermore, some authors related ad- and/or absorption to symptoms of dehydration (Ebeling and Wagner, 1959) and to weight loss (Alexander et al., 1944). Detailed mechanisms of dehydration are unknown.

The current study was designed to determine the mode of action and efficacy of diatomaceous earth. We tested efficacy of the diatomaceous earth product Fossil Shield® against four species of stored product pests [Sitophilus granarius (L.), granary weevil, Tribolium confusum du Val, confused flour beetle, Tenebrio molitor L., mealworm, and Plodia interpunctella (Hübner), Indian meal moth].

Section snippets

Materials and methods

Insects used in the study were obtained from the Federal Biological Research Center for Agriculture and Forestry in Berlin (Institute of Stored Product Protection). T. confusum, T. molitor and P. interpunctella were reared on bran enriched with yeast and sugar, whilst S. granarius was reared on wheat, in a climate chamber at 25±1°C and 62±2% r.h.

The diatomaceous earth product Fossil Shield® has a particle size of 5–30 μm and is composed of 73% amorphous SiO2, 3% aerosol and other mineral

Effect of Fossil Shield® treated plywood plates on T. confusum and T. molitor

Fossil Shield® significantly reduced the survival of adult T. confusum and T. molitor after 7 days exposure on plates without food (ANOVA 2-way; p<0.0001; Fig. 1); after 14 days survival was nil. There was no significant difference in survival between doses of 2 and 4 g/m2 in T. confusum (Tukey’s-Test, p=0.005), but only after 7 days exposure to 4 g/m2 did all T. molitor beetles die.

Adult T. confusum exposed to Fossil Shield® for 7 days without food lost 20% of their initial weight, in contrast

Discussion

Exposure of T. confusum and T. molitor adults to Fossil Shield® treated wooden plates at 25°C, 62% r.h., was lethal within 14 days, if they could not feed. The addition of food counteracted the effectiveness of dusts against T. confusum, as evidenced by the longer survival times of treated beetles when given nourishment and the need for higher dosage rates to achieve control. The delayed or reduced effect of dusts against T. confusum given nourishment could be attributed to metabolic water

Acknowledgements

We thank Dr Reichmuth and the Institute of Stored Product Protection from the Biological Research Center for Agriculture and Forestry for supporting this work. Furthermore, we thank Dr James R. Burleigh, the site coordinator for the AVRDC-Periurban-Project in the Philippines, for discussing this work.

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