Ingas Diplomarbeit
             

Morphological and physiological effects of amorphes diatomaceous earth against selected stored product insects

Summary

Increased problems by application of chemical pesticides within pest protection caused the demand for non-polluting, biological specific active countermeasures. This work aims to test the possible use of diatomaceous earths against several stored product insects. It will also examine the morphological and physiological effects of amorphous diatomaceous earth against insects.

Fig. 1: Diatomaceous earth, Fossil Shield
		Fig. 2: Diatomaceous earth, Dryacide
Fig. 3: Diatomaceous earth, Silico-Sec

Diatomaceous earth namely Fossil Shield, Dryacide and Silico-Sec reduced the number of adults Sitophilus granarius in wheat. Fossil Shield was the most effective dust against the granary weevil. After two weeks, 100 % mortality was observed of the adults treated with dosage of 2 g diatomaceous earth per kilogram wheat, at 11 - 12 % corn moisture content and relative humidity of about 62 %. The rise in grain moisture and relative humidity decreased the effectiveness of dusts. It was observed, that the diatomaceous earths reduced the number of adult Granary weevils and increased the mortality rate at development stages, but they did not completely prevent the hatching of the next generation.

Adults of Tenebrio molitor as well as Tribolium confusum showed sensitive reaction to Fossil Shield and Dryacide on treated wooden plates. Without addition of food, with the dosages 2 g/m² and 4 g/m² were after 14 days all beetles died. A lower mortality of the adults was observed with addition of feed substrate. Fossil Shield had no effect against the larvae of the mealworm.

Fig. 4: T. confusum, untreated		Fig. 5: T. confusum, treated with Dryacide

Fossil Shield was only effective against the egg larvae of Plodia interpunctella. This diatomaceous earth showed none effect on older instars (L3 und L4). Two weeks old larvae of T. confusum were more sensitive against treatment with diatomaceous earth as instars of P. interpunctella of the same age.

After contact with diatomaceous earth, the individuals of S. granarius, T. molitor and T. confusum had lost on weight and reduced their water content respectively. Granary beetles significantly lost weight and body water after one day exposure to Fossil Shield. This showed that silica dusts break the water barrier of the cuticle. However, it increases the transpiration rate of water across the cuticle and the insect diet past several times through dehydration. Less effective was diatomaceous earth at higher relative humidity toward stored product insects, because for this reason exists a lower permeability for water across the cuticle. This delays or rather prevents the drying action of diatomaceous earths. As well, an addition of food affected the effectiveness of dusts against insects. A lot of stored product beetles will be able to produce metabolic water of feed substrates. This ability protects and weaves contrary to the loss of water.

Diatomaceous earths can be effective primarily through the sorptive properties of the hygroscopic particles. One day after dust treatment a significant weight loss of T. molitor pupae was recorded. The exposure to the silica dust Fossil Shield caused partly an incomplete metamorphosis and death of the individuals. Pupae, were dipped in Fossil Shield or abrasive aluminium dust and rolled for further five minutes, reduced not significant higher their weight as unrolled exuvial stages.

The morphological analyses of treated insects under the Scanning Electron Microscope showed, that the dust covered the hole integument. Partly, the silica particles were sank in the cuticle. After application of Fossil Shield at the cuticle of adults T. molitor, does not recognized any wax layer. Also it could not be found once blocked by stigma of the mealworm. The dust particles accumulated around the respiration openings and they partly overgrow.

The effect of amorphous diatomaceous earths get influence through the sorptive or abrasive characteristics of the particles and through the physiology and morphology of the individuals respectively.

Fig. 6: Cuticula of T. molitor, untreated		Fig. 7: Cuticula of T. molitor, treated

Fig. 8: Cuticula with sunk diatomaceous earth particle

Fig. 9: Mouthparts of S. granarius, untreated		Fig. 10: Mouthparts of S. granarius, treated
Fig. 11: Stigma opening of T. molitor, untreated		Fig. 12: Stigma opening of T. molitor, treated

The diploma-theses was realized in the institute of Stored Product Protection at the Federal Biological Research Center for Agriculture & Forestry in Berlin and in the institute of Applied Zoology / Chemical Ecology at the "Freie Universität Berlin".