Efficacy of diatomaceous earths, applied as structural treatments, against stored product insects and mites

Abstract

Diatomaceous earth (DE), a silicon-based dust that can desiccate insect and mite pests, acts by removal of water-proofing waxes found in the cuticle. Three DE products marketed for stored product protection were launched in the UK from 2001 onwards. Following on from previous research, that enabled recommendations for use on grain, this project aimed to investigate their use to control residual pests in empty grain store structures.

Initial laboratory experiments using methods based on an international protocol, tested UK DE products against a range of insect, mite and moth species. DE treatment was applied either as a dry dust, or an aqueous slurry. From the results of the first phase of testing, the most tolerant species and the most appropriate doses were identified. These were adults of the grain weevil (Sitophilus granarius), the cosmopolitan food mite (Lepidoglyphus destructor) and larvae of the Mediterranean flour moth (Ephestia kuehniella), with doses of 10-20 g/m2 against the weevil/ moth and 1-3 g/m2 (dry dust) or 20 g/m2 (slurry) against the mite, depending upon DE product. The second phase of experiments involved more in-depth testing of the DEs against these most tolerant species on a more challenging substrate, wood. This time efficacy was tested over a longer 12 week period to indicate the DE's persistence and the most appropriate dose for field validation. As a result of this, it was decided that dry-dust treatments should be evaluated at a dose of 10 g/m2 for a storage season (harvest to early summer).

Trials took place in two identical stores containing a nest of six 20 tonne steel bins, into which ca. 100,000 insects were released in July 2003. The effect of treatment was assessed by monitoring the number of insects trapped, and by counting the number killed on contained bioassays. This exercise was repeated again with a second release of insects in March. The trial concluded that under the most ideal situation where best practice was followed, the least tolerant species (saw-toothed grain beetle) could be killed within 2 weeks and the most tolerant (grain weevil) within 5 weeks.

The project concluded that DE is ideal for treating empty stores if used as part of an integrated strategy as follows:

• Clean out stores between harvest, ensuring no food residues remain in cracks and crevices.
• Monitor the empty store with traps.
• Apply DE as a dry dust at 10 g/m2 if insects are found (only apply as a slurry where practice dictates).
• Where practical, leave for at least 5 weeks before filling with grain.
• Monitor the store after treatment.
• If large numbers are found in a particular area, investigate and, if necessary, re-clean and re-treat.