Stored product pests fall into two main insect orders:
- Coleoptera (beetles)
- Lepidoptera (moths).
Within each group, some species are considered as primary pests, they attack the whole grain, while others follow the initial damage as secondary pests. Losses can amount to 10% or more of the commodity through spoilage or secondary fungal infection.
Understanding Primary and Secondary Pests
Primary coleopteran pests include grain weevils (Sitophilusgranarius, S.zeamais, S.oryzae),the lesser grain borer (Rhyzoperthadominica) and the saw-toothed grain beetle (Oryzaephilussurinamensis.)
Secondary beetle pests include the flour beetles (Triboliumconfusumand T.castaneum)
The main lepidopteron pests are secondary; they feed regularly on processed foods so are more common in domestic kitchens and larders.
A variety of pests infest stored grain. This may occur in succession – primary then secondary. Eggs are laid in the grain or part-processed food (flour, bran etc.) where the larvae feed through to the pupal stage.
Moth larvae usually fly to a hidden site away from the food source to pupate, adults then mate and eggs are deposited on or in suitable food.
Emerging beetle adults feed on grain or find shelter in the structure of the silo/warehouse prior to invading new food sources.
Cool temperatures (10°C) reduce the rate of reproduction. While low humidity and ventilation reduce insect activity and viability, they also suppress fungal development.
Pheromone traps aid species control, notably moths, and indicate if and when to treat. Egg to adulthood may take weeks or months, so an infestation is not instantly apparent.
Treat empty grain stores between batches of grain to prevent infestations (regulated under 91/414, not BPD). Treat grain with a residual insecticide at the point of entry into the silo.
Admixture means active ingredient is not subject to photo degradation, while microbial activity in the grain is low, favouring long residual protection.
Fumigation (eg in ship holds) with methyl bromide was popular in the past (no residual action but effective control).