Grain Weevil (Sitophilus granarius)

Grain weevils are pests of stored grain and seeds. They develop inside whole grain kernels as small, white, wrinkled, grub-like larvae. There is generally no external evidence that the larvae have been eating and growing inside the seed until after about one month when the adult weevil chews through the seed coat and emerges. Weevil-damaged grain can be readily recognised by the presence of large holes which are the exit holes of the emerging adults.

Identification:
Adult weevil, 2.5 - 5 mm in length. Dark brown-black in colour. It has a long slender snout and is a major pest of whole grains.

Biology:
Females drill a tiny hole in the grain kernel, deposit an egg in the cavity, then plug the hole with a gelatinous secretion. The egg hatches into a young larva which bores toward the centre of the kernel, feeds, grows, and pupates there. New adults bore emergence holes from the inside, then leave to mate and begin a new generation. Female granary weevils lay from 36 to 254 eggs. At 27 - 30°C, eggs hatch in wheat with a moisture content of 13.5 to 19.6 percent in 3 days. There are 4 larval instars, and the developmental period is from 3 to 5 weeks. Pupation within the kernel requires 5 to 16 days. The life cycle is about 30 to 40 days during the summer, and 123 to 148 days during the winter, depending on temperature. The grain weevil is long-lived, surviving for 7 to 8 months as an adult. The female lays very few eggs at temperatures below 16 °C, but can survive for 2 months or more at about 2 °C.

Control:
Infestations in domestic or food storage situations can usually be traced back to a cereal source. Since these beetles can infest so many types of grain products, inspect thoroughly. There may be several infested products, discard any food that is infested. Infested areas should be thoroughly vacuumed paying particular attention to cracks and crevices, contents of the vacuum cleaner should be disposed of in an outside bin. Store new food products in sealed glass or plastic containers. After cleaning, treat the infested areas using a residual insecticide labeled for crawling insect to ensure that all adult beetles and larvae have been killed.

Products to control Grain Weevil: