Scientific name: Sitophilus granarius
There are 3 different species:
- Sitophilus granaries (Grain Weevil)
- Sitophilus oryzae (Rice weevil)
- Sitophilus zeamais (Maize Weevil)
Each of these species varies considerably in size but has a distinctive elongated snout which is adapted to the size of its preferred grain. Typically, they reach 2-4mm in length and have a long cylindrical body which is dark brown or nearly black in colour.
Grain weevils are encountered in all temperate and warm-temperate climates. They are widely distributed around Europe. Both adults and larvae are cold-hardy.
Rice and Maize weevils are widely distributed in tropical and sub-tropical areas and will be carried to temperate areas on imported commodities.
The maize weevil breeds on maize in the field but the Rice weevil only breeds in stored grain. Both insects will not normally overwinter in unheated premises or grain stored at normal temperatures.
Grain weevils do not fly but instead, infestations often occur after being imported in grain and cereal products, also from the fabric of vehicles used to transport grain or buildings to store it.
The female will lay a single egg inside the grain, where larva and pupa stages will occur, once developed, the weevil bores its way out leaving a hole in the grain.
The Grain weevil can only breed in grain with moisture content of more than 9.5% and at temperature within the range 13-35C.
Risks and Symptoms
Grain weevils are primary grain pests, infesting undamaged grain and attacking other hard cereal products such as macaroni and spaghetti. Weevil-damaged grain can be readily recognised by the presence of large holes which are the exit holes of the emerging adults. They are regarded as primary pests of grain since they are able to infest otherwise undamaged grain.
Both the adults and the larvae feed on the grain causing holes and also contamination with their excretions. Grain quality and marketability is reduced.
The grain weevil can only breed in grain with a moisture content of more than 9.5% and at temperatures within the range 13-35 °C. The female lays about 200 eggs at a rate of 2-23 per day depending upon temperature and humidity, placing each one in a small hole bored in the grain and sealing it in with a mucilaginous plug of saliva. At 18-20 °C the eggs hatch in 8-11 days to give small, white, legless larvae which feed in the endosperm in the grain.
Only one larva develops in small grains such as wheat and rice but large grains such as maize will support the development of several. Larvae are never free living and develop entirely within the grain. They moult four times, finally pupating within the grain after 6-8 weeks. The adults emerge after a further 5-16 days and will live for about 9 months. If disturbed they will feign death by drawing their legs up to their bodies and remaining still.
At 15 °C and a grain moisture content of 11% the full life-cycle takes 6 months. The life-cycles of the rice and maize weevils follow a similar course to that of the grain weevil.