The adults are 3-4 mm in length and are dark brown to nearly black in colour with a shiny appearance. They have 8 segmented antennae; a prothorax with distinct elongated punctures and the hindwings are absent. They are unable to fly.
Weevil-damaged grain can be readily recognized 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.
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 2-3 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 of 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.
Traditionally grain weevils were the prime pest of stored grain in the UK on account of their ability to bore into seeds as well as being able to survive the winter in unheated buildings. Whilst they are encountered in all temperate and warm to temperate climates throughout Europe their importance in the UK has declined but they are still regularly found.
The following damage may be caused:
Reduction in the weight and quality of grain as a result of the larvae feeding on the endosperm. The germ (embryo part of the seed) is not always attacked so germination may take place, producing a weak seedling which is vulnerable to attack by moulds, bacteria and other insects. The larvae and adults both feed on grain
Tainting with white, dusty excreta which contaminate the product as well as rendering it unpalatable
Heating of the grain, accelerating development of the insects and making the commodity liable to caking, moulding and even germination. Temperatures may be attained which actually kill the insects
Management of storage pests
Use trapping methods to identify and measure insect infestations
Clean stores thoroughly before harvest and seal all building cracks and crevices
Store grain at <15 % MC and <15 °C
Where appropriate apply insecticides to the fabric of stores or apply grain protectants
Grain weevil adult
Grain weevil damage
Illustration of a grain weevil adult