Stored Grain Insects



Granary Weevil

Sitophilus granarius



Adult granary weevil (Sitophilus granarius)
Courtesy of Degesch Inc.

The granary weevil (Sitophilus granarius) is dark brown-black in colour and is about 2.5 - 5 mm in length. In small grains, such as millet or milo maize, weevils are small in size, however in grains such as corn they are larger. They possess a long slender snout and cannot fly. In the larval stage the weevils are legless, humpbacked, white to creamy white, with a small, tan head. Weevils in the pupa stage have snouts like the adults. The granary weevil is closely related to the rice weevil, but lacks the pale markings of its elytra. The pits on the thorax are elongate rather than round or irregularly shaped.


Developmental stages in granary weevil
Courtesy of Degesch Inc.

Life cycle
The egg, larva, and pupa stages of these weevils occur in the grain kernels and are rarely seen. Feeding is done within the grain kernel, and adults cut exit holes to emerge. Emergence holes of the granary weevil are fairly large and tend to be more ragged than smooth and round. 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 center 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 and 75 - 90 % rh, 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 granary 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.

Damage caused by granary weevil
Courtesy of Degesch Inc.

The granary 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. Grain weevils are important pests of farm-stored grain. They are frequently regarded as primary pests of grain since they are able to infest otherwise undamaged grain. Grain weevils will also attack other hard cereal products, e.g. macaroni and spaghetti. Fine cereal products are unsuitable for breeding purposes unless they become caked. Granary weevil infestations can result in the reduction in the weight and quality of grain as a result of the larvae feeding on the endosperm. The germ is not always attacked so germination may take place, producing a weak seedling which is vulnerable to attack by moulds, bacteria and other insects. Both larvae and adults will feed upon grain. The grain can also be tainted with white, dusty excreta which contaminate the product as well as render it unpalatable. Heating of grain also occurs inturn 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. Weevil-damaged grain can be readily recognised by the presence of large holes which are the exit holes of the emerging adults. Some idea of the huge numbers of weevils that can be generated is provided by the results of one study in which, 5 weeks after wheat was infested with larval forms, adults were seen to be leaving the grain at a rate of 100 per kg per day.

Adult granary weevil
? Agriculture Western Australia

The simplest and most effective measure is to locate the source of infestation and quickly get rid of it. If practical and regulations allow, dispose of heavily infested foods in wrapped, heavy plastic bags or in sealed containers for garbage removal, or bury deep in the soil. If you detect an infestation early, disposal alone may solve the problem. Properly ventilate the storage area to discourage these moisture-loving stored product pests. Be sure to store only clean, dry grain with a moisture content of 12 percent or less to reduce weevil problems.

Grain insects are declared under the Agriculture and Related Resources Protection Act. Limited chemical controls are available to farmers and emphasis is placed on clean hygienic storage and cleaning of machinery. Farmers are encouraged to purchase and maintain sealed farm silos to increase the effectiveness of fumigation.


MPEG of Sitophilus granarius (approx 6 Mb)