Insect Note #10 (Revised)

Dated 6/94
Placed on the Web 4/95 by the Department of Entomology, NCSU

Control of Weevils in Stored Peas and Beans

Bean Weevil

This wedge-shaped insect is dark grayish-brown, thickly covered with grayish yellow hairs and has dark bands across its back. It measures about 1/10 inch long and has six reddish-brown legs.

Female bean weevils deposit whitish eggs singly or in clusters among the dry peas or beans. White, legless larvae or grubs hatch from the eggs and penetrate the seed where they remain throughout the rest of the larval, pupal and early adult stage. Young adults soon chew their way out of the seeds, promptly mate, lay eggs and begin a new generation. More than one larva may develop in each seed.

Cowpea Weevil

Cowpea weevils are about the same size as bean weevils. They are reddish-brown with four dark spots on the wings. Life stages and growth habits are similar in both weevils, except that cowpea weevil eggs are glued to the seed. When the young larvae hatch, they quickly bore into the seed and usually pack the empty egg shell with frass (a form of excrement). The cowpea weevil breeds only in cowpeas. It may lay eggs on other seeds but they seldom develop.

Control

Heat Treatment

Place beans or peas in a shallow pan and heat at 120 degrees F for 30 minutes or 145 degrees F for 15 minutes. Be sure that seeds are left long enough for the heat to penetrate to the center of each bean for the required period of time to kill all stages of the insects. If seeds have a low moisture content, germination should not be affected. After heat treatment, place beans or peas in a tight container that will prevent the reentry of insects.

Cold Treatment

Peas and beans which are to be used as seed or for food may be preserved in a freezer or refrigerator. Four days at 0 degrees F kills all life stages of the insect. Storage in a refrigerator at about 40 degrees F stops insect development but normally does not kill the insects. If desired, the peas or beans may be removed from the freezer after a week or so and kept in tight insect-proof containers until used for food.

Chemicals

1. Fumigation - Normally not practical unless large quantities are involved. Most fumigants are very poisonous, so be careful! Place seed in a tight container and fumigate with approved fumigant. If, for example, you use ethylene dichloride, apply one tablespoonful per bushel of beans or peas. Since gas given off by the liquid is heavier than air, it should be applied over the top layer of seed. Cover fumigated seed and do not disturb for 24 hours.

   After 24 hours the seed should be aired by pouring from one container to another. This step is essential if seeds are to be planted. Poor germination may result otherwise. After fumigation, store seeds in a tight container or they may become reinfested. Be sure moisture content of seed is low or germination may be reduced. Check fumigated seed in 30 days. If there are signs of insect activity, retreat.

   If peas or beans are infested when they come from the field, or are infested prior to any treatment, it is advisable to fumigate. Since seeds are susceptible to insect attack following fumigation, they should be placed in insect-proof containers.

2. Dusts - Treat harvested peas and beans with malathion (premium grade) 5% dust using 1 ounce per bushel of seed. Be sure to cover every seed with insecticide. This treatment will prevent insects from infesting the seed for one season. It will not control an active infestation. The malathion dust treatment will kill weevils when they emerge from seeds and thus prevent further damage. For seed only - do not use for feed or food.

3. Sprays - Sprays containing malathion (premium grade) may also be used. Use 1 tablespoonful of malathion 57% emulsifiable concentrate in 1/2 cups of water and apply at the rate of 2 1/2 tablespoonsful of solution per bushel of peas or beans. For seed only - do not use for food or feed.