Beneficial Insects Thwart Storage Losses

Scientists have identified several species of parasites and predators that control specific pests in stored grain. With worldwide postharvest losses at about 9%, and as high as 30% in some tropical countries, an alternative method is critical where pests cannot be controlled by fumigants.

Research at the Agricultural Research Laboratory in Savannah, Georgia, led by Research Entomologist John Brower may provide that alternative.

Brower is studying three insect pests that infest wheat, rice, corn, and other cereals. They are: the granary weevil (Sitophilus granarius), the rice weevil (S. oryzae), and the lesser grain borer (Rhyzopertha dominica). The adult insect lays eggs in the grain bins, and the resultant larvae burrow into and feed upon the kernels until they emerge at maturity. The larvae of one insect can infest up to 300 kernels. The lesser grain borer can lay up to 500 eggs and is more destructive than the others because it continues feeding as an adult. Damaged or infested grain then becomes more susceptible to other insects and is problematic during processing.

The Anisopteromalus wasp is a potentially effective control of weevils, such as Sitophilus in wheat. It seeks out infested seeds, drills a hole in the kernel, and lays an egg in the pest larva. The wasp larva feeds on the body fluid of the pest, consuming its host within one week. This wasp efficiently locates even small numbers of pests.

In a small-scale storage situation, 30 pairs of Anisopter-omalus wasps released into Sitophilus-infested grain debris reduced weevil populations by 90 percent.

They are too highly specialized to become pests themselves, and do not feed on other pest species, according to Brower. This narrow host-range can be a drawback in controlling different grain pests. Therefore a combination of predators is needed to be effective.

Brower is also studying the larger grain borer, Prostephanus truncatus, which is a serious pest of stored corn in Central America and Africa. Brower has found that several species in the Pteromalidae family attack P. truncatus within the corn kernel at different stages of maturity and with varying success. These predators are: Anisopteromalus calandrae, Lariophagus distinguendus, Choetospila elegans, and Pteromalus cerealellae.

Predatory insects are being considered for use in conjunction with other controls, such the application of pesticides and improved storage containers and facilities. Controlled storage climates and improved sanitation should also reduce pest populations.

At the Rice Research Unit in Beaumont, Texas, combined biological and chemical control methods in a 15-month test reduced rice weevil populations by 67%, and lesser grain borers by 89%. Some secondary pest populations such as the flat grain beetle were reduced by up to 97%.

A proposed regulation, now being considered by the FDA, would allow parasites or predator insects to be added to stored grains as a pest control.

Replacing Fumigants with Beneficial Insects, Agricultural Research, Feb. 1991.

Brower, J. H. Biological Control of Prostephanus Truncatus (Horn) with Parasitic Hymenoptera, 5th International Working Conference on Stored Product Protection. September 9-14, 1990.

For more information:

John H. Brower
P.O. Box 22909
Savannah, GA 31403