Feature Article

Biological Control of Stored-Product Pests

Wasps in the corn flakes? Pirate bugs in the peanuts? Let's hope we don't find any insects in the foods we buy at the grocery store. However, there are many natural enemies of stored-product insect pests, and they can be used as biological control agents in stored commodities. Various predators, parasitoids, and microbial pathogens of storage pests have considerable potential for providing needed pest management alternatives for postharvest agriculture.

"Stored products" include all durable agricultural commodities that can be dried and stored in bulk, such as cereal grains, flour, oil seeds, legumes, nuts, dried fruits, animal products, and packaged, baked or processed foods. Of these, cereal grains make up the majority of commodities maintained in storage, and represent an important component of the world food supply. Recent postharvest losses have been estimated at $5 billion per year in the U.S. (mainly due to insects and microbes, which usually work in concert), with these being proportionally higher in developing countries. Insecticides can be used to protect grain, but few are labeled for use on grain, and insects are evolving resistance to some of these. In the U.S. there are two residual insecticides and one fumigant gas that are used routinely to treat grain. Regulatory loss of any of these could cause serious problems for grain pest management. Researchers at universities and government agencies are looking for alternatives to chemical pesticides (including biological controls) for grain protection and to develop methods of Integrated Pest Management for stored products.

Insects have infested stored food products since the beginning of human civilization. Interestingly, many of the pest species found in stored products do not normally occur on wild host plants or in non-human habitats, but are fully adapted to the storage system, much like house flies and certain cockroaches are adapted to human habitations. Thus in addition to grain-eating beetles and moths that are expected in stored grain, one also can find habitat-specific and host-specific natural enemies that utilize these pest insects as food. Most pests and their natural enemies have world wide distributions because they have been spread by people. Common storage pests include the Indian meal moth and Mediterranean flour moth that infest grains, flours, animal feed, and people's pantries; red and confused flour beetles in the genus Tribolium, together with sawtoothed and flat grain beetles in the genera Oryzaephilus and Cryptolestes, that are found in many grain products; and the more serious internal grain feeders such as rice, maize and granary weevils in the genus Sitophilus, and the lesser and larger grain borers, bostrichid beetles that riddle stored grains with their tunneling activities. Populations of all of these pests can be impacted by one or more species of natural enemy.

Taxonomic groups of natural enemies in stored products are similar to those in other agroecosystems, although numbers of species are lower than in outdoor habitats. Important natural enemies include parasitoid wasps in the families Braconidae, Ichneumonidae, Pteromalidae and Bethylidae, and predatory pirate bugs. A collection of other predators can be found in some situations, and these include assassin bugs, hister beetles, pseudoscorpions, and predatory mites. Pathogenic bacteria and viruses occur naturally within storage insect communities. Bacillus thuringiensis, the bacterium commonly known as Bt, affects stored product moth larvae and is available commercially for use on grains. Species-specific insect baculoviruses have been studied and at least one type is available for use against the Indianmeal moth.

Certain parasitoids and predators of storage pests are notable for their potential as biological control agents in the U.S. Females of the parasitoid wasp Bracon hebetor seek out and sting wandering-stage larvae of pyralid moth pests such as the Indianmeal moth and the Mediterranean flour moth. The wasp lays a group of eggs on the body of the paralyzed caterpillar, and wasp larvae hatch out and feed gregariously on the host. Because B. hebetor's development time from egg to adult is half that of its host, and several adult wasps can be produced from one parasitized host, this species is considered an excellent candidate for biological control since its population can increase rapidly and overtake that of the host. Other important wasps that can control internal-feeding grain beetles are the pteromalids Anisopteromalus calandrae and Choetospila elegans. A female of these species forages through grain, selects a kernel that contains a larva or pupa of a grain beetles (such as the rice weevil or the lesser grain borer), inserts her ovipositor through the kernel and stings the beetle larva inside. Then she lays an egg on the host that eventually develops into an adult wasp. Pirate bugs such as the warehouse pirate bug, Xylocoris flavipes, and the larger pirate bug, Lyctocoris campestris, are generalist predators whose adults and nymphs feed on any life stage of pests that can be subdued. Pirate bugs have the greatest impact on external feeding species of beetles and moths. The species discussed here are easy to rear and manipulate in the laboratory and some species are now commercially mass-produced for sale to stored grain managers. None of these species feed on or adversely affect grain, and the adult natural enemies occur outside grain kernels and hence can be cleaned from grain before milling.

Research during the 1970's and 1980's demonstrated the efficacy of parasitoids and predators in controlling storage pests. Releases of the warehouse pirate bug, Xylocoris flavipes, resulted in a 79-100% suppression of moth populations in small storages of peanuts, up to 99% reduction of sawtoothed grain beetle populations in 35-quart lots of corn, and a 90-98% suppression of red flour beetles in a simulated peanut warehouse. Rice weevils in wheat spillage in small rooms were suppressed 96% by the parasitic wasp Anisopteromalus calandrae. When the egg parasitoid Trichogramma pretiosum and the larval parasitoid Bracon hebetor were released together in simulated peanut warehouses, they suppressed Indianmeal moth populations by 84% and almond moth (Cadra cautella) populations by 98%; B. hebetor alone suppressed almond moth populations by 97.3%. Several other studies have yielded similar results that suggest these natural enemies should provide effective biological control of storage pests. However, these experiments were conducted under laboratory conditions in which the commodities, storage containers or structures, and numbers of pest insects were under strict control.

Recently scientists with USDA's Agricultural Research Service have been conducting operational field trials of parasitoids and predators in large metal bins of grain identical to those used on farms and at country elevators. Results of these studies confirm that natural enemies can be used in bulk grains to reduce pest populations, and some recommendations about application methods can be made. In one of these field experiments at the U.S. Grain Marketing Research Laboratory in Manhattan, Kansas, six cylindrical bins were each filled with 3000 bushels of wheat and infested with known numbers of grain beetles. After 20 days three of the bins were treated with about 500 parasitoid wasps and the other three bins received no wasps. More beetles were added monthly for three months to insure established pest populations, but no more parasitoids were added after the initial introduction. Populations of the lesser grain borer, a serious internal-feeding beetle pest of small grains, were severely reduced by the addition of the parasitoid wasp Choetospila elegans to treated bins. After nearly 200 days of storage, the control bins (with beetles only and no wasps) contained an average of 2.06 lesser grain borers per kilogram of wheat, while bins that received the wasps had only 0.05 lesser grain borers per kilogram of wheat. The addition of parasitoids 20 days after the wheat was infested apparently allowed for the proper life stage (late instar larva) to be present for parasitization, and gave the parasitoid population a boost compared to the growth rate of the pest population. Promising results have also been obtained from a similar USDA research project in which maize weevils infesting corn were controlled by the addition of parasitoid wasps.

The use of natural enemies against stored-product pests is legally permissible in the U.S., but the regulatory process applied to these organisms resulted from an interesting series of events. As research results on the efficacy of storage natural enemies became generally known, a commercial insectary began producing some of these species for distribution. An organic food company that had purchased these organisms was undergoing a routine inspection by a government health agency when the inspector observed that the company was adding insects to their grain. Since insects represent "filth" in food and should be eliminated, not increased, the company was ordered to stop sale of the affected grain. The U.S. Food and Drug Administration (FDA) and the Environmental Protection Agency (EPA) soon became involved. Although biological control agents are considered "pesticides" because they are used to mitigate pests, in 1982 EPA exempted them from the regulations imposed on chemical pesticides. However, EPA was only considering biological control agents applied to crops in the field. As "pesticides" added to food, stored-product biological control agents were subject to regulation under the Federal Food, Drug and Cosmetic Act, which sets tolerance levels for chemical pesticides that may occur in food. Finally, in 1992 the EPA exempted biological control agents from having a requirement for tolerance levels in commodities, and essentially allowed their use in any stored-product situation where they are not expected to become a component of food.

There is great potential for using biological control to control pests in stored products. Research is continuing to determine the proper prescriptions for use of natural enemies in stored grain. Behavioral, ecological and physiological data are being collected that will facilitate effective deployment of parasitoids and predators. Storage situations other than grain bins, such as feed mills, food warehouses, and food factories may be targeted areas for biological control in the future. As part of an IPM system for stored product management, biological control should help reduce the use of pesticides on food and provide for high quality food products.

- Tom Phillips, USDA-ARS, Hilo, HI (formerly of the USDA Stored Product Insect Lab, Univ. of Wisconsin - Madison)

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