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Finding the needle in the haystack: using parasitic wasps to kill hidden stored-grain pests.

Using parasitic wasps to kill hidden stored-grain pests

Move over Sherlock. USDAARS entomologists have turned detective to solve an agricultural mystery.

The case involves parasitic wasps which find and attack insect pests in multi-ton masses of stored grain. The researchers are searching for clues to how the bugs pull off their seek-and-destroy capers.

Entomologists Ralph Howard and James Baker hope their studies will lead to methods for helping the wasps naturally control insect pests in stored grain. They conduct their work at the Biological Research Unit in the Grain Marketing and Production Research Center, Manhattan, Kansas.

Alternative pest control strategies are being created and evaluated in response to a shrinking pest management arsenal. Many insects are developing resistance to chemical insecticides. Also, residual insecticide development for grain and grain products has slowed nearly to a stop.

Howard and Baker believe that parasitic wasps could be the answer for the grain industry. The flying insects are self-targeting, which means they can be released in one area of a storage facility to hunt down pests. This saves the grain manager from having to treat an entire grain mass. Other advantages are that wasp populations are self-perpetuating and female wasps kill many pests by laying their eggs on them.

A natural pesticide

Recent pilot studies by USDA scientists Paul Flinn and David Hagstrum, also of the Manhattan center, show that beneficial insects such as wasps can control selected pest species to stay within economic threshold levels. Flinn and Hagstrum found that parasites reduced dead insect fragment counts more than 90% in finished commodities. The parasites kill pests in the larval stage, which prevents them from developing the hard exoskeleton that often remains intact or in fragments in the grain.

Although parasites are a naturally occurring component of the grain storage system, they often cannot keep up with pest population growth. Grain managers can give the parasites an edge by boosting their numbers in the grain.

Parasites are compatible with other non-chemical controls such as sanitation and aeration. Some are also resistant to currently used insecticides.

Before parasite use becomes practical. more knowledge about the insects' biology and effective use in management programs is needed. Howard and Baker are developing technology for using several species of parasitic wasps to control insect pests that attack stored wheat and corn in the United States. These pest species fall into two main categories:

* Primary pests, which live and feed for at least part of their life cycle inside intact grain. They include the maize weevil, the rice weevil and the lesser grain borer.

* Secondary pests, which live and feed on broken or damaged grain. They include the Indian meal moth, the saw-toothed grain beetle, the rusty grain beetle and the red flour beetle.

Each pest group is vulnerable to a specific complex of parasites that use different approaches to locate and attack them.

The most destructive stored grain pests are the primary pests that feed inside grain kernels. How do parasitic wasps find these hidden creatures? It is likely that a complex series of events leads wasps to pest larvae. For example, clues within a grain mass could direct a parasite toward an infested kernel. These clues would help the wasp pinpoint a small number of infested kernels within millions of uninfested kernels. They would also show where on the kernel surface to probe to sting a hidden host.

Discovering the clues to such behavioral sequences is difficult. To start, the researchers listed some possible ways a wasp might locate its target. They include:

* sounds or vibrations produced by pests chewing inside kernels,

* a sound difference a wasp notices when it taps its antennae on a solid grain kernel versus a hollow one,

* higher levels of respiration gases, such as carbon dioxide, emitted from infested kernels,

* increased temperature of infested kernels,

* chemical marks left on grain by a pest where it entered a kernel, or from feces ejected from a kernel during host development and

* visual cues.

German scientists have shown that at least one parasite species is attracted to the fetes ejected from kernels infested with developing maize weevil larvae.

USDA scientists in Manhattan - and Gainesville, Florida - have shown that sounds produced by insects feeding or chewing on grain are detectable by microphone arrays in a grain mass. These sounds can be related to the number of pests present.

Heat produced by pest larvae releases energy in the infrared portion of the energy spectrum. USDA scientists at Manhattan recently detected internal insect pests in grain using near infrared spectrometers. Perhaps wasps similarly detect heat from their hosts, or hot spots in a grain mass where high concentrations of pests occur.

A parasite's vision is an unlikely factor in dark grain masses except for locating pests in the top layers of a mass. However, visual cues may be used in lighted warehouses or grocery facilities.

Secondary pests

At least two of the parasitic wasps that attack beetles feeding on broken or damaged grains act as bloodhounds, following chemical trails left by wandering host larvae. When the trail is disturbed or weak, the wasps may lose the scent. But when they find it again and catch the doomed beetle, they begin a behavioral sequence. They grasp the larvae behind the head, sting it to paralyze it, then hide it before laying one to three eggs on its surface.

When the eggs hatch, the wasp larvae feed for about three days on the paralyzed beetle until nothing remains but a shrunken carcass. The wasp larvae then drop off and spin a silk cocoon to protect them for two weeks while they transform from pupae into adults.

Identifying needs

Educating growers. grain managers and consumers to the benefits of alternative pest management will be an ongoing challenge.

Part of the education will help grain managers monitor their stores to control pests before they become problems. They will also learn to identify pest problems and select economical and efficient control strategies. If parasites are one of these options, the managers should know which parasites to introduce at what time and in what numbers.

Parasitic wasp use must also cross some hurdles before managers find it beneficial. One obstacle is the current inadequate number of parasites available for purchase from reliable U.S. commercial suppliers.

Fulfilling a need

Cereal grains are an important dietary component for much of the world. Biological control agents, including parasitic wasps, offer a promising strategy for the future. Engineering and educational efforts could help grain managers implement these new technologies.

An example is a computer program recently developed by USDA's Flinn. The software guides grain managers through a step-by-step decision making process. It also answers questions and implements controls quickly and cost-effectively.

Flinn and others hope their research will unravel the parasitic wasp mystery and help them develop more tools to make such alternative pest management a viable option.

Ralph W. Howard and James E. Baker are entomologists at the USDA-ARS Grain Marketing and Production Research Center, 1515 College Ave., Manhattan, KS 66502, USA; 785-776-2706, fax 785-5375584,,
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Author:Howard, Ralph W.; Baker, James E.
Publication:Resource: Engineering & Technology for a Sustainable World
Date:Mar 1, 1999
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