Virus allows wasps to kill crop pests.

Parasitic wasps are gaining renown among gardeners and farmers as premier insecticidal agents because of their ability to selectively attack only targeted plant pests, not beneficial insects. Wasps use the pests as both hosts and a food source for their young. But the key to the parasites' success, new research at Texas A&M University in College Station shows, is a virus left behind with each of the eggs they inject into a host.

"Most of the parasitic wasps that lay their eggs inside a host have viruses," says entomologist Brad Vinson. Each of the egg-harbored viruses that he's found appears to be genetically complex and specific to a particular wasp. Since there are several thousand species of such wasps, he says, "we may be talking about many thousands of kinds of viruses."

The best characterized of these newly discovered polydnaviruses is one associated with Campoletis sonorensis, a less than half-inch long wasp that attacks two larvae -- the tobacco budworm and another that's variously known as the cotton bollworm, tomato fruitworm or corn earworm. Once in a larva, this virus appears to move into the insect's "fat body" -- a structure with a function somewhat analogous to that of the human liver.

"We know the virus affects the immune system," Vinson says, apparently by altering hemocytes, a blood cell similar to the human white blood cell. Moreover, the virus appears to cause endocrine system changes that can curb a larva's appetite, keep it from molting or prevent its pupation (passage into that dormant stage when it would metamorphose into an adult, capable of reproduction). Right now, Vinson is working with virologist Max Summers to pin down biochemically how the virus achieves these functions.

Since they have to be injected, it appears polydnaviruses can't infect larvae except with the wasp's help, Vinson notes. But, he says, "If we understood its genes enough to know how [the virus] affects immunity or prevents pupation, these genes might be cloned and inserted into other viruses" that are virulent in the field -- making them more effective natural insect-control agents.