Derailing a Natural Defense

It could be happening in your garden. A Manduca sexta caterpillar, better known as a tobacco horn worm, munches away on the leaves of your prized tomato plant. Suddenly, a tiny wasp, Cotesia congregata, lands on the back of the unsuspecting caterpillar, puncturing its bluish green hide with its stinger. The wasp injects eggs into the caterpillar through the tube-like stinger, or ovipositor, and a few days later, wasp larvae emerge from the eggs and begin to grow. When the larvae near maturity, they chew their way through the dying caterpillar and spin cocoons on its back.

Not a pretty thought, but this kind of parasitism happens all the time in the insect world. In fact, several kinds of parasitic wasps are used by horticulturists in integrated pest management (IPM) programs as a way to control populations of plant-eating insects. "Parasitism is a pretty bad deal for the host insect, but it makes for some very interesting research," says graduate student Naomi Lovallo, who is studying the Manduca caterpillar and Cotesia wasp in the laboratory of entomologist Diana Cox-Foster. For more than a decade, Cox-Foster has studied an enzyme released by insects when they become parasitized or infected. This enzyme, which is called FAD-glucose dehydrogenase (GLD), is released by hemocytes, or blood cells. "These blood cells attempt to surround and destroy invaders in the insect," Lovallo explains. "What normally happens is that an insect's system recognizes the presence of a parasite, or some other invader, and the hemocytes rush in and glom around the intruder, isolating it and killing it."

Larvae of the tiny wasp Cotesia parasitize a Manduca caterpillar, or tobacco horn worm. Researchers are looking at what causes the caterpillar's immune system to fail to respond to its invader. This research could be important in the development of biocontrols for plant-eating pests.

But when a Cotesia wasp pierces a Manduca with its ovipositor, something very different happens. Along with its eggs, the wasp deposits a little something extra–a virus. "Cotesia has a polyDNA virus incorporated right into its genetic structure," Lovallo explains. "When the wasp injects its eggs, it also injects this active virus which blocks the host's normal immune response. Scientists know that the virus is responsible for blocking this response, but they don't know how it works. That's where my interest lies."

Using Cotesias, Lovallo parasitizes Manduca caterpillars and studies the derailment of the caterpillars' immune response. "Early on, we hypothesized that the virus is doing one of two things–either it is causing a caterpillar's immune system to fail to recognize that it is being invaded, or it is blocking the ability of hemocytes to target the invader," Lovallo says. "We found that the caterpillar's GLD levels do increase in response to the presence of the parasitic eggs, so the caterpillar's system must realize something is wrong. The virus has to be confusing the immune system in some other way."

Lovallo has begun looking at how the virus may be affecting the hemocytes' targeting ability. To do this, she mixes yeast and agar to make a target, then covers the target with saline solution and adds a few drops of blood from a parasitized caterpillar. For comparison, she also does the same experiment using the blood of an unparisitized caterpillar. This allows her to study the response of the blood cells using a high-powered microscope equipped with a video camera to record the behavior. "From what we're learning by studying the videos, it seems more and more likely that the hemocytes aren't able to locate their target," she says. "Instead, they move around and appear to attach to one another–a behavior called aggregation. At the same time, we're detecting an increase in GLD, plus some cellular debris. That tells us that the GLD may be destroying some of the hemocytes. In other words, these cells, which normally surround and kill an invader, are aggregating and may be killing each other."

One thing that intrigues Lovallo is why the cells aggregate in this way at all. "If they can't identify an invader, it makes sense that they would not respond to it," she says. "It isn't clear why they are responding to each other in this way. That tells us that the mechanism of this virus is anything but passive. Rather than simply preventing an immune response, it appears that the virus is causing the immune response to turn inward, on the blood cells."

Lovallo, who is finishing her master's work and will be continuing the project for her Ph.D. studies, hopes to eventually learn just what the virus is up to. "There are two families of parasitic wasps that are known to use viruses to evade the host-immune response," she says. "So little is known about this mechanism and about virus evolution in general that a person could easily spend a lifetime identifying pieces of this puzzle."

Since these viral mechanisms are extremely effective, Lovallo's research could lead to further use of parasitic wasps for biocontrol in IPM, as well as answer some basic ecological and evolutionary questions about parasitism and viruses. "I got my bachelor's degree in wildlife ecology from the University of Wisconsin, and I've always been very interested in this kind of symbiosis," she says. "What I'd eventually like to do is work as an insect physiologist on a research team that is interested in an ecological or agricultural problem. Who knows–maybe someday geneticists will be able to develop a transgenic wasp containing a virus that's targeted toward a specific insect pest. Meanwhile, the research can help answer questions for both basic and applied science. There's a lot of common ground, and that makes this work very satisfying."

–Rose Pruyne