Tiny Tyrant

It is well known that certain wasps suppress the immune systems of their caterpillar hosts so they can successfully raise their young within those hosts. Now researchers at Penn State show that, in addition to suppressing caterpillar immune systems, wasps also suppress the defense mechanisms of the plants on which the caterpillars feed, which ensures that the caterpillars will continue to provide a suitable environment for the wasps' offspring. Futhermore, the plants on which these caterpillars feed ultimately yield larger and higher numbers of fruits.

According to Gary Felton, professor and head of entomology, a type of virus, called a polydnavirus, resides within the ovaries of the female wasps and, when injected into caterpillar hosts, is responsible.

"The polydnavirus suppresses glucose oxidase in the saliva of caterpillars, which normally elicits plant defenses," said Felton. "Suppressing plant defenses in this way benefits the wasp and the virus by improving the wasp's development and survival within the caterpillar."

The plant, too, indirectly benefits from the polydnavirus. "Plants that are attacked by caterpillars with the parasitoid wasp and virus have increased fitness," said Felton. "This translates to possible increases in fruit number and fruit size. In the case of a crop such as tomato, this is a potential increase of yield when biological control agents such as these wasps are present.

The team--which included Ching-Wen Tan, doctoral student in entomology--placed parasitized and nonparasitized caterpillars onto tomato plants. After allowing the caterpillars to feed on the plants for 10 hours, the researchers harvested the remaining leaves and examined them for enzyme and gene expression activity associated with a defense response.

"Using molecular and biochemical techniques, we found that parasitized caterpillars induced significantly lower enzyme activity and defense-gene expression among the tomato plants than the nonparasitized caterpillars," said Tan.

The team also determined that the caterpillar's saliva, which was reduced in glucose oxidase by the polydnavirus, was responsible for inducing these lower defense responses in the plants.

"Our study demonstrates the important role that microorganisms play in plant-insect interactions," said Tan. "The ability of polydnaviruses, which possess less than a couple of hundred genes, to so dramatically affect wasps, caterpillars, and plants is remarkable."

--Sara LaJeunesse