"It's a simple way to visually observe how the tissue is affected," said Kurt Saltzmann, a Purdue research assistant professor of molecular entomology and co-author of the paper. "It's one of those things you can see immediately."

Researchers saw signs that resistant plants were producing more lipase, a protein that degrades lipids, or fats, in the cell surface. It is believed that lipase acts as a defense by providing the small surface holes that deliver toxic proteins to deter larvae from feeding.

In an unexpected twist, however, the researchers found cases in which avirulent fly larvae, which should trigger defense mechanisms from the plants that lead to the larvae deaths, could survive in some cases. A virulent larva that attacks a resistant plant that has already initiated defense mechanisms can reverse those defenses. In that case, both the virulent and avirulent flies would be able to feed on the plant.

"By having this rescue happen, it keeps some avirulent flies in the population," Williams said. "This may be an advantage to the plant to some degree. It probably extends the durability of resistance."

The next step in the research is to determine which genes are responsible for turning on wheat defenses and how those could be activated to respond to virulent flies.

"We need to better understand the mechanisms that occur when a virulent larva infests a host plant in order to build better defenses for those plants," said Subhashree Subramanyam, a research associate in agronomy and paper co-author.

The U.S. Department of Agriculture-Agricultural Research Service funded the research.