Japanese beetles feast on more than 300 species of plants, leaving lacy, bare leaves in summer gardens and agriculture crops. But when it comes to eating the leaves of cherry trees, these horticultural pests are very particular about the varieties they eat and those they avoid. Horticultural researchers at North Carolina State University have learned that Japanese beetles will avoid eating leaves of some Prunus trees cherries and related species that contain high levels of certain chemicals. This information could lead to the development of plants with stronger resistance to Japanese beetles.
In the research project, hungry Japanese beetles were placed in a petri dish with a fresh leaf from one of 27 varieties of Prunus. After 24 hours of feeding, the researchers measured how much of the plant the Japanese beetle had eaten.
The researchers found that Japanese beetles ate larger amounts of Prunus leaves with a low cyanide potential, and less of Prunus leaves with high cyanide potential, according to Patton, who worked on the project with a graduate student in horticultural science. Some varieties were virtually immune to the pest.
Cyanide potential is the measure of cyanide-containing compounds found in plant leaves. Using biochemical analyses, the interdisciplinary research team determined which plant varieties produced high levels of these compounds. The compound prunasin, found in high concentrations in some Prunus species, converts to cyanide when combined with certain enzymes, Patton said. When Japanese beetles eat plants with high prunasin concentration, these enzymes cause prunasin to convert to cyanide.
The researchers chose to investigate Prunus trees because of casual observations that Japanese beetles preferred some varieties over others, said Dr. Tom Ranney, N.C. State associate professor of horticultural science based at the Mountain Horticulture Crops Research and Extension Center in Fletcher. Feeding on plants with high concentrations of prunasin did not kill the Japanese beetles, but discouraged them from eating those plants. Because Japanese beetles feed on so many plants, they would simply move on to another plant.
Japanese beetles are significant pests, both to farmers and to backyard gardeners, feeding on a wide variety of plants. The insects first came to this country in 1916 by way of New Jersey. Since then, the beetles have become widespread in states east of the Rocky Mountains, and most recently have been found west of the Rockies. "They're expanding their range and may become a worldwide pest," Ranney said.
The idea of naturally occurring resistance in plants is not new; "Plants and insects have been involved in chemical warfare for the past 350 million years," Ranney said. If scientists can learn more about natural properties in plants that make them resistant to certain pests, they can use this knowledge to develop new plant varieties with increased resistance.
The high concentrations of cyanide-producing chemicals in Prunus species will not discourage all pests from feeding on the trees, he said. One exception is the Eastern tent caterpillar, which is virtually immune to the effects of high cyanide concentrations and has a fondness for black cherry trees, which contain high levels of prunasin. The accumulation of cyanide concentrations in the caterpillar's body discourages predation by ants.
"Japanese beetles will eat just about anything. If you can discourage them from eating certain plants, you can probably discourage [them from eating] others," Patton said.
With the information from this study, Ranney said researchers have some information they need to develop cultivars that are resistant to Japanese beetles. Through traditional breeding methods or molecular biology techniques, Prunus species could be developed with greater resistance to Japanese beetles.
Patton said that more needs to be learned about the genetics of transmitting traits for high prunasin concentrations. Based on this study, the researchers were able to identify trees within the Prunus genus that are able to fend off attacks by Japanese beetles. Other members of the N.C. State team that conducted the research are Dr. Jim Burton, associate professor of horticultural science, and Dr. Jim Walgenbach, professor of entomology.
Resistance in host plants is an important part of Integrated Pest Management, Ranney said. By reducing the use of synthetic pest controls, host-based resistance can be one of the most environmentally safe forms of pest management.