February 27, 2009

Each year, Kentucky grape growers struggle to control two important insect pests, the green June beetle and the Japanese beetle. In a recent study, Derrick Hammons, a University of Kentucky doctoral student, showed that even minor feeding by Japanese beetles greatly increases the likelihood that green June beetles will wreak havoc on clusters of ripening grapes.

A paper summarizing Hammons' 3-year study recently was published by the Proceedings of the National Academy of Sciences. Collaborators include Melissa Newman, UK animal and food sciences professor, Daniel Potter, UK entomology professor and Hammons' advisor, and Kaan Kurtural, Bronco Wine Co. Viticulture Chair at California State University, Fresno.

"Derrick's paper is the first documentation of an invasive insect directly aggravating injury by other insects to a high value crop," Potter said. "His recognition by PNAS continues the tradition of UK entomology graduate students who are recognized as excellent scientists and contributors to our profession. The sky's the limit for graduate students here."

The green June beetle is a native pest to Kentucky, but the Japanese beetle is an invasive pest. The populations of both beetles peak around the same time during the summer, but the Japanese beetle becomes active a little earlier and stays later than the green June beetle.

"A nice thing about studying Japanese beetles is that they are here every year, but the adults are only active for about six weeks, so you have to be prepared to work quickly," Hammons said.

The project was supported by a graduate student grant Hammons, a London, Ky. native, received from the U.S. Department of Agriculture's Sustainable Agricultural Research and Education program and another grant he and his collaborators received from the UK New Crop Opportunities Center.

The idea for Hammons' study began in 2005 when he noticed groups of Japanese beetles and green June beetles were feeding together on clusters of ripening grapes at the UK Horticultural Research Farm in Lexington. During the next two years, he studied the triggers that attract the insects to the fruit by using traps baited with grapes alone or grapes being fed upon by one or both beetle species. Green June beetles were only weakly attracted to intact fruits. However, grapes that were bitten and fed upon by Japanese beetles attracted dozens of green June beetles. Clusters being fed upon by both species were the most attractive.

Sugary juice and pulp of grapes and other ripe fruits, such as blackberries, apples and peaches, are a favorite food source for green June beetles. But because their jaws are blunt and spatula-shaped, green June beetles have trouble biting into fruits that are not already wounded in some way. In contrast, Japanese beetles have sharp, opposable teeth and can readily bite into fruit. If a Japanese beetle comes along first and breaks the fruit's skin, then green June beetles will come and feed on the fruit until there's no more left. When abundant and not controlled, green June beetles can destroy 95 percent or more of a vineyard crop.

Not only do Japanese beetles make it easier for green June beetles to get to the pulp, but Japanese beetle-damaged fruits actually attract green June beetles from a distance. As it feeds, the Japanese beetle contaminates the fruit cluster with yeasts carried on its body or in its saliva and feces. The yeast interacts with the fruit and ferments. The green June beetle can sense this and is strongly attracted to the odor of the fermenting fruit.

Due to the damage the Japanese beetle can do to grapes, efforts are being taken to keep it from spreading to large grape-producing areas such as California and France.

"The Japanese beetle is an A-1 quarantine insect for Europe, and it has already been eradicated several times in California, a major grape-producing state," Potter said.

While the green June beetle is mainly found in the Southeast, there is a beetle native to California that has similar habits. Hammons and his colleagues believe that if the Japanese beetle were to reach California and other grape-producing areas out west, it could facilitate this beetle's feeding too.

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