Issue No. 16, Article 1/July 14, 2006
Japanese Beetles--The Onslaught Continues
Without question, the Japanese beetle has captured the most attention in the insect world in corn and soybean thus far in 2006. The presence of western corn rootworms is a given annually, but the huge numbers of Japanese beetles are diverting some attention even from rootworms, at least for the time being. However, the recent stormy weather may expose some rootworm larval damage by causing damaged plants to lodge. As I've indicated in previous articles in the Bulletin, the full extent of rootworm problems is not yet realized.
Back to Japanese beetles. By now you should have had ample opportunity to hear one or more specialists talk about Japanese beetles on radio programs, to read articles in newspapers and magazines, and maybe even to see some video on the topic. We have tried our best to use mass media to spread the word about the potential impact of Japanese beetles on corn and soybean production, not to mention on trees and ornamentals around the home. In areas where corn has not completed pollination, Japanese beetles still pose a threat. The published economic threshold is three or more beetles per ear before pollination is complete. After pollination is complete, the threat to corn disappears. For flowering soybeans, the economic threshold is 20% defoliation, declining from the 30% defoliation threshold for vegetative-stage soybeans. Insecticides for control of Japanese beetles in corn and soybean were listed in issue no. 13, June 23, 2006, of the Bulletin in "Get Ready for . . . Japanese Beetles." One specific change in that list is that Asana XL, a pyrethroid, now is registered for control of Japanese beetles in corn. The rate of application is 5.8 to 9.6 oz per acre. Asana XL is restricted for use by certified applicators. In addition, several generic insecticides are labeled for control of Japanese beetles in corn and soybean.
Japanese beetles on corn silks (photo courtesy of Ron Hines, University of Illinois).
Japanese beetles and injury to soybean leaves (photo courtesy of Ron Hines, University of Illinois).
More expletives and superlatives about Japanese beetles in Illinois are unnecessary. However, some facts about their history and biology may provide useful information about their presence for the remainder of the growing season. I obtained this information primarily from the review article "Biology and Management of the Japanese Beetle" by Daniel A. Potter and David W. Held, Department of Entomology, University of Kentucky, published in the Annual Review of Entomology, 2002, vol. 47, pages 175-205. Some of the history of the Japanese beetle in Illinois is condensed in the article "Japanese Beetles and Western Corn Rootworms: Old Insect Foes Present New Challenges" by Michael E. Gray, Jared Schroeder, and Kevin L. Steffey, published in the Proceedings of the 2003 Illinois Crop Protection Technology Conference, pages 77-85. You can view the article online (Adobe PDF).
The Japanese beetle was first discovered in North America in 1916 in New Jersey. The insect spread naturally and inadvertently by human commerce throughout the eastern United States. By 1998, the Japanese beetle was established in all states east of the Mississippi River except for Florida, as well as in parts of Wisconsin, Minnesota, Iowa, and Nebraska.
In Illinois, the Japanese beetle first appeared in the 1930s. As numbers of Japanese beetles increased in eastern counties, federal and state governments initiated an attempt to eradicate the pernicious pest. The eradication effort failed, obviously, and earned a place in IPM history by being featured in the book Silent Spring by Rachel Carson, published in 1962. The efforts to eradicate Japanese beetles in Illinois and elsewhere are described in the chapter "Needles Havoc" (pages 85-100). Although many scientists have rightfully challenged the "facts" Rachel Carson provided, the unsuccessful effort to eradicate the Japanese beetle and the unintended consequences are undeniable.
The Japanese beetle completes one generation each year. Adults, the creatures we currently are battling, emerge in June in Illinois. After mating, females lay eggs in the soil from June through August. Larvae (grubs) hatch from the eggs in 10 to 14 days and feed on plant roots and decaying vegetation. Larvae have three instars, developing as follows: first instar, two to three weeks; second instar, three to four weeks; third, and final, instar, the overwintering stage. Most grubs are third instars by mid-September, and they feed well into October. The grubs move downward in the soil in late autumn, usually overwintering between 2 and 6 inches deep. They begin moving upward in the soil in March when the soil temperatures exceed 50°F. The grubs feed for another four to six weeks before pupation.
Virgin Japanese beetle females are mated almost immediately after they emerge from the soil. Gravid (pregnant) females begin to lay eggs soon after mating, probably before feeding. After initial oviposition (about three days), females fly to host plants (more than 300 species) to feed and mate again. Females alternate between periods of feeding and of oviposition, typically entering the soil a dozen or more times and depositing 40 to 60 eggs over four to six weeks.
The large aggregations of Japanese beetles that people witness are the result of a combination of female sex pheromone emitted by unmated females, plant volatiles released from beetle-injured leaves, and possibly an aggregation pheromone. Flight activity is greatest on clear days and is depressed by overcast or windy conditions and by rainfall.
Unfortunately, Japanese beetle adults have few natural enemies, though vehicle windshields take out some small portion of the population. In all seriousness, however, in the absence of natural enemies, insecticides are the only solution for large densities defoliating soybean or clipping cornsilks.
We will continue to search for information that might be useful during our encounters with Japanese beetle adults. If we are willing to accept the downsides, let's hope for a colder winter this year. The grubs are susceptible to freezing.--Kevin Steffey and Mike Gray