Japanese Beetle Injury in Soybean:
Can Control Be Justified?
Graduate Research Assistant
Department of Crop Sciences
University of Illinois
The Japanese beetle, Popillia japonica, was accidently introduced into the United States in 1916 at Riverton, New Jersey. It is believed to have been carried from Japan on the roots of Japanese iris. Currently, this pest is widespread in many states east of the Mississippi River and portions of Iowa, Minnesota, and Nebraska. Japanese beetle was first reported in Illinois in the early 1930s. The United States Department of Agriculture along with the Illinois Department of Agriculture have been instrumental in initiating tactics to keep infestations from reaching economic thresholds.
Japanese beetle adults are a familiar, but unwelcome summertime insect in Illinois. The green and copper-colored adults feed on many different plant species. Males emerge from the soil first, followed by the females a few days later. Females lay eggs beginning in mid- to late June and continue through August; there is one generation per year. Females lay 40–60 eggs during their four to six week adult lifespan. Egg development is highly favored by moist weather conditions; extremely dry conditions can have detrimental effects on egg survival. Japanese beetle larvae are soil-dwelling white grubs with a brown head capsule. Japanese beetle grubs can be identified by examining the hair pattern on the last abdominal segment called the ‘raster.’ A V-shaped hair pattern is diagnostic of a Japanese beetle white grub.
Soybeans were harvested on a record 76.4 million acres in 2009, the largest production ever in the United States. Japanese beetle adults are strongly attracted to soybean plants; their leaf-feeding injures soybeans by defoliating a portion of the plant. Japanese beetle feeding on soybean occurs during the plant’s reproductive stages. Feeding beetles are most active during mid-morning to late afternoon with some feeding in the evenings. Clear weather conditions with temperatures between 84 and 95 degrees Fahrenheit are optimal for their feeding.
Our two-year Japanese beetle sampling study is funded by the Illinois Soybean Association and involves on-farm cooperators across the state. Sampling has focused on identifying Japanese beetle abundance at the edge (i.e., within 10 rows of the edge) versus the interior of soybean fields. Leaf defoliation and grain yield have also been measured and related to beetle abundance.
In 2009, both sweep samples and visual counts showed higher populations of Japanese beetles at soybean field edges compared to the interior. Leaf samples also showed that significantly more defoliation occurred at field edges compared to interior samples. Japanese beetle populations in soybeans were also strongly influenced by the crops bordering a field. Field edges that bordered cornfields contained significantly more Japanese beetles edges adjacent to other crops or roads. We hypothesize that the various crops surrounding a soybean field influence Japanese beetle density and the likelihood that control is needed.
Our findings suggest that integrated pest management tactics will be important in control of Japanese beetle adults in soybeans. However, scouting just the field edges will not tell the whole story. An abundance of Japanese beetles along the edge does not mean there is an abundance over the entire field. At the beetle densities measured in 2009, our results revealed that soybean yields were not significantly different between the soybean field interior and the edge. Recommending targeted edge applications of insecticide for Japanese beetle management depends on more than the abundance difference we measured; the economic impact of beetle feeding in soybean must justify the expense.