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Issue No. 15, Article 3/July 1, 2005

Japanese Beetle Invasion

"Two thousand, two thousand one, two thousand two, . . ." That's pretty much the sound coming from Ron Hines's office these days. I'm sure Ron, like many other people around the state, feels like he's up to his elbows in Japanese beetles. We've heard quite a few horror stories of "swarming" Japanese beetles this past week. Truth be told, Ron is up to his elbows in Japanese beetles. The daily and weekly trap catch totals from his Japanese beetle traps are staggering--3,365 beetles in 6-1/2 hours in Pope County, with a weekly catch total in that same county of 17,826. And don't forget Massac and St. Clair counties, which racked up 6,383 and 8,770 beetles, respectively, this past week.

Severe infestations of Japanese beetles are occurring in many areas of Illinois. They are easily spotted feeding on ornamental and garden plants as well as on corn and soybeans. The beetles defoliate the host plants, chewing tissue between the veins, leaving a lacelike skeleton. In corn, Japanese beetles can interfere with pollination by clipping silks. However, in soybeans, the greatest concern is defoliation, which occurs mainly during the reproductive stages of plant development. The pest may also feed on flowers and interfere with soybean pollination.

Figure 1. Defoliation key.

Distribution of Japanese beetles in fields may be highly clumped. Because of the pheromone they emit, it is not unusual to find high densities of the beetles aggregated in certain areas of the field. This clumping may also be because of the attractiveness of the beetles to areas of the field that have flowers and tender new growth, such as the tops of soybean plants and silks of corn plants. This uneven distribution makes it extremely difficult to assess injury across a field.

In soybeans, an insecticide treatment should be considered when defoliation reaches 30% before bloom and 20% between bloom and pod fill. When estimating defoliation in soybean fields, remember to sample random leaves in at least five different areas of the field. Insecticides recommended for control of Japanese beetles in soybeans can be found in Table 4.

Japanese beetle defoliation on soybeans (Photo courtesy of Ron Hines).

In corn, estimate the number of beetles per ear, and examine the ears to determine the extent of silk clipping. Just as with soybeans, look at a representative portion of the field. If sampling occurs only near areas where Japanese beetles are clumped, densities across the field may be overestimated. Even though densities may appear to be high, the average beetle density may be below levels of economic concern. An insecticidal treatment should be considered during the silking period if

  • there are three or more beetles per ear, and
  • silks have been clipped to less than 1/2 inch, and
  • pollination is less than 50% complete.

Table 5 lists insecticides labeled for the control of Japanese beetles in corn.

Deciding if and when to spray for Japanese beetles can be extremely difficult. Hindering management decisions this year and causing even more anxiety are the drought conditions. The lack of moisture and subsequent stress placed on the plants has caused uneven crop development in many locations. Areas within fields may have begun tasseling early in the week, while other areas are still days away from beginning pollination. Problems arise when those plants that are pollinating are experiencing beetle feeding and silk clipping.

Japanese beetle on corn silks.

What do you do? First things first--don't throw out the thresholds! Even though this is an atypical year and thresholds are typically developed under several years over "normal" growing conditions, they still offer validity concerning when to treat. Our best advice is to take the thresholds into consideration and use your own judgment and experience. Decide if treatment is needed immediately based on the injury in the field, or if it could be delayed a couple of days. Because Japanese beetles emerge over time, an insecticide applied early may not last long enough to control later-emerging beetles. Also, when temperatures are high, pyrethroid insecticides (e.g., Ambush, Asana, Capture, Pounce, Warrior) may lose some efficacy. And when insecticides are applied during silking, the lengthening of the silks exposes silks on which insecticides are not present.

There are lot of things to mull over before deciding to treat for Japanese beetles. Patience will be a key this summer, as all of these factors may challenge control of Japanese beetles.--Kelly Cook

Kelly Estes

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