Japanese Beetles Win "Insect-of-the-Week" Award

July 9, 1999

· Japanese beetles present in very high numbers in eastern Illinois counties

· Making decisions about control in soybeans and corn

Our telephone lines were abuzz last week with reports of Japanese beetles occurring in "hordes," "swarms," and "impressive" or "huge" numbers. These are just some of the descriptions we heard. One individual stated that driving along country roads in Iroquois County is like driving through a hailstorm as the Japanese beetles popped against the windshield. (A potential control tactic?)

We have received reports of heavy infestations of Japanese beetles from southern Macon County to Kankakee County and almost everywhere in between and up and down the eastern border of Illinois. The beetles are defoliating soybeans and flying about in cornfields, ready to jump on silks when they emerge. In addition, homeowners in the affected area are watching the beetles tear up ornamentals, flowers, and fruit and vegetable plants. We have tried to forward these calls to either Phil Nixon at (217)333-6652 or Ray Cloyd at (217)244-7218, both entomologists in the Department of Natural Resources and Environmental Sciences. On a positive note, Japanese beetles also have defoliated many weeds.

I know that it is difficult to hold back when so many Japanese beetles are apparent and some spots in soybean fields have been defoliated to the stems. However, once again, I urge people not to overreact. Although we have had a history of overreacting to this showy, noisy insect pest in Illinois, we need to remain rational about its control. I am not suggesting that the spraying to date has not been justified; I am simply urging objectivity.

Examples of levels of insect defoliation of soybean leaflets.

Defoliation of soybeans by Japanese beetles looks frightening in some areas, but the average percentage of defoliation within a given field should be assessed before you make the decision to spray an insecticide. I received one report from an entomologist that although the tops of plants in some areas of soybean fields in Iroquois County have been defoliated to the stems, most plants had only 1 to 5 percent defoliation. When you assess the level of defoliation in a soybean field, don't just pick out the spots where Japanese beetles are numerous and the injury looks severe; your estimate of potential damage likely will be too high. Instead, try to follow this standard procedure:

· Without looking at the plants, stretch out your arm and collect, at random, 20 leaflets each from the top, middle, and bottom thirds of scattered plants in the field, for a total of 60 leaflets. However, if the plants are reasonably large and the canopy has filled in, focus on gathering leaflets from the tops and middles of the plants.

· Compare the leaflets with the leaflets in the photo above, which illustrates insect-produced defoliation at six increments (5, 10, 20, 30, 40, and 50 percent).

· Record your estimates of the percentage defoliation for each of the 60 leaflets and determine the mean (add the estimates and divide by 60). The result is the overall level of defoliation in the field.

Recall from last week's Bulletin (issue no. 15, July 2, 1999) that treatment with an insecticide is not warranted during vegetative growth of soybeans until defoliation by insects reaches or exceeds 40 percent. However, as soybeans begin to flower, the economic threshold drops to 15 to 20 percent. But use your best judgment; the value of soybeans at this time should have some bearing on decisions to treat or not treat soybeans to control Japanese beetles. If an insecticide seems justified, the following products are suggested: Ambush 2E* at 6.4 to 12.8 ounces per acre; Asana XL* at 5.8 to 9.6 ounces per acre; Penncap-M* at 3 to 4 pints per acre; Pounce 3.2EC* at 2 to 4 ounces per acre; Sevin XLR Plus at 1 to 2 pints per acre; and Warrior T* or 1E* at 3.2 to 3.84 ounces per acre. (Use of products followed by an asterisk is restricted to certified applicators.)

As corn begins to tassel and silk and Japanese beetles are present, there is some threat that the insects will interfere with successful pollination. Again, control may be warranted if silks are clipped to less than 1/2 inch, fewer than 50 percent of the plants have been pollinated, and beetles are feeding. The rule-of-thumb threshold is three or more beetles per ear. Suggested insecticides are Penncap-M* at 2 to 4 pints per acre, Sevin XLR Plus at 2 to 4 pints per acre, and Warrior T* or 1E* at 2.56 to 3.84 ounces per acre. (Use of products followed by an asterisk is restricted to certified applicators.)

Bob Nielsen, agronomist at Purdue University, has some interesting information about corn pollination on his web site (http://www.kingcorn.org/chatchew.htm). He has described the fundamentals of the pollination and fertilization processes, some of which might help you make some decisions about silk clipping in corn:

"Pollen grain germination occurs within minutes after a pollen grain lands on a receptive (moist) silk. A pollen tube, containing the male genetic material, develops and grows inside the silk, and fertilizes the ovary within 24 hours. Pollen grain can land and germinate anywhere along the length of an exposed silk. . . . Generally, silk length on injured [clipped by insects] ear shoots must be at least 1/2 inch to ensure that a sufficient length of uninjured silk tissue is exposed for pollen germination to occur. . . . Silk receptivity to pollen grain germination exists up to 10 days after silk emergence. After 10 days, silk receptivity decreases rapidly. Silk elongation continues until pollination is successful, although elongation eventually ceases as unfertilized silks senesce."

Why are there so many Japanese beetles this year?

We entomologists often have difficulty answering questions like this with a great deal of confidence. However, whatever has supported the survival of Japanese beetles also has supported the survival of several beetle pests this year, including grape colaspis and corn rootworms. During the spring, we received several reports of people finding Japanese beetle grubs feeding on corn roots, and now we are reaping the benefits of their survival. We have also had lots of reports of grape colaspis larvae injuring corn and grape colaspis adults feeding on corn foliage and silks. Early indications suggest that corn rootworm damage may be rather extensive this summer, and numbers of adults may be high. Good weather during egg laying late last summer? Mild winter? Suitable conditions this spring? Yes, probably all of these factors contributed to the success of some of these pests this year. We will keep plugging away to determine whether we can make better predictions in the future.--Kevin Steffey

Author: Kevin Steffey