Upon my return from a 2-week vacation, I learned that Japanese beetles were introducing themselves to cornfields again this year. According to several reports, the numbers in some fields have been "scary." Ron Hines, senior research specialist at the University of Illinois Dixon Springs Agricultural Center, has been capturing Japanese beetle adults in one of his traps in Pope County since the third week of June. He reports that this trap now is capturing more than 100 beetles per day. He has reported captures of Japanese beetles at three other trap locations (Massac, Pope, and Pulaski counties) since July 1.
Gone are the days when Japanese beetles in Illinois were a concern among growers almost exclusively in eastern counties in Illinois and states to our east. Within the past 5 to 10 years, Japanese beetles have become established throughout most of Illinois. The National Agricultural Pest Information System located at Purdue University has created a map of the "Reported Status of Japanese Beetle (JB), Popillia japonica in US and Puerto Rico (2002)." You can view the map here. The map clearly shows that Japanese beetles have become established in almost all of Illinois, with some establishment indicated farther west (e.g., Iowa, Minnesota, Oklahoma).
In areas where this pest is a relative newcomer, people tend to overreact to its presence. After all, these insects are rather large, quite colorful, and somewhat noisy, and large clumps of them attract attention. However, as is frequently the case with newly recognized insect pests of field crops, their numbers usually look worse than they really are. People who have experienced significant silk clipping by Japanese beetles in the recent past probably will cringe when I say this, but I urge everyone not to overreact. Take a deep breath before and after you scout for Japanese beetles in corn, and make a decision that seems economically justifiable. There's no reason to spray an entire field of corn when Japanese beetles are clipping silks only in small areas of a field.
Sampling for Japanese beetles to make a decision about whether an insecticide application is necessary is a challenge. Entomologists throughout the Midwest recommend sampling for most insect pests by scouting entire fields, stopping to take samples that represent the whole field. However, through the action of pheromones emitted by Japanese beetles, the distribution of these insects within fields usually is highly clumped. Many have noted that clumps of Japanese beetles are particularly noticeable in field edges. This probably occurs as the very active beetles move from their areas of emergence from the soil to the most attractive flowering plants (and pheromone-emitting females) in the vicinity. Consequently, densities of Japanese beetles within a field will be overestimated if you sample only where clumps of these pests occur. Although the infestation of Japanese beetles may look overwhelming in one or a few areas within a field, it's entirely possible that the average density of the beetles for the field is below a level of economic concern.
Over the years, the standard economic threshold to prevent Japanese beetles from interfering with corn pollination has been three beetles per ear. When densities of Japanese beetles reach or exceed three per ear before pollination is complete and significant silk clipping is obvious, insecticide application may be warranted. However, this threshold is debatable, its origin lost in antiquity. So use your best judgment.
If an insecticide to control Japanese beetles is warranted, select a product from among those suggested in Table 1. Science and experience have taught us that pyrethroid insecticides, in general, lose some efficacy when temperatures are high. Also keep in mind that while Japanese beetles continue to emerge and move around, initial efficacy of almost any product will seem questionable because of beetles immigrating into sprayed fields. Keep this information in mind when you assess the efficacy of any product applied to control Japanese beetles in corn.--Kevin Steffey