Scout soybeans for beetles

By Tom J. Bechman
It’s a rite of passage every summer. Insects invade your soybean fields. The questions always boil down to ‘how many’ and ‘is it enough to warrant spraying?’
This month’s panel of Indiana Certified Crop Advisors provides advice on one insect that shows up almost every year- Japanese beetles. Their bark may be worse than their bit, or, in this case, their bite might be worse than it looks, except during key reproductive periods for the plants.
While you’re inspecting them, though, you may find other pests as bad as even more damaging. Scouts advising keeping your eyes open for soybean aphids, especially in the northern half of the state.
Question: Japanese beetles are having a field day in a couple of my soybean fields. How do I know if I should spray them or not?
Traci Bultemeier, accounts manager, DuPont Pioneer, Ft; Wayne: The field should be sampled by inspecting five plants in each of five areas throughout the acreage to determine percent defoliation of the plants. During the R1 to R5, or reproductive stages, plants are more susceptible to yield damage through defoliation. Energy and resources are being put into flowers, pods and pod fill. At this point in the season, at 15% defoliation or more, you’ve reached the economic threshold for insecticide application. Unlike with many other insects, spot treatments may provide acceptable control. Through scouting is important to make sure this is a viable option.
Jesse Grogan, agronomist, LG Seeds, Lafayette: Japanese beetles are foliar feeders that congregate in localized areas. They usually emerge in July and have one generation per year. Typically they are heavy along field borders and easily detected. They congregate in cooler, cloudy or rainy weather and feed excessively to cause extensive damage to individual plants. Single beetles do not eat much alone.
They fly and move about when temperatures are in the mid 80’s to 90s. Control with insecticides is determined by overall insect defoliation in each field. Treatment threshold is 30-40% (average across field) prior to bloom and 25% through pod fill to maturity.
That’s quite a lot of defoliation and can easily be overestimated. Use available charts to determine percent defoliation (of the entire plant). These charts are available in the Purdue University Corn & Soybean Field Guide, paper or iPad versions. Field borders can be treated if Japanese beetles are causing severe damage in these localized areas.  
Andy Like, co-owner, Daylight Farm Supply, Evansville: You should reference Purdue’s defoliation threshold table that accounts for growth stage and percent defoliation to determine if an application to control the beetle is necessary. Typically, insect defoliation appears to be worse that it really is, especially when soybeans are in the vegetative stage. When you’re scouting the field look for other pests that are damaging the beans as well. Try to determine if the combined sub-threshold levels of all the pests in the field justify an application to control them.
Key points
Leaf loss from inset feeding more critical during reproductive phases
Check Purdue’s defoliation table which lists expected yield loss per amount of defoliation
Watch for other insects besides Japanese beetles while scouting
Visible feeding- Note holes and tattered leaves. Insects are feeding here during the reproductive stage, but thye overall amount of defoliation is minimal. Japanese beetles are often top feeders