In soybeans: It’s important to scout flowering soybean fields for the presence of the Japanese beetle and the extent of defoliation. Estimate the number of beetles per foot of row and examine the plants to determine if any blooms are being destroyed.
Estimate the percent defoliation in at least five different areas of the field. Insecticidal treatments should be considered when defoliation reaches 30% before bloom and 20% before bloom & pod fill.
In corn: Estimate the number of beetles per ear and the examine ears to determine the extent of silk clipping. Be sure to look at a representative portion of the field. Populations of Japanese beetles are usually clumped together as a result of the pheromones they emit that cause them to aggregate.
There are usually clumps of Japanese beetles near field edges, probably due to beetles moving from areas of emergence to host plants within the area. If sampling only occurs near areas where these clumps of Japanese beetles occur, densities across the field may be over estimated.
Even though densities may appear to be extremely high, the average density of beetles may be below levels of economic concern. An insecticidal treatment is should be considered during the silking period if:
- there are 3 or more beetles per ear,
- silks have been clipped to less than ½ inch, AND
- pollination is less than 50% complete
*Note*
When scouting for Japanese beetles, be cautious of false Japanese beetles. False Japanese beetles are usually found in areas with sandy soils. False Japanese beetle rarely cause economic injury to crops, but it is important to correctly identify them.
About the same size as Japanese beetles, false Japanese beetles are similar in appearance to Japanese beetles, but are more dull in color whereas Japanese beetles are shiny and more brightly colored.
False Japanese beetles do not have the characteristic arrangement of the six white tufts along their abdomen as do Japanese beetles.
Control of false Japanese beetles is not recommended in agricultural crops in Illinois.
Management:
Management of the Japanese beetle in corn and soybeans is very challenging. Both the adults and larvae have the potential to cause injury to these crops in different ways. It is difficult to predict where grubs will be present from year to year.
Heavily infested areas with grubs do not guarantee the crop planted in that same area will suffer severe injury due to adult feeding.
Even though adults tend to aggregate, they are very mobile. Therefore, controlling one life stage will not necessarily eliminate problems with the other.
Management of adult beetles focuses on the prevention of defoliation in soybeans and silk clipping in corn from exceeding economic levels. Scouting entire fields is essential in determining the extent of injury of a crop.
Japanese beetle injury to either corn or soybeans will add to other stresses that the crop is experiencing (such as moisture stress). Consider adjusting the economic thresholds accordingly. A reduction in economic thresholds for insect injury when plants are suffering from other stresses may be warranted.
The potential for lower yields usually results in an increase of economic thresholds. There are many insecticides labeled for use on corn and soybeans to control Japanese beetles. When making a decision on regarding the use of an insecticide, take into account several factors: the density of Japanese beetles, amount of injury (silk clipping in corn, defoliation in soybeans), level of crop stress, yield potential, and cost of control.
Japanese beetle traps are most useful for detecting Japanese beetle emergence and monitoring population fluctuations. Since the trapping of adult beetles involves the use of pheromones, traps generally attract beetles from surrounding areas. Their use as a tool for control is not recommended.
Predicting future infestations of Japanese beetles would most likely be frustrating and often futile. However, one can generally anticipate economic densities of Japanese beetles after mild winters, followed by early planting (the first 2 weeks of April). Infestations also are the greatest in east-central Illinois where there is a history of repeated problems with this insect.
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