Agronomy Update Japanese Beetles
Brian Weller, Lead Agronomist
In the last week we have seen a dramatic increase in the Japanese Beetle populations that have been defoliating soybeans. The economic threshold for Japanese beetle attacking soybeans is 20 % or more, for soybeans from flowering through pod filling stages.
Japanese Beetles are about ½” long and have metallic green to what some call bronze or almost chestnut colored wing covers. These beetles also have 6 tufts of white hair on the sides of their abdomen.
The life cycle of the Japanese beetle is to overwinter as larvae, and in the spring, eat dead or decaying plant material and then emerge as adults in late June into July or early August, the adult stage lasts for approximately 60 days. The females lay eggs in turf grass, the eggs grow into larvae and after going thru 3 instar stages will burrow to the 4-10” depth to overwinter, then emerge again next spring.
Japanese beetle adults prefer to feed in areas with more sunlight but will move down into the canopy as leaves are defoliated. I have been told that affected leaves have an odor that attracts more Beetles which makes the problem that much worse. Female Beetles tend to leave the feeding areas at dusk and seek out turf to lay their eggs.
If you have fields that have Soybean Aphids returning and are also seeing defoliation of 20% or more, with Japanese beetles or other defoliators (Green Cloverworm, Thistle Caterpillar) present, it may be in your best interest to control these pests.
The picture below is from the University of Minnesota Extension Service and shows levels of defoliation on soybean leaves by pest.
Many of the insecticides that we would use on Soybean Aphids are very effective on Japanese Beetle and have residual that would be very effective on Aphids. With that being said, if you were to spray to control Japanese Beetle it would still be advisable to check to monitor Soybean Aphid levels.
If you have questions regarding this or other Agronomic issues please contact myself or your local Central Farm Service Agronomist.