As the name suggests, Japanese beetles (Popillia japonica Newman) are native to Japan, and were first discovered in the US during the early 1900’s. They can be found in all states east of the Mississippi, but have also moved west with occasional infestations affecting crops in Missouri, Iowa, Nebraska, and California.
The beetles are known to affect over 300 species of plants. Damage can occur from both larvae (grubs) feeding on the the roots and adults feeding on leaves, silks, flowers, and fruits of many different crops later in the season. In field crops, economic damage from Japanese beetle damage is relatively infrequent. When heavy infestations are found in field crops, their impact will primarily be from the adults clipping silks in corn or defoliation in soybeans.
Adult females target the soil in sods and wet soils with significant residues for depositing their eggs. The emerging grubs feed on roots in close proximity to where they hatched, whether perennial grasses, germinating and emerging crops; or tender transplants. Wetter areas of fields, down-drained fields, and grass sods in heavier soils will be preferred egg laying sites for the adults.
The life cycle of the Japanese beetle is completed in a single year. They have 3 instars of larval development. Adults lay their eggs during their feeding cycle. These eggs hatch in the fall and begin their first two instars of development while feeding on roots and moving deeper into the soil. The beetles overwinter as late stage grubs, after burrowing to a soil depth of 3-6”. As temperatures rise in the spring, the larvae will travel upward in the soil to feed on roots and continue development before pupating out. Adults will begin emerging in June, with activity increasing through July, August, and into early September. During this time, they will feed and breed, seeking out preferred sites for egg laying in the process. Once in flight, adults can travel great distances to seek out preferred plants and sites for feeding, breeding, and egg laying.
Identifying the adult Japanese beetle is fairly easy, thanks to their shiny metallic green and bronze-colored wings. They are fairly large and dense in size, reaching about ½” (13-25mm) in length. The larvae, however, can be confused with the grub of June beetles or other grubs, as they too are relatively large (1/2”-1” in length).
Japanese Beetle Management and Control:
Managing against Japanese beetle damage is somewhat challenging as their activity can vary from year to year. They can travel in numbers across many miles, and the time of year they feed is aligned to crop development. This means that practical applications of insecticides or repellants can be futile because, in heavy years, re-infestation can occur fairly soon after application. In higher-value small fruits and ornamentals, scouting and control can be justified more commonly than in field crops.
Scouting for Japanese beetles should occur in higher risk areas and preferred crops during the early stages of adult emergence. In cultivated fields, you’ll typically find the heaviest populations on the outer edges, especially if the field is bordered by preferred egg laying sites. In corn, thresholds are based on how young the silks are when heavy feeding is observed. If the corn has advanced past 50% pollinated, silk clipping may not be that significant; but when well over threshold, treatment may be justified if populations are extreme. In soybeans, thresholds are predicted by % of leaf area damaged, not in terms of beetles per plant. Consult your Certified Crop Advisor or regional entomologist to determine your region’s recommended threshold for your specific crops.
Proactive controls through weed control in preferred egg laying sites around fields can be somewhat effective. Japanese beetles do have some preferential weeds and grasses in which to oviposit, aside from sods. Weeds like wild raspberry, blackberry, Virginia creeper, wild grape or sassafras seem to be magnets for Japanese Beetle adults, as they will feed on these weeds, then lay their eggs in close proximity to them.
Pheromone traps are sold widely for Japanese beetles, both for monitoring and control. In terms of monitoring, the large adults can be easily seen, so traps aren’t really needed. In terms of control, the use of pheromone traps can actually draw more beetles to the area and, in the process, cause more feeding and egg laying in close proximity to the traps.
Soil applied insecticides for controlling grubs of the beetle may have a short term benefit for controlling the first flush of adults. But over the course of the season, due to the mobility of the adults across wide areas, infestation will still occur in years of heavy populations. Further, the sod or area where heavy grubs exist create a preferred egg laying location, which will attract more adults during feeding time. Finally, the size of the grub limits your choices of soil insecticides that actually have adequate levels of efficacy.
The best approach for Japanese beetle control is to watch at-risk fields closely at predicted adult emergence, especially in years with wet springs and on sites where soils are poorly drained. Familiarize yourself with thresholds for your area; then treat if economic loss is predicted.