The first detection of Japanese beetle in the United States was in 1916 in a plant nursery in New Jersey. After a hiatus of many years, Japanese beetles seem to be making a comeback in our region. Historically, July is a month of misery when Japanese beetles abound, and with a relatively cool spring these mischief makers have arrived right on time. Japanese beetles are extreme foodies with more than 300 kinds of trees, shrubs, vines, and herbaceous plants on the menu. Among their favorites are sassafras, lindens, maples, apples, cherries, grapes, and, sadly, roses. In a series of careful studies, Dan Potter and his colleagues in Kentucky found that roses with large, light colored blossoms, particularly yellow or white, were more attractive to Japanese beetles than varieties with smaller, darker blossoms of red or orange. In the tree realm, researchers noted that lindens with densely hairy leaves were less preferred than scantily haired varieties. Maples with purple or deep red leaves were preferred over those with green leaves.
You may have noticed that Japanese beetles often attack one plant severely, leaving a lucky neighbor relatively unscathed. Apparently, when beetles initiate an attack, specific odors are released by the damaged plant. These send a signal to other beetles something like "good food, eat here.” This foliar attractant is compounded when female beetles release a chemical message called a sex pheromone. The sex pheromone says to the guy beetles “how'd you like to spend a little time with me?” A rambunctious love fest and feeding frenzy erupt, and, in the process, your plant takes a beating.
Clever chemists have been able to synthesize both the floral attractant and the sex pheromone and place them in a lure. Attach the lure to a few plastic fins for beetles to bump into, and a funnel to direct them into a plastic bag and, voilà; you have a Japanese beetle trap. Japanese beetle traps capture beetles by the thousands, but traps may not be all that effective in protecting your plants. Plants near the traps may actually sustain more damage as beetles lured to the vicinity mill around awaiting their turn to hit the fins and be captured. It is best to place these traps far away from valued plants you want to protect.
Japanese beetles lay their eggs in soil, so if they are a chronic problem in your garden or landscape, the best way to get relief may be to reduce the numbers breeding in your lawn, especially if you have irrigated turf. One promising “green” approach is to apply insect pathogenic nematodes, tiny roundworms that attack and kill beetle grubs. Nematodes enter the grub and release a lethal bacterium. There are many different species and strains of nematodes. Dave Shetlar of the Ohio State University suggests that products containing strains of Steinernema carpocapsae are a bit less effective against beetle grubs than species in the clan named Heterorhabditis. You must wait until late July or August when grubs are in the soil, if you go the nematode route.
There are several potent soil insecticides that can be applied in late July through August that are very effective in killing tiny grubs as they hatch from eggs and feed near the soil surface. If you opt for the chemical route, choose wisely. We now know that at least one neonicotinoid insecticide applied to turf grass can be taken up by clover growing in turf. Bumble bees foraging in this clover may be harmed. When using any insecticide, always read the label and follow the directions carefully and pay particular attention to warnings pertaining to beneficial insects like bees.
Many insecticides are available to control adult Japanese beetles on plants, but multiple applications may be necessary if you cannot tolerate damage by these critters. As with turf applications, be cognizant of beneficial insects foraging on plants. Read and follow label precautions. Another nifty way to help reduce damage is to simply knock the beetles from your plants into a bucket of soapy water. If you do this early in the season of beetle misery when beetles are first observed, you may reduce the chemical cues that incite a feeding frenzy. Beetle removal may be most successful in early morning or late evening when beetles are less active. There is a strange kind of justice in drowning this pest in soapy water or capturing them in traps. Save the bodies of the little rascals captured in your bucket or trap. The earthly remains of so many beetles make a wonderful addition to a compost pile that can later be used to nourish your garden.
A little swim in a bucket of soapy water puts a quick stop to Japanese beetles cavorting on my sassafras.
Excellent references including “Assessing Insecticide Hazard to Bumble Bees Foraging on Flowering Weeds in Treated Lawns” by Jonathan L. Larson, Carl T. Redmond, and Daniel A. Potter,
and publications in the links below were used to prepare this episode.
For more information on the biology and management of Japanese beetle, including resistant plants, please visit the following web sites: