Last week I heard from a friend on the Eastern Shore of Maryland who said that her entire garden was being consumed by hordes of ravenous Japanese beetles. I was a bit surprised at the news. You see, the lore about Japanese beetles suggests that in a year following a summer drought, numbers of Japanese beetles are reduced due to poor survival of eggs and young grubs. Last year we enjoyed the kind of drought that could really set these buggers back. I have seen only a few Japanese beetles in my yard this year. My unfortunate friend went on to say that her entire community had irrigation for their lawns and landscape beds. Well, so much for the drought. Say hello to Japanese beetles. July is a month of misery when Japanese beetles are about. They eat more than 200 kinds of trees, shrubs, vines, and herbaceous plants. Among their favorites are 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 beetles than roses 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. A comprehensive listing of susceptible and resistant varieties is contained in one of the references below.
You may have noticed that Japanese beetles often attack one plant severely, leaving a 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 they 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.
If Japanese beetles 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 your have irrigated turf like my friend. 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 a bit less effective against white 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. Two very potent soil insecticides imidacloprid and halofenozide (active ingredients), applied in late July through August will be very effective in killing tiny grubs as they hatch from eggs and feed near the soil surface. When using any insecticide, always read the label and follow the directions carefully. Many insecticides are available to control Japanese beetles but multiple applications may be necessary if you cannot tolerate damage by these critters. 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. Save the bodies of these little rascals. The earthly remains of so many beetles make a wonderful addition to a compost pile that can later be used to nourish your garden.
Special thanks to Sherald Reagle for providing the inspiration for this Bug of the Week. For more information on the biology and management of Japanese beetle visit the following web sites.