The Japanese beetle population typically fluctuates somewhat from year to year. Members attending the July Cumberland County Master Gardener meeting were asked whether the number of Japanese beetles sighted around Crossville in 2010 was more or less than seen last year. Those attending thought there were fewer this year.
Japanese beetles are about 3/8 inch long by 1/4 inch wide and oval shaped with attractive, metallic greenish-bronze colored wing covers. Because adult beetles chew both leaves and flowers on hundreds of different plants and the young grubs attack turfgrass and crop roots, Japanese beetles can do major damage to garden and landscape plants. Have you observed many in your yard this season?
Like most insects, these pests take on different body shapes during their lifecycle starting with eggs laid underground by fertile female beetles. In Tennessee, that happens while adult beetles are active during June, July and August. Tiny white grubs hatch from the eggs in about fourteen days (June through late August or early September depending on conditions). The smallest grubs then molt to larger second-stage grubs, all the while eating roots of turfgrass in lawns or roots of garden flowers and vegetables like beans, beets, sweet corn, onions, and tomatoes. Dry weather at this early phase lessens the grub survival rate. At this point while grubs are small and feeding, is when chemical grub treatments are most successful. Apply grub-killers containing the active ingredients trichlorfon (Dylox), carbaryl (Sevin©) or Imidacloprid (Merit) to lawns in June, July, August or early September. Milky Spore is a natural grub control that is relatively expensive and has limited effectiveness.
Third-stage grubs develop in late September and during October. They go deep into the soil to over-winter. The following spring, third-stage grubs move back up after soil warms in April. These larger grubs do not eat, so controls that must be ingested aren’t effective. Each grub changes to a pupa where the adult beetle forms. Newly morphed Japanese beetles start emerging in June. Adult Japanese beetles live to feed and mate. They fly about following the smell of flowers to local gardens then release a scent into the air to attract more Japanese beetles to the food source. They also emit a second pheromone—to attract the opposite sex for mating purposes. Once fertilized, female beetles drop to the ground, burrow down and lay eggs starting the cycle anew. Females continue to feed, mate and produce eggs while alive.
Fewer adult beetles in your yard means not as many signals are sent out to bring others there. A simple way to get rid of beetles is to take a small bucket containing about 2 inches of soapy water out to your yard in early morning or late evening when it’s cooler and insects move more slowly. Hand pick Japanese beetles from plants. Drop them into the bucket. (Don’t use plain water. Soap breaks water’s surface tension so beetles in the solution drown.) An effective chemical spray is Sevin©. Apply sprays late in the day when honeybee activity is reduced. Don’t use insecticides near water gardens or ponds containing aquatic life such as fish, snails, frogs, etc. as any overspray can kill these creatures. Bags that trap Japanese Beetles have been found to attract more beetles than would otherwise visit your property. A trap placed in a vacant lot at least 20 yards away could be used to lure Japanese Beetles away from your gardens and shrubs but otherwise is not a recommended control.
Plateau Gardening is written by Master Gardeners for those tending home landscapes and gardens in Tennessee’s Upper Cumberland Region. UT Extension Cumberland County at P.O. Box 483, Crossville, TN 38557 (484-6743) has answers for horticulture questions, free publications and details on how to become a Master Gardener. Send email comments or yard and garden inquiries to Master Gardener Rae, firstname.lastname@example.org.