Japanese Beetles on the rise or fall, but still there

Published: June 29, 2012

Japanese beetles were accidentally introduced to the United States in 1916 by way of New Jersey. Since that time, they have become one of the most devastating landscape pests in the eastern United States. As little as 10 to 15 years ago, Missouri was somewhat free of this critter with only a few scattered pockets of beetles being found in a few counties. Presently, they appear in most Missouri counties and annul counts (by county) can be monitored through the Integrated Pest Management website at: https://ipm.missouri.edu/pestmonitoring/jb/viewall.cfm

Large areas of turfgrass and pastures provide desirable habitat for developing grubs with no effective natural enemies. Grassed levees around crops of corn and soybeans in river bottoms may lead to destruction of these plants. Needless to say, we have abundant habitat for this particular insect and numbers this year are being reported above and below last year’s numbers. We are not sure as to why some areas have lower numbers than last year. Perhaps dry weather is a factor.

While the grubs are damaging to several plant roots (turfgrasses included), the adults beetles also feed on 300 to 400 plant species. Listed in table 1 are just a few plants likely to be damaged by Japanese Beetles. Lindens and roses seem to be the most popular. Some lindens (Tilia tomentosa ‘Sterling’ and Tilia Americana ‘Legend’ are less susceptible than other lindens. Some cultivars of crabapple (Baccata v. jackii, Jewelberry, Harvest Gold, and Louisa) are relatively resistant. Information for tables 1 & 2 can be found at the Cooperative Extension Service, University of Kentucky, Entfacts 451.

Adult beetles are 3/8” long, metallic green beetles with copper-colored wing covers. White tuffs of hair protrude along the underside of the wing covers. This is a dead-ringer characteristic for Japanese beetle. Adult beetles will usually start their feeding at the top of a plant and work their way down. Adults will feed on the upper side of leaves between leaf veins giving a skeletonized appearance. Odors from damaged leaves may serve as an attractant for drawing more beetles to desirable food sources. Adults can be highly mobile, up to several miles; however, most will make short flights to feed or lay eggs. There is no guarantee that adults will lay eggs in and around a site where they feed.

Egg lying begins immediately following emergence in the spring and mating. Clusters of beetles can be noticed on vegetation during the early day, usually in full sunlight. Females fly down later in the day to burrow 2 to 3 inches into the soil to deposit 40 to 60 eggs (mid to late June). Egg hatch occurs in July and grubs grow very quickly, achieving nearly full size by August. Grubs continue to feed on roots of turfgrasses and other plants in home lawns, gardens, parks, golf courses, and cemeteries.

Soil moisture is important for the survival of eggs and small grubs during the summer months. Females prefer moist soils to lay eggs. Irrigated lawns, sports fields, and golf courses will often have higher grub populations, especially during droughty periods. Older grubs move deeper into the soil profile where moisture exists, becoming more tolerant of droughty conditions.

Most individuals are familiar with white grub damage. Root pruning by grubs will create brown patches of dead turf that easily pulls up and separates from the soil. Many often refer to it, “like rolling up a carpet.” Early lawn damage from feeding grubs includes slightly discolored patches that may be sunken and beginning to wilt. These normally show up in late July to early August, but some years as late as September. This is when to investigate the existence of grubs in these patches and treat when numbers reach 5 to 10 per square foot.

Damage reduction and control can be accomplished several ways. Just as we listed plants susceptible to adult beetles, we also have a list of plants that are less likely to be damaged by adult Japanese beetles. They are listed in table 2.

Selection of less desirable landscape species is a good first step to plant selections in Japanese beetle infested areas. Removing beetles by hand may provide some protection for small plantings when numbers are low. However, the presence of beetles left unscathed will only attract more beetles. Shaking small plants is one way to remove beetles. Protecting small plants with cheese cloth is another.

Many garden centers sell traps. Sex attractant hormones lure beetles to the traps and can attract thousands of beetles a day. Unfortunately, research indicates that traps attract far more beetles than are actually caught. If traps are used, place them far away from landscape plants and gardens.

Several over-the-counter and commercial insecticides are labeled for adult and larval (white grub) Japanese beetles. Products containing pyrethroids such as cyfluthrin (Bayer Advanced Lawn & Garden Multi-Insect Killer), acelepryn (Acelepryn), bifenthrin (Talstar One, Onyx), clothianodin (Arena), deltamethrin (Deltagard), imidacloprid (Merit), lambda-cyhalothrin (Scimitar, Spectracide Triazicide), permethrin (Spectracide Bug Stop Multi-Purpose Insect Control), and thiamethoxam (Meridian) offer good control for professionals and homeowners. Carbaryl (Sevin) is also effective for both adults and grubs. Pyrethroid products will provide 2 to 3 weeks protection, while carbaryl provides only 1 to 2 weeks protection. For those wanting an organic approach, Neem products like Azatrol or Neem-Away will provide 3 to 4 days deterrence of feeding. Sequential applications of all products may be needed under extended periods of activity. Always follow label directions and note any precautions for bees. On food crops, follow the recommended pre-harvest interval before harvest begins.

Predicting the outcome of such a large outbreak of Japanese beetles is difficult. Since adult beetles are quite mobile, controlling grubs in a lawn may not protect landscape plants from adult feeding. Because you notice adult beetles in your landscape, does not necessarily mean that you need to treat a lawn. Treating landscapes plants will offer some protection from adult beetles when noticed. Perhaps the best approach for lawns is one of, “wait and see.” Others will want to treat immediately.

Reference:
Japanese Beetles in the Urban Landscape, M.F. Potter, D.A. Potter, and L.H. Townsend. University of Kentucky Cooperative Extension Service – College of Agriculture, EntFact 451.