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Chicago Urban Gardening

The day to day experiences of a University of Illinois Extension Urban Horticulture Educator in Chicago, Illinois

Watch for Japanese Beetles

Posted by Ron Wolford -

Japanese beetle adults will probably emerge this week in southern Illinois. Typically, they emerge there around June 18, but emergence appears to be delayed this year. As of June 18, they had not emerged in Kentucky, and they should emerge in southern Kentucky a week or so before southern Illinois. Perhaps the cool spring has slowed down their emergence. Japanese beetle adults were emerging in North Carolina on June 11, so they will eventually show up.

When they do start to emerge in southern Illinois, we expect them about a week to 10 days later in central Illinois, and a week to 10 days after that in northern Illinois. An entomologist at Eastern Illinois University in Charleston needs to collect recently emerged adults. Please contact me, Phil Nixon, at (217)333-6650 or at pnixon@uiuc.edu when Japanese beetle adults are first seen in southern and south central Illinois so that I can let him know when to start looking for them.


We had high numbers of Japanese beetle adults in southern and northern Illinois last year, and nothing has occurred since then to decrease their numbers. Central Illinois had lower numbers than predicted last year, and we have still not figured out why.


Reduced numbers of Japanese beetles in this area of the country are typically affected by two climatic conditions. Once the larvae hatch from eggs in late July to early August, they need 11 inches of water through the fall before descending deeper into the soil for the winter. They tunnel downwards when the turfgrass root zone temperature drops to 60oF. Thus, a summer into fall drought can cause a reduction in beetle emergence the next year. Of course, irrigation helps the grubs to survive, so the drop in numbers may not be so severe in housing developments where a high percentage of the lawns are irrigated frequently.


Deeply frozen soil during the winter is the other climatic condition that reduces the number of Japanese beetles. Most Japanese beetle grubs migrate only about eleven inches deep into the soil for the winter. They can tolerate freezing temperatures during the winter for 2 to 3 weeks before dying. Several years ago, the soil in central and northern Illinois froze 18 inches to 3 feet deep and stayed that way for about 6 weeks. The following summer, only about 1/4 to 1/3 the number of Japanese beetle adults emerged, compared to the previous summer.


We had adequate rainfall last summer and fall for the Japanese beetle grubs to survive. Temperatures did not get cold enough last winter without protective snow cover to freeze the soil very deeply. Based on that, we are expecting a normal to high emergence of beetles this year.
Male Japanese beetles emerge before the females. They are able to detect females tunneling near the surface of the turf before they emerge, resulting in their being numerous in these areas. As soon as a female emerges, many males will try to mate with her, creating a ball of beetles a couple of inches in diameter. This is referred to as "balling." These become obvious on the closely mowed turf of golf greens and tees.


Japanese beetle adults feed on the foliage of many trees and shrubs, preferring linden, crabapple, willow, birch, and rose. They feed first on the upper, sunniest leaves, eating through the upper surface, the epidermis, of the leaves. Frequently, they eat holes through the leaves but also may eat just the upper epidermis and internal mesophyll cells, leaving the lower epidermis intact. Leaves damaged in this way will initially appear whitish but turn brown as the lower epidermal cells dry and die. Thus, damaged trees have missing and/or brownish foliage at the top of the canopy that gradually descends as the beetles feed on lower leaves. This damage is primarily aesthetic, with little obvious effect on plant health.


Because the damage is primarily aesthetic, not treating attacked trees or shrubs is a viable option. By treating only those plants most obvious in the landscape, such as those near entryways and in front yards, you reduce the amount of insecticide applied into the environment and keep the cost to the client lower.


These insects typically feed on one host for about 3 days, and then fly 3/4 to 1-1/2 miles away to another host. They repeat this every 3 days for about 6 weeks. They are much more attracted to previously damaged foliage, particularly that fed upon by other Japanese beetles. Thus, reducing leaf damage during the first couple of weeks after emergence frequently results in markedly less damage through the balance of the season even with no further control efforts.


Carbaryl (Sevin), cyfluthrin (Tempo), permethrin (Astro), and other pyrethroid insecticide foliar sprays provide protection from damage for 10 days to 2 weeks. These would need to be repeated twice at 2-week intervals to provide season-long protection. However, if clientele are willing to pay for only one treatment, applying soon after beetle emergence is likely to provide the best results. Azadirachtin, sold as Azatin, Ornazin, and Neem, has been shown to be an effective repellent in some situations and less so in others. It is most effective when applied just before the beetles emerge.


Clientele can hand-pick the beetles, and this is most effective during the first 2 weeks after emergence. Use a wide-mouthed jar, and partially fill it with rubbing alcohol (isopropyl alcohol) or soapy water. Holding the jar opening under the beetle, poke at the beetle. It will fold its legs and fall into the jar, where it will be killed. This is most effective in the late afternoon to evening or in the early morning. During the middle of the day, disturbed beetles tend to fly off into your face rather than drop into the jar. By harvesting the beetles every day or two, one can greatly reduce the amount of damage to the plants.


Japanese beetle traps contain a pheromone to lure males and a floral lure to attract female beetles. They have a bright yellow color in the lure area that also attracts both sexes. Research has shown that these traps increase the amount of damage to landscape plants. Apparently, the traps attract beetles to the area; but once in the area, many beetles attack nearby plants rather than fly all of the way to the trap. If traps are used, be sure that clientele understand the risks of increased damage and place the traps 50 feet or more from plants that you want to protect.

Author: Phil Nixon: Extension Specialist, PAT/Ornamental Household Insects



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