Japanese beetle adult
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Japanese beetles feed during daylight hours from early through midsummer on the leaves of many trees and shrubs. They prefer sunny areas and will feed heaviest on the upper leaves of the plant, which turns brown.
Japanese beetle grubs (as well as several other white grubs) feed on the roots of grasses, vegetables, and ornamental plants. In general, 10 to 12 grubs per square foot or more will eat enough roots to cause dieback of the turf. The turf will turn brown and can be easily turned back like a carpet because few uneaten roots remain. The area not only becomes unsightly, but its usefulness may be reduced in golf courses, football fields, and other athletic turf. Heavily used areas or turf in dry years may become severely damaged with relatively low numbers of white grubs. Similarly, turf areas that are used lightly or irrigated may retain a good appearance with a relatively high number of grubs per square foot. The ability of the turf to replace eaten roots can compensate for high grub numbers.
Various animals are attracted to white grubs for food. Raccoons will roll back large areas of turf a foot or more wide to eat the grubs in the root zone. Armadillos dig holes several inches across and deep in searching for grubs. Skunks will typically dig up small, 2- to 3-inch diameter divots of sod to eat the grubs underneath. A single skunk may make 100 of these small holes per night. Insect-eating birds, such as starlings, will peck through the turf to obtain and eat the grubs. When a flock works over an area in this way, the result is a brownish area that on closer inspection is seen as damaged turfgrass plants. All of these animals will damage turf that appears to have as few as 3 to 5 white grubs per square foot, thus causing injury in areas where grub numbers are below as well as above turf-damaging levels.
Japanese beetle has a 1-year life cycle. Adult beetles emerge in late June in southern Illinois and early July in central and northern Illinois. They are 1/2 inch long, heavy-bodied, oval beetles that are metallic green with coppery wing covers. Visible along the sides of the body below the wing covers are a series of white spots formed by clumps of white hair. There are 2 similar spots of white hair below the wing covers on the rear end of the beetle. The beetles fly when disturbed, making a buzzing sound. Mating occurs in early morning and early evening, and the female burrows into the soil to lay her eggs. The adults hide in the soil from late evening through the night, remaining in the soil on cold, wet days. The adults hide in the soil from late evening through the night, remaining in the soil on cold, wet days. The adult beetles live for 4 to 6 weeks.
Eggs hatch in the soil about 2 weeks after being laid. The eggs hatch into larvae or grubs that are white, C-shaped, and have 6 legs and brown heads. Fully-grown grubs will be about 1 inch long. Grub feeding lasts from August into the fall. The grubs descend for the winter when the temperature in the root zone drops below 60 degrees F. They ascend to the root zone in the spring when the soil warms to 50 degrees F and feed for a short time before pupating. Adults emerge from these pupae from late June to early July as indicated in the preceding paragraph.
Japanese beetle adults can be controlled by handpicking or using insecticides. Because the beetles are numerous and cause damage for about six weeks and most insecticides last two weeks or less, repeated applications are necessary. To reduce insecticide use and cost, treat only heavily attacked ornamentals where the damage will be readily noticed, such as near building entrances. The health of even heavily attacked ornamentals is unlikely to be seriously affected because the damage occurs in the second half of the growing season after the leaves have already produced much of the food for the plant. Thus, treatment may not be necessary behind buildings, along alleys, and in other less obvious landscape areas.
Japanese beetle traps have been shown to attract beetles from two or more blocks away. However, once they have been attracted to the trap's vicinity, they frequently feed on nearby ornamentals rather than coming all the way to the trap. As a result, studies have shown that landscapes with Japanese beetle traps are likely to experience more damage than those without traps. Also, due to the long and frequent flights of adult beetles, reducing grub numbers in the turf or beetle numbers in the landscape has little effect on the number of adult beetles in the landscape the next year.
Japanese beetle grubs: Refer to the white grubs page for management suggestions.