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Rhonda Ferree's ILRiverHort

Rhonda Ferree's Horticulture Blog
Japanese beetles
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Japanese Beetle Myth-information


Japanese beetle adults are above ground eating leaves for about 6 weeks from mid-June into August. After mating and feeding females lay eggs in moist, actively growing lawns. Eggs hatch into large "C" shaped grubs that feed on plant roots. In fall grubs burrow into soil. Next year they form adults and start the vicious cycle again.

Horticulture educator Sandy Mason shares frequent myths about Japanese beetles below.

Myth #1: Japanese beetles will go away in a few years.

As with all insects their populations cycle from high to low numbers. It is doubtful they will ever completely go away. Weather conditions such as cold winters and dry falls more than likely determine the cycles. The eggs and young grubs can be killed in dry soils. Therefore stopping or reducing irrigation during July can result in fewer grubs as long as there is little rain. At least it's one good thing about drought.

Myth #2: Control the grubs and you will not have Japanese beetle adults.

Unfortunately controlling Japanese beetle grubs does not significantly reduce the number of adult beetles in your yard the following year. The beetles are good fliers and easily fly a couple miles in a single flight. They may travel 10 to 15 miles from where they lived as grubs.

Myth #3: Milky spore will control Japanese beetle grubs.

Yes and no. Many variables are involved with bacterial milky spore. It has been effective in some areas and not in others which may be due to soil conditions, product variability, or grub resistance. It can take 2-3 years for spores to build up in soil. Milky spore is only effective against Japanese beetle grubs and does not kill other turf damaging grubs.

Myth #4: Japanese beetle traps are effective controls.

Japanese beetle traps do an excellent job of attracting, but not a good enough job at trapping. The majority of beetles are attracted but not caught in the traps according to research at the University of Kentucky. Therefore plants can suffer more damage with traps than if no traps are used at all if the plants live along the beetles' flight path or around the traps.

Myth #5: My tree will die if the beetles eat all its leaves.

Woody plants can tolerate some feeding and many will releaf once the beetles are gone. Woody trees and shrubs usually can tolerate three years of significant feeding before showing visible signs of decline.

Adapted from article by Sandra Mason, Extension Educator, Horticulture, slmason@illinois.edu



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