The Homeowners Column

The Homeowners Column

Japanese Beetle Myth-information

Photo of Sandra Mason

Sandra Mason
State Master Gardener Coordinator
slmason@illinois.edu

Recently I was surveying the area for Japanese beetles. Actually I was driving my car around and counting the hits on my windshield. The beetles were bouncing off my car like popcorn. I consider myself a fairly tolerant person when it comes to bugs, weeds and things. But this time I enjoyed every crack of their evil beetle bodies. I knew the green slime left on my car was the last remains of my roses. I wanted to turn my car around and run over their dead beetle bodies. A single death wasn't enough. Japanese beetles arouse our barbarian instincts.

Japanese beetle adults are present in high numbers and devouring leaves for about 6 weeks from the end of 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.

Here are some frequently shared myths about Japanese beetles.

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.

Woody plants can tolerate some feeding and many will releaf once the beetles are gone. Japanese beetle adults can be controlled with foliar applications of carbaryl (Sevin) or cyfluthrin (Tempo). However, Sevin is toxic to bees and other beneficials.

Pyola, canola oil with pyrethrins, shows some control. Neem or azadiractin is an effective repellent if applied before the beetles become numerous. Imidacloprid is sold as a systemic to kill adult beetles. For best effectiveness it should be applied in April or May when the plants are actively growing. Beneficial nematodes have shown mixed results.

Picking beetles off by hand in early morning is another alternative. When disturbed, beetles fold their legs and drop to the ground. Hold a bucket containing rubbing alcohol or soapy water below the infested leaves. Move the plant and the beetles will drop to their death. Just make it a part of your early morning ritual; cup of coffee, cup of beetles. Just don't confuse the two.

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