Issue 1, April 17, 2009

Japanese Beetle

The soil drench application of imidacloprid, sold as Merit and other brand names, should be applied to susceptible plants at least two months before adult beetles emerge. With emergence in southern Illinois occurring around the end of the third week of June, central Illinois in late June, and northern Illinois in early July, it is time to make these applications.

Research on emerald ash borer and other pests has shown that application within two feet of the trunk is most effective. Because imidacloprid adsorbs onto dead organic matter, mulch or sod should be removed before a drench application is made. Otherwise, the insecticide will glom onto the dead organic matter and will not be picked up by the tree roots. Once the insecticide has soaked into the soil, the organic matter can be returned. Using a soil needle to inject the insecticide below the mulch or sod eliminates these steps.

Although Japanese beetle adults feed on a couple hundred species of trees and shrubs, those most at risk of severe damage in Illinois are linden, crabapple, birch, willow, and rose. Feeding is heaviest at the top of the tree and moves downward through the season. The beetles sit on top of the leaves and eat holes through the leaf laminae between the veins. They may also eat the upper surface or epidermis of the leaf, consume the interior mesophyll, and leave the lower epidermis which is initially light-colored. These exposed cells soon die and this window-feeding results in thin, brown leaves.

The low winter temperatures that we experienced in central and northern Illinois this past winter may have greatly reduced the number of overwintering larvae. Most Japanese beetle grubs migrate no deeper than eleven inches into the soil for the winter. They survive freezing temperatures, but can only tolerate them for about three weeks. In central Illinois, the soil froze to about 18 inches last winter; those in northern Illinois froze about two feet deep. They remained frozen for about six weeks. Based on previous experience, this will likely result in only about one-third of the number of beetles emerging than expected. Time will tell whether this prediction is accurate.--Phil Nixon

Phil Nixon

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