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White Grubs

July 20, 2005
It is time to apply insecticides to control white grubs. The pattern of emergence of Japanese beetles is variable this year around the state. In many parts of the Champaign-Urbana area, numbers are small even though the first beetles were seen a month ago. The Bloomington area has had large numbers since mid-June. The early emergence may be due to the unusually early hot weather in May and June. Similar patterns of few or many beetles seem to be occurring in patches throughout the state. Be prepared for many more beetles to emerge later in July or perhaps even in August.

We may be seeing a developing pattern of the delayed emergence that has been reported in Ohio and other states east of Illinois. There, many beetles are emerging later in July or even in August, resulting in very small young grubs mixed with much larger ones in early fall. The smaller grubs overwinter, take longer to develop in spring, and emerge as adult beetles later in summer, perpetuating the longer adult emergence and larval hatching period. In Illinois, there appeared to be a somewhat extended adult emergence pattern into late July of 2004, but two different size classes of white grubs were not readily apparent.

Masked chafers (annual white grub adults) emerged as is typical in early July. The first adult that I know about was seen on July 2 in central Illinois. Apparently, the warm spring did not cause an early emergence of these species. The number of masked chafers appears to be low this year.

Regardless of the emergence pattern of the adult stages of various white grubs, the dry weather pattern over most of the state will result in increased egg-laying in irrigated turf. Even recent widespread rainfall from Hurricane Dennis is unlikely to change that pattern. Application of imidacloprid (Merit) or halofenozide (Mach 2) during July to irrigated turf should provide control. Both of these insecticides last long enough to control any late-hatching Japanese beetle grubs. Mach 2 is more water soluble and will flush down into the root zone if irrigation or rainfall occurs within 3 days after application. After 3 days, ultraviolet light apparently breaks down enough of the exposed chemical on the grass blades to reduce grub control. Merit should be watered in with at least 1/2 inch of water immediately after application to move it into the root zone where the grubs will contact it.

Author: Phil Nixon


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