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This spring many problems with white grubs have occurred in several different crops. The mature stage of white grubs may be any one of several May and June beetle species. Some are considered annual grubs with a one year life cycle beginning in early summer when eggs are laid and larvae emerge to feed and eventually overwinter in the soil. The life cycle is completed the following spring and summer when the grubs mature, pupate in the soil, and emerge as beetles to feed, mate, and lay eggs for the next generation. Many additional white grub species exhibit a perennial life cycle in which the larval stage or grubs remain in the soil for 3 or 4 years before emerging as beetles. Many white grub species do not have common names, but are identified only by scientific Latin names. Some common annual grub species include northern and southern masked chafers, Japanese beetles and green June beetles.

White grubs have always been present in field crops with most damage reported from corn and soybean fields. In recent years white grub problems also have been observed in wheat and grass pastures during spring. In all cases damage is often seen as large circles of dead, dying, or stunted (sometimes yellow) plants. In wheat, severe grub damage can be seen as large circles of weeds in fields during late spring although feeding by the grubs occurred either the previous fall or during early spring when the wheat plants were destroyed and replaced by weeds.

Traditionally, effective management of white grub infestations has been accomplished through pre-plant or planting-time insecticide applications which were often applied for control of other insect species. With the recent adoption of Bt corn varieties and insecticide seed treatments, the amount of insecticides used during early spring has been reduced in Missouri. White grub is listed on the label of most insecticide seed treatments which research shows provide protection from insect pests for about 45 to 60 days following plant emergence. It has also been observed in Missouri that white grub populations can be reduced by the use of insecticide treated seed. Data gathered from submitted grub samples and field visits suggest that a majority of white grub problems in Missouri originate from grubs with annual life cycles. Perennial grub problems do occur, but account for about half as many infestations in most years as annual grubs.

So why are steadily increasing number of May and June beetles being captures in our light traps and higher numbers of white grub larvae being observed in Missouri crop fields? No one knows for sure, but several factors can influence numbers of white grub beetles and larvae. There is a normal cycle for most insect populations which fluctuates over a several year period. Weather, soil conditions, insecticide use, crop rotations, crop residue management, tillage practices, beneficial insects, pest life cycles, soil type, organic matter content and many other factors may also influence pest numbers in any one year.

Rescue of corn or soybean crops from white grub infestations is difficult after the crop has been planted. Few insecticides are available for rescue applications for white grubs in field crops. In fields with frequent white grub problems soil insecticides, broadcast insecticide applications at planting, and seed treatments all offer some control. At this time in the season, white grub problems in field crops are difficult to control unless the crop is replanted. Two factors are often present at the location of white grub infestations in field crops. They include the presence in field borders of willow, cottonwood, or other trees often associated with river bottom areas and drainage ditches. The beetles congregate on these trees, especially willows, to feed and mate. The female beetles will then return to surrounding fields to lay eggs. The second factor often associated with white grub infestations is the presence of cattle and/or previous feeding of hay bales in the crop field experiencing white grub problems.

Wayne Bailey
BaileyW@missouri.edu