Damage from the grubs of Japanese Beetle in field corn has been observed for the past several years in river bottom fields located just north of St. Charles. Typically damage from Japanese beetle is caused by adult foliar feeding on soybean or silk feeding on corn ears. Feeding on seedling corn plants by Japanese beetle grubs is rare, but rapidly becoming more common as the beetles move into most counties of the state. A question which is often asked is whether seed treatments are effective controls for grubs of this pest? If so, do different rates provide different levels of protection? The answers to these questions are difficult to answer as field insecticide trials are lacking in this area. Additionally, over 100 annual grub species and 160 perennial grub species can occur in Missouri. Each of these grubs may react differently to insecticides based on their abilities to survive exposure to specific pesticides or by their abilities to avoid pesticides in the environment. In general, I believe seed treatments on corn and soybean do reduce numbers of most grub species, although the reduction in numbers may not reduce populations to below threshold levels. For example, I observed side by side corn fields which were planted with the same corn variety although one had a seed treatment at the 250 rate and the other did not have a seed treatment applied. Both fields exhibited grub damage and larvae were present. The one without a seed treatment required replanting whereas the field with the seed treatment showed less stand loss and did not get to a plant population where replanting was necessary. In another situation where two side by side fields exhibited grub problems, the problems were less in the field with the 1200 rate as compared to the 250 rate. Thus seed treatments will help reduce numbers of most grub species, but the populations may still cause significant economic loss.
In contrast, the grubs of Japanese beetles do not seem to be affected by seed treatments at the 250 rate. Grub damage to emerging corn seedlings in the St. Charles area seems to occur to both untreated and treated seed. The damage usually occurs early season on seedlings that are just emerging. One reason why the grubs of the Japanese beetle may not be significantly affected by seed treatments is that there may be a dose to size response as well as a behavioral response. The grubs of this species go through four instars or larval stages as they grow to maturity. Unlike most grubs which often complete much of their growth in the spring, the Japanese beetle grub gains most of its growth in the fall of the year in which the eggs are laid. In the spring this grub is already large in size and does minimal feeding and growing prior to pupation in the spring. If a dose response is present with this species, then it should require a greater level of insecticide exposure to kill a large grub as compared to a small grub. In addition, the Japanese beetle grub completes its feeding very early in the spring and may not be feeding when most insecticide seed treatments are effective on other soil insects. This would explain why most damage by Japanese beetle grubs in corn occurs very early in the season to plants which are just emerging. Although many other factors, such as reduced use of soil insecticides, may also allow the Japanese beetle grubs to escape the effects of insecticides, the maturity of this grub occurring early in the season may explain much of its ability to survive seed treatments. Until insecticide trials are conducted specifically on the grubs of the Japanese beetles, no definite answer to the questions asked will be known.
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REVISED: December 1, 2011