As reported in an earlier article, corn rootworm egg hatch in 2012 was slightly ahead of a normal year. Adults were first detected in Illinois last week and around Iowa this week. We’ve been getting many questions about when or if it is appropriate to treat the adults. Three factors must be taken into account before a foliar insecticide application is warranted.
First, consider the growth stage of the plants. Adult corn rootworm will feed on leaves and cause some scarring; however, this will have little if any effect on yield. Adults can cause yield loss if they are present in the field when corn is silking, which is a critical time period to protect plants. Adults are strongly attracted to silks and will mass on plants to feed and mate (Photo 1). Adults that trim back silks to the husk during pollen shed will interfere with pollination.
Next, consider current growing conditions. Weather plays a huge role on how plants respond to silk feeding. Plants do not tolerate as much feeding during pollination in hot and dry weather. For example, under ideal moisture conditions, plants could tolerate 15 beetles per plant, but that number is reduced to just five per plant under drought stress (Photo 2).
Finally, consider plant maturity. Late-planted fields or late-flowering hybrids compared to neighboring fields are generally attractive to adult corn rootworm. Green silks will still be developing in these fields when older fields have brown or drying silks. Adults may migrate and aggregate in these later-maturing fields.
It is always a smart idea to scout for insects in corn fields during pollination. But with adult corn rootworm becoming active before silking this year, we highly recommend keeping an eye on fields to protect yield. A foliar insecticide may be warranted if there are five or more beetles per plant, silks have been clipped to less than ½ inch of the ear tip, and pollination is not complete. Also take into consideration other insects that may be feeding on the silks at the same time (e.g., Japanese beetle).