Bryan Jensen
UW Extension and IPM Program
Although northern and western corn rootworm beetles have been emerged for a while, we are rapidly approaching that time period (early/mid-August through early September) when oviposition will be in full swing. Never before have beetle counts been more important than at the present time. By establishing the level of adult infestation during the current year, you can determine whether rotation or preventative treatments will be necessary in the following year’s corn crop. Not only will #’s/plant give you an idea if the field needs to be treated next year, but counts will give insight for the best treatment options for resistance management.
To determine the larval damage potential in continuous corn, count the number of beetles on five non-consecutive plants in each of 10 random areas of the field. First, grasp the ear tip tightly enclosing the silks in the palm of your hand and count beetles on all other areas of the plant. The silks often have the most beetles on the plant, so a tight hold on the ear tip keeps beetles from dropping out. Pull leaves away from the stalk to examine leaf axils and expose hiding beetles. Once the entire plant is examined, open your hand slowly and count the beetles that come out of the silks as you strip the husk away from the ear tip. Record the total number of beetles and divide by the number of plants counted (50). The grower will need to manage corn rootworm larval populations if you find an average of 0.75 beetles per plant during the egg laying period. Because beetles are mobile, a minimum of 2 counts, 7-10 days apart will be needed to make a reliable no-treat decision. However, if on the first scouting, beetle numbers are > 0.75 beetles/plant, a second if not third count would be useful to help decide on the appropriate treatment option next year.
Diversifying your management practices is an important resistance management tool. Having the field data (i.e., beetle counts) will increase your comfort level when making recommendations and can give you a point of reference for future years.
