Begin Scouting for Corn Rootworm Beetles Now!

     Southern               Northern                    Western
Corn Rootworm         Corn Rootworm        Corn Rootworm

Figure 1.   Adult Corn Rootworm Beetles
Source: Ohio State University

As a corn producer you have two reasons to scout for corn rootworm beetles during July and August. One reason is to insure that silk-clipping is not interfering with pollination, Figure 2. The second reason to scout, is to assess the beetle population to help you correctly chose planting time insecticide treatment for next year’s corn acres.

Figure 2. Adult Corn Rootworm Beetles feeding on silk

Source: Ohio State University

Beetle emergence occurs in late June and early July. Beetles that hatch before silk emergence feed on corn leaves, scraping the surface tissue off the leaf and leaving a white parchment-like appearance. Silks and pollen become the favored food once the silks emerge and pollen is released. No real thresholds for silk-clipping damage have been established based on the number of beetles per plant. However, once the population per plant is in the 6-8 beetle range and silk growth is restricted, treatment must be considered. Control should also be considered when silks are clipped off to within an inch or less of the ear tip before 50% of the pollination is completed. Once all plants have ceased releasing pollen, the window for treatment to improve ear fill is over.

The second reason to scout for rootworm beetles is that those same beetles will soon be laying eggs. These eggs overwinter in the soil, hatch into corn rootworm larvae in the spring and feed on corn roots from early June until late July. A typical symptom of damage from corn rootworm larvae is lodged corn plants (Goosenecking) from root pruning. Fortunately, not all-continuous cornfields will have economic infestations of corn rootworms. Weekly scouting of adult rootworm populations during July and August are needed to provide you with the information to decide if a rootworm insecticide is needed next year.

Begin scouting when beetle activity begins and continue until activity stops. Late planted or late maturing fields are more susceptible to corn rootworms, moving in from nearby earlier maturing fields to feed and lay eggs. Beetle populations can be monitored by counting beetles per plant or by using sticky traps. The threshold for continuous corn is 0.75 beetles per plant. The threshold in a first year cornfield is 0.45 beetles per plant. And the threshold for a sticky trap is six beetles per trap per day. Rotating fields out of corn or using an insecticide at planting will help prevent economic damage. Fields remaining below the threshold level do not need to be treated with a rootworm insecticide next year.

During recent years in the Western Corn Belt states, producers have begun using adult beetle control programs. These programs drastically reduce the egg laying population if the timing is correct, in effect, reducing the beetle population below the threshold before egg laying begins.

In some areas of the Corn Belt, adult rootworms have altered their behavior and started laying eggs in soybean fields. In these areas soybean fields also need to be scouted for threshold populations. Sticky traps and sweep nets are the accepted methods for assessing populations in soybean fields which will then be followed by corn. Current thresholds are 100+ beetles per 100 sweeps and 6+ beetles per sticky trap per day.