Corn Rootworm Adults Active

Corn rootworm adults have been very active these last few weeks. You may have noticed some of the leaves in your corn field taking on a dull grey appearance. When you hold these leaves up to the light, you can see wide scratches or feeding tracks running parallel with the leaf veins (Figure 1). These are caused by the corn rootworm adult feeding. They typically prefer to feed on the silks and pollen but when not available, they can feed on the leaves. You may also see them feeding on soybean leaves which is a normal part of their diet as well and is not an indication that the rotation variant is in those fields.

Though the leaf feeding injury in corn may look bad in areas, it is not economical. The bigger concern is when these very active adults move onto the corn silks. Spraying for adults is extremely rare in Ontario though this year may have some exceptions. Scout these fields for the next few weeks.

Inspect ears and silks randomly across the field. If the silks are clipped to less than ½ inch on the majority of the ears control may be necessary. It is especially important in a dry season when the plant may not be able to grow the silks back as quickly, which can impact proper pollination. Bt rootworm hybrids only protect the roots from larval feeding and have no activity on the adults feeding on the silks.

Now that the adults have emerged, it is also a good time to assess the root injury from the larvae in continuous corn fields and also determine if control will be necessary next year if those fields are going into corn again. For root feeding, bring a shovel and dig around a few plants, ensuring that you take a large enough root ball that you are not clipping or cutting off parts of the root system with your shovel. Shake off excess soil and soak these roots in water overnight to wash off all of the soil from the roots. Look for signs of clipping and tunneling in the roots. Determine if the product you used this year gave you adequate control. In heavy rootworm years, you may find that the high rate of Poncho (1250) did not give adequate protection to the roots. If this has happened, you may need to consider switching to a rootworm Bt hybrid next year which has been shown to give better control under heavy rootworm pressure. Don't be surprised though if you also see some root feeding even on the Bt rootworm hybrids as these hybrids only produce a low to moderate dose of Bt.

If you are considering going with continuous corn next year, look for adult activity in your field now. Scout 10-20 plants in 5 areas of your field. One adult per plant in the next two weeks indicates that egg laying activity was high enough in your field to warrant treatment at planting if corn is planted again in that field next year.

Figure 1. Adult rootworm feeding injury on corn leaves (Photo credit: Marlin Rice, Iowa State University)