Crops: Crop Scene Investigation - 10 Solved: What's really stressing John's corn?

Saturday, May 10, 2008

by SHAWN BRENNEMAN

Rootworm larvae damaged the roots in John's corn causing the plants to look severely drought- stressed.

The key to solving this puzzle was getting as much information as possible from the field history. Typically, rootworm is a pest found in corn after corn. While John's previous crop had been soybeans he'd had significant volunteer corn pressure.

Rootworm adult beetles lay their eggs in corn fields in August. If the field is planted in corn again the next year, the larvae hatch and feed on the roots. If the field is planted into other crops, like soybeans, they starve and die.

In John's field, however, the high amounts of volunteer corn provided a food source for the larvae - as did the grassy weeds he had trouble controlling. There are a few species of annual grasses, such as green and yellow foxtail, that rootworm can feed on to survive.

The adult beetles emerging from John's soybean field would have laid their eggs in the same field. When he planted corn the next year, the larvae hatched and had fresh corn roots to feed on once again.

When John and I took a closer look at the affected plants we found larvae tunnelling and scar marks on the roots. Reduced root growth from rootworm feeding meant the plants were retrieving less water. As root feeding and injury increases, the potential for lodging, stress, stalk rots and yield loss also rises.

Crop rotation is the best way to manage corn rootworm. There are also options for control of the pest in continuous corn fields. Soil-applied insecticides and seed treatments provide good control. More recently, transgenic corn varieties offer in-plant protection that works much the same way as Bt works for corn borer - it expresses a protein in the plant and when the corn rootworm larvae ingests the protein the rootworm dies.

Congratulations to Jane Enright, Renfrew, for her correct answer. BF

Shawn Brenneman, CCA, is a Field Agronomist for NK® Brand, Syngenta Seeds.