Northern corn rootworms in corn and soybean

Northern corn rootworms are extremely abundant across much of Iowa this year. Lodging of first-year corn is widespread and in some counties the potential for yield loss is expected to be very high. Beetles also were abundant in some fields during late July and August and prevented pollination by clipping silks.

Northern corn rootworms typically have a 1-year life cycle and rotating corn with another crop has been a very successful management strategy to prevent lodging and yield loss from this insect. Unfortunately, this rootworm has successfully adapted to the corn-soybean rotation in Iowa and a 2-year life cycle of some populations is common. This 2-year life cycle is called extended diapause because some of the eggs remain dormant in the soil for nearly 2 years before the larvae hatch. For instance, for the eggs that were laid in 1999, some larvae hatched in 2000, but the remainder did not hatch until 2001.

The question is why is the lodging so prevalent and widespread this year? Possible reasons include two consecutive winters that were favorable for higher than average egg survival in the soil. The winter of 1999-2000 was relatively mild and the winter of 2000-2001 had an insulating blanket of snow that, at least in Ames, covered the ground for 99 consecutive days. This snow cover was followed by a cool spring that slowed corn root development, and some fields were then hit with strong rains and wind in July. Plants with small root systems and substantial root injury from the larvae then fell over.

In recent weeks, I have been asked many times whether northern corn rootworm females will lay eggs in soybean and cause lodging problems in corn next year? I asked corn rootworm experts Jon Tollefson, Iowa State University, and Mike Gray, University of Illinois, if they have any evidence that northern corn rootworm females are laying eggs in soybean, and their answer, based on current knowledge and experience, was "no."

This article originally appeared on page 182 of the IC-486(22) -- September 17, 2001 issue.

Updated 09/16/2001 - 1:00pm