Adult northern corn rootworms have been extremely abundant in soybeans this August (see image of beetles on corn silk, below). Beetle populations in soybeans at the extension field lab in Boone County averaged 45 per 20 sweeps; however, Brad Buchanan, crop consultant in Cedar Rapids, reports 50--150 per 20 sweeps in some eastern Iowa fields. Their abundance has prompted many questions regarding what they are doing in soybeans and if they will present a threat to rotated corn next year. Here are several of the questions we have encountered, and our responses to those questions.
Do northern corn rootworms feed on soybeans?
The larvae do not feed on soybean roots--they arerestricted solely to corn and several species of grasses. The adults, however, will feed on soybean leaves but apparently they do not like soybean leaves. Adult beetles were collected from soybeans in Boone County the first week of August and placed on soybean leaves in Petri dishes. After 96 hours, 20 percent of the beetles had died without feeding on the leaves, and those that did feed ate very little. The most feeding injury caused by these adults in this experiment was very minor, not amounting to more than 1/2 square inch (see injured soybean leaflet, below).
Do northern corn rootworms lay eggs in soybeans?
We have no evidence that they are laying eggs in soybeans, so as far as we know, the answer is still no.
But the abundance of adults in soybeans certainly makes us pause and wonder if the insect might be changing its behavior.
Could northern corn rootworms change their behavior and lay eggs in soybeans?
Sure, corn rootworms have surprised us with several behavioral changes that make us think that if northerns laid eggs in soybeans it would not be a total surprise. During the last 25 years, we have witnessed the development and increase in extended diapause in northern corn rootworms, and now the spread of this behavior across the entire state. Also, the western corn rootworm has changed its behavior and some populations in Illinois, Indiana, and extreme eastern Iowa now lay eggs in soybeans.
Should these adult northern corn rootworms in soybeans be killed with an insecticide?
No. Again, we have no evidence that they are laying eggs in soybeans and their leaf injury is insignificant.
Should an insecticide be used in first-year corn if northern corn rootworms were abundant in the soybeans this year?
An insecticide should be used only if there is a confirmed extended diapause problem (i.e., lodging from rootworm larvae) in the neighboring corn fields. We would not recommend insecticide use only based on the presence of northern corn rootworms in soybeans.
What is Iowa State University doing to address this northern corn rootworm "situation" in soybeans?
We may sample soil from some of these high population soybean fields this winter and look for corn rootworm eggs, but this procedure is labor intensive, costly, and rootworm eggs can be very easy to miss unless a large number of samples are collected. We would be interested in knowing where continuous soybeans (2 or 3 years) will be planted to corn in 2006. In these fields, we could place emergence cages to trap adults and confirm, or deny, that northern corn rootworms were laying eggs in the soybeans. Then, we would have an answer to the question that has many of us bugged.
Injury to soybean leaflet from adult northern corn rootworm. (Marlin E. Rice)
Adult northern corn rootworms feeding on corn silks. (Marlin E. Rice)
This article originally appeared on pages 168-169 of the IC-494(22) -- August 22, 2005 issue.