By Dr. Brent Tharp, Wyffels Hybrids Agronomy and Product Training Manager
Geneseo, IL (October 7, 2019) - I've been getting a lot of comments on late season corn rootworm (CRW) beetle activity in soybean and weedy prevent plant fields. Those comments are usually followed by: What are they doing? Should I be concerned about them next year?
What are they doing?
They are foraging. CRW beetles prefer feeding on pollen and nectar sources, which aren't readily available in corn fields once silks dry up and turn brown. CRW beetles that emerge in corn fields after brown silk will go search for better food sources. Many weeds species found in ditches, soybean fields or prevent plant fields contain late season pollen and nectar which make them desirable for CRW beetle activity.
Should I be concerned about them next year?
The answer depends on the species of CRW beetle foraging in these fields.
Northern corn rootworm (NCR) beetles are various shades of green and contain no markings. They typically lay eggs in corn fields and have not been documented to lay eggs in soybean or weedy fields. So if you're seeing NCR beetles in soybean and weedy prevent plant fields we would not expect them to be an issue next year.
In some cases, a percentage of NCR eggs laid in corn fields have been shown to diapause (hibernate) for a year, thereby hatching and causing root damage when the field is rotated back to corn in two years. This behavior has primarily been confined to areas west of the Mississippi River, and we're not able to predict when or where NCR diapause will occur each year. The threat of diapause NCR should not be influenced by late season appearance of NCR beetles.
Western corn rootworm (WCR) are yellow with black stripes of varying thickness. Traditional WCR only lay eggs in corn fields. However, a variant of the WCR will lay eggs outside of corn fields. Variant WCR behavior has been identified in eastern IA, southern WI, north and central IL, and northwest IN. You can't visually distinguish a variant WCR from a traditional WCR. So, if you have seen WCR beetles foraging in weedy soybean fields or in prevent plant fields then there is a chance they are laying enough eggs to require management in next year's corn. However, many soybean fields in IL that have been monitored with sticky traps in August and early September have shown low numbers the past few years which indicates a lower risk of variant WCR activity.
CRW beetles with spots are Southern corn rootworm beetles and are not considered a threat to Midwest corn production.
If you have seen late flushes of NCR in soybean or prevent plant fields, the risk of damage to corn planted into those fields next year is low. If you have seen late flushes of WCR, there is some risk of rootworm damage to corn planted next year. If you are uncomfortable taking that risk in fields with late season WCR activity, plant a SmartStax hybrid or use a soil insecticide proven to control CRW with a VT2P or conventional hybrid.