Erin Hodgson, Department of Entomology and Adam Sisson, Corn and Soybean Initiative
Some people base the corn rootworm hatching date on the calendar, accumulating degree days or when they see fireflies. The first two are more reliable methods for predicting hatching dates. Corn rootworm egg hatch in Iowa can occur from late May to mid June depending on soil conditions, but the average hatching date is around June 6.
Research suggests about 50 percent of larval hatch occurs between 684-767 degree days (base 52 F soil). The cool spring weather in 2011 has slowed down predicted egg hatch and will be behind the normal date (Fig. 1). The southwest Iowa region is experiencing 50 percent larval hatch now and other parts of the state will approach it within 7-10 days depending on the temperature.
Fig. 1. Accumulated degree days (base 52F, soil) Jan. 1-June 21, 2011. About 50 percent of larval hatch occurs between 684-767 degree days.
Shortly after egg hatch, young larvae will begin feeding on root hairs and inside roots. As they develop, larvae will begin feeding on root tips. A severe infestation can destroy nodes 4 to 6 which interferes with water/nutrient uptake and makes the plant unstable.
Saturated soils will diminish overall corn rootworm pressure, and the high adoption of Bt corn should decrease populations in most fields this year. However, every field should be scouted for corn rootworm larval feeding regardless of the seed selection (i.e., scout even if Bt proteins are used). Continuous corn fields and areas with persistent corn rootworm populations are the highest priority for inspection.
Sample for larvae by digging up corn plants and washing the roots in a bucket; larvae should float to the top of the water (Fig. 2). Sample corn plants in different areas of the field to estimate infestation levels. Determine the number of larvae per plant and/or the nodes pruned.
Fig. 2. Corn rootworm larvae are creamy white with a small, brown head. Photo credit Marlin E. Rice.
Corn rootworm rescue treatments should be made in June, but are not very practical or effective, as the corn is too tall for the product to sufficiently penetrate the soil and reach the larvae. But sampling and evaluating root systems this year will help assess corn rootworm management and seed selection for 2012. Here are two websites that may be of interest for corn rootworm management:
Erin Hodgson is an assistant professor of entomology with extension and research responsibilities; contact at email@example.com or phone 515-294-2847. Adam Sisson is a program assistant with responsibilities with the Corn and Soybean Initiative. Sisson can be contacted by email at firstname.lastname@example.org or by calling 515-294-5899.