When will corn rootworm eggs hatch in your fields? Some people base the corn rootworm hatching date on the calendar, or on accumulating degree days or when they see fireflies. The first two are the 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, says Iowa State University Extension Entomologist Erin Hodgson.
Hodgson, along with Adam Sisson of the ISU Extension Corn and Soybean Initiative, provide the following information as to what's happening this year.
Research suggests about 50% of larval hatch occurs between 684 to 767 growing 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% larval hatch now (June 23, 2011) and other parts of the state will approach the 50% mark within 7 to 10 days, depending on the temperature.
Fig. 1. Accumulated degree days (base 52F, soil) Jan. 1-June 21, 2011. About 50% of larval hatch occurs between 684-767 degree days.
Soon after egg hatch, young larvae start feeding on corn roots
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 root nodes 4 to 6, which interferes with water and nutrient uptake and makes the plant unstable.
Saturated soils will diminish overall corn rootworm pressure, and the high adoption of Bt corn which has traits for rootworm protection 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 corn with Bt proteins has been used). Continuous corn fields and areas with persistent corn rootworm populations are the highest priority for inspection.
Scout corn, even if you planted corn with Bt rootworm protection
Sample for larvae by digging up corn plants and washing the roots in a bucket. The 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 number of nodes pruned.
Fig. 2. Corn rootworm larvae are creamy white with a small, brown head. Photo credit Marlin E. Rice.
Corn rootworm rescue treatments of insecticide should be made in June, but are not very practical or effective, as the corn is too tall for the insecticide 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 your corn hybrid seed selection for 2012. Here are two websites that may be of interest to help you with corn rootworm management: