The good news is those areas of Iowa which have been receiving too much rain lately and continue to receive a lot of rain should experience some drowning of corn rootworm larvae. "The larvae stage is susceptible to drowning and we did see a lot of that occur last year in many parts of the state," says Mark Johnson, Iowa State University Extension field agronomist in central Iowa.
Once the eggs hatch and the small larvae start to feed on corn roots, you need to assess this larval feeding activity to make future rootworm management decisions. It's best to begin looking for damage about seven to 10 days after rootworm egg hatch reaches its peak for your area, he says.
The optimum time for scouting and evaluating cornfields for rootworm injury should occur from middle of July through early August, adds Erin Hodgson, ISU Extension entomologist. Primary fields of concern and where to start first are continuous cornfields and areas and fields with Bt corn rootworm trait performance problems.
Rootworm egg hatch in Iowa in 2015 is slightly behind normal
Corn rootworm egg hatch in Iowa typically occurs from late May to the middle of June, with an average peak hatching date of June 6, says Hodgson. In 2015, the average hatching date will be slightly behind normal and approximately the same time as in 2014. Development is driven by soil temperature and measured by growing degree days. Hodgson provides the following scouting guidelines and management suggestions.
Research suggests about 50% of egg hatch occurs between 684 to 767 accumulated degree days (base 52 degrees F, soil). A few areas of Iowa had already reached peak corn rootworm egg hatch a week ago, on June 9 (See Figure 1), particularly around Muscatine. Many other regions will reach 50% egg hatch this week, by June 20.
To generate degree day accumulation on corn rootworm egg hatch for your area, use this website. To create an accurate map, make sure to set the start date to January 1 of the current year and the end date to today, and set the plot parameter to "soil growing degree days (base = 52)." Be aware the website is having some technical difficulties with the soil temperature probes this year.
A severe corn rootworm larval infestation can destroy nodes 4 to 6; each node has approximately 10 nodal roots. Root pruning can interfere with water and nutrient uptake and make the plant unstable (Photo 1). A recent meta-analysis showed a 15% yield loss for every node pruned.