Corn rootworm egg hatch in Iowa typically occurs from late May to the middle of June, with an average hatching date of June 6. Development is driven by soil temperature and measured by growing degree days, says Erin Hodgson, Iowa State University Extension entomologist. She provides the following information to keep in mind and help you if you are checking corn rootworm infestations this year. Research suggests about 50% of egg hatch occurs between 684 to 767 accumulated degree days (base 52 degrees F, soil). Cooler spring temperatures mean slower egg maturation in 2014. But a few areas of Iowa are approaching 50% corn rootworm egg hatch now (Fig. 1), particularly around Muscatine. Many other regions will be reaching 50% egg hatch within the next two weeks.
To generate up-to-date information for your area, go to mesnoet.agron.iastate.edu for current degree day accumulation for corn rootworm eggs. 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)."
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 (See photo 1 in this article). A recent meta-analysis showed a 15% yield loss for every node pruned.
Several environmental factors influence the survivorship of corn rootworm eggs. Crop rotation or the use of Bt corn should decrease populations in most fields. Regardless of the agronomic practices, every field should be scouted for corn rootworm injury (i.e., dig and rate corn roots even if Bt proteins are used).
Continuous cornfields and areas with Bt performance issues are the highest priority for inspection. Assess corn rootworm feeding and adjust management strategies if the average injury is above 0.5 on a 0-3 rating scale. Aaron Gassmann, Iowa State University corn entomologist, has a webpage for additional corn rootworm management information, including an interactive node-injury scale demonstration and efficacy evaluations at www.ent.iastate.edu.
Best management practices to control corn rootworm
Continued use of the same rootworm-resistant trait in corn hybrids, an increase of corn-on-corn areas, plus a lack of refuge acres, all have caused rootworm resistance to become an issue in some growing areas. Best management practices, including rotation, are imperative to effectively controlling corn rootworm (CRW) populations.
Farmers can no longer merely rotate between corn and soybeans as the only effective management strategy. Both the Northern Corn Rootworm and the Western Corn Rootworm have developed methods to counteract the effects of rotation on their life cycles. The Northern species uses a tactic called Extended Diapause that allows its eggs to lay dormant during the time fields are in soybeans. The eggs will then hatch after the field is planted to corn, and the young worms will feed on the corn roots. Adult females of the Western species have actually learned to lay their eggs in soybean fields, thereby allowing them to hatch the following spring when the field is planted to corn.
With these changes in what used to be considered the "normal" life cycles of rootworms, managing CRW today requires increased diligence. Corn rootworm management is a complex issue, notes Gassmann. There are many factors and management options that must be considered. He encourages farmers to dig some roots and check for signs of rootworm damage this summer. The newly hatched larvae can be found feeding near the corn roots on hairs and root tissue. Once they grow larger the rootworm larvae are forced to enter the roots to meet their growing demand for food.