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Issue No. 16, Article 1/July 11, 2008

Insects Taking Their Toll in Corn Whorls

During the past week, reports of insects causing injury by feeding in corn whorls increased markedly from the previous week. We had already provided information about corn earworms feeding in corn whorls ("Corn Earworms Found Feeding in Corn Whorls," issue No. 15 of the Bulletin, July 3, 2008), but we learned shortly thereafter that fall armyworms and Japanese beetles had joined the party in some areas. Fall armyworms have been observed feeding in corn in southwestern, western, central, and eastern counties (e.g., fourth and fifth instars in a field in Champaign County on July 7), and the numbers of Japanese beetles being found, in southern Illinois in particular, are extremely high. Visit "The Hines Report" for some eye-popping numbers of adult Japanese beetles captured in traps in Franklin, Marion (43,957 in the week ending July 8!), Pulaski, Sangamon, and St. Clair counties. We also have received several reports of large numbers of Japanese beetles in areas throughout central Illinois. And on top of all of this, a familiar face--first-generation European corn borer--has been observed in very large numbers in fields of non-Bt corn in some areas this year. Robert Bellm, University of Illinois Extension crop systems educator in Edwardsville, visited a field in Greene County during the week of June 30 that was a "tunneled and shot-holed mess."

Injury to corn caused by first-generation European corn borer larvae (University of Illinois Extension).

So, what gives? Well, apparently this gang of corn defoliators is thriving on current and past weather conditions. Based on the numbers of Japanese beetles being observed, the 2007-2008 winter temperatures caused little mortality of overwintering grubs. The same can be said for overwintering European corn borer larvae. In addition, the stormy weather this spring has had considerable impact on the situation. During most years at this time, corn has pollinated or is pollinating all over the state. Consequently, during most years, we focus primarily on insects that clip silks (corn rootworm adults, Japanese beetles) rather than on insects that feed in corn whorls, injury that is usually fleeting. However, delayed planting and delayed corn development this year have synchronized V6-VT stages of corn with a host of defoliating insects. Some of the insects (corn earworm, fall armyworm) were aided by weather patterns that helped the adults arrive relatively early and in large numbers in Illinois.

Unfortunately, our knowledge about the effects of insect defoliation in corn is sadly lacking, particularly in comparison with the nearly exhaustive research focused on insect defoliation of soybeans. During a July 7 teleconference, extension entomologists in the north-central states briefly discussed this issue, and the consensus was that we know very little about insect defoliation of corn. For a benchmark, we can use information on yield loss as a response to the percentage of leaf area destroyed by hail (for hail insurance adjustment). However, defoliation caused by hail usually occurs all at once, whereas insect defoliation occurs over time, during which corn development advances through several growth stages. Nonetheless, some information from the leaf-loss table (page 68) in the Corn Loss Adjustment Standard Handbook, 2007 and Succeeding Crop Years might be helpful. The handbook, identified as FCIC-25080 (11-2006) and published by the USDA Federal Crop Insurance Corporation, is available online (Adobe PDF). As most people know, corn at earlier growth stages suffers less loss of production from defoliation than corn at growth stages closer to tasseling and silking. For example, 10% or 11% lost production occurred at the following combinations of growth stage and percentage leaf area destroyed:

  • 8-leaf stage: 95% leaf area destroyed
  • 9-leaf stage: 85% leaf area destroyed
  • 11-leaf stage: 65% leaf area destroyed
  • 12-leaf stage: 55% leaf area destroyed
  • 14-leaf stage: 45% leaf area destroyed
  • VT stage: 25% leaf area destroyed

It is likely that the rule-of-thumb threshold for fall armyworms feeding in the whorls of corn (75% of the plants with whorl damage) is derived from these hail damage adjustment figures.

Fall armyworm injury to vegetative stage corn (University of Illinois Extension).

Fall armyworm larva; note the inverted white "Y" on the head and pronotum of the insect (photo courtesy of Andy Knepp).

In general, defoliation of corn caused by insect pests almost always looks worse than it is in its effect on corn yields, even with the current high value for corn. So we encourage you to carefully consider the amount of injury, the pest species involved, and the stages of the insects present before making a decision to apply an insecticide. Some people have expressed concern that insects might destroy developing tassels (not really a major issue), eventually clip silks (Japanese beetle), or eventually infest corn ears (corn earworm, fall armyworm), and they want to make control decisions based on these suppositions. I suggest, however, that the focus on insect control decisions should be on what is true rather than on what might happen. The decision to protect the pollination process from silk clipping caused by Japanese beetles (refer to "Preparing for Silk-Clipping Insects" in this issue of the Bulletin) should be made at a more appropriate time, not when corn is in vegetative growth stages. Japanese beetles are highly mobile and could just as likely move out of a given cornfield as stay put. Also, the corn earworm and fall armyworm larvae currently feeding in corn whorls soon will pupate and develop into adults that may or may not lay eggs in the same field.

If you determine that an insecticide application is necessary to protect yield, several insecticides are labeled for control of all of the aforementioned insect pests of corn (Chapter 1, pages 4-10, of the 2008 Illinois Agricultural Pest Management Handbook). Please follow all insecticide label directions and precautions. If you intend to tank-mix an insecticide with a herbicide, consult the labels of both pesticides beforehand.--Kevin Steffey

Kevin Steffey

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