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Issue No. 10, Article 1/May 30, 2008

Insect Issues During This Period of Slow Crop Growth and Development

Off to our slowest crop-growing season in years, the focus has been, justifiably, primarily on crop growth and development in cool, wet conditions, with less of a focus on pest issues. According to the most recent report from the Illinois Department of Agriculture (Illinois Weather & Crops, May 27), even though 87% of corn has been planted (similar to percentages at similar times in 2003 and 1999, but ahead of 2002 [71%]), only 62% has emerged. The 5-year average of percentage corn emergence at equivalent times of year is 90%. So far as we know, insects have been responsible for only a small percentage of the slower-than-average emergence of corn. And soybean planting and emergence are even farther behind--39% planted (lower than any percentage planted over the past 10 years [except 2002, 20%]) and only 4% emerged (43% is the 5-year average). We have written several articles about the overall effect of late planting and slow growth of crops on the potential for insect problems, so I won't belabor that issue. Rather, following is a snapshot of insect-related issues thus far, with more in-depth articles to be provided in the weeks to come.

Black cutworm. We have received several reports of black cutworm activity in cornfields, most of which have originated from western Illinois counties. Several people have reported good protection against cutworms by Cruiser and Poncho seed treatments, apparently providing excellent control of the younger instars. However, some fields of corn have been sprayed with insecticides to prevent additional black cutworm injury. Knowledgeable sources, though, have indicated that some insecticide applications have been made unnecessarily. Obviously, deciding to treat a cornfield with an insecticide to control black cutworms is a judgment call, and the high price for corn justifies some reduction in the economic thresholds (refer to "First Reports of Black Cutworm Injury" in issue No. 9, May 23, 2008), but reducing thresholds, even justifiably, still has limits. We are aware of at least one situation in which 0.25% or less cutting by black cutworms was observed, but the field was sprayed anyway. Spending money to spray a cornfield simply because the price for corn is high makes no sense if the return on investment is $0. Early evidence of leaf feeding or slight cutting activity does not guarantee that an economic level of damage will occur. So we encourage people to be patient and use good judgment when making decisions to control insects, including black cutworms.

Other creatures in the soil. While scouting cornfields for black cutworms or just keeping an eye on emergence, people have encountered a variety of other creatures in the soil. Thus far we have received few reports of injury caused by grape colaspis, white grubs, or wireworms, to name a few, but continued slow growth will expose corn seedlings to these insect pests into June. Remember, insects are developing slowly, too.

Given the broad array of problems in cornfields at the moment, it is possible to find nonpest creatures and incorrectly blame them for poor emergence or slow development. The entomologists at Purdue University wrote a nice article with some good photos about non-insects (e.g., millipedes, juvenile earthworms) in issue No. 8, May 23, 2008, of their Pest & Crop newsletter. We, too, encourage accurate identification of soil-inhabiting invertebrates so that money for insecticides is not spent unnecessarily.

And keep your eyes open for slugs. Ron Hammond, extension/research entomologist at Ohio State University, wrote a "Slug Update" in issue 2008-15 (May 27-June 2, 2008) for the Crop Observation and Recommendation Newsletter, indicating that reports of slug activity have increased in Ohio. Given the current weather conditions, don't be surprised to encounter slugs feeding on corn or soybean seedlings.

European corn borer. Several people have noted emergence of European corn borer adults, marking the time of year when the first generation will begin mating and laying eggs. Obviously, the females will be hard-pressed to find cornfields where first-generation larvae will survive. Recall that European corn borer larvae do not survive very well on corn in the 6th-leaf stage or smaller (i.e., with higher concentrations of DIMBOA, a compound that deters feeding and establishment of small corn borer larvae). However, if the cornfields in a given area have differential heights, focus your attention on the tallest fields, especially those planted to non-Bt corn hybrids. Also, keep your eye on the non-Bt corn refuges for harborage of European corn borers.

Bean leaf beetle. Like the difficulty of first-generation European corn borers establishing on small corn plants, bean leaf beetles will have a tough time finding early emerging soybean fields where they can deposit eggs to initiate the first generation. The bean leaf beetles being observed now in alfalfa and clover fields and in noncrop areas are the adults that overwintered and have emerged from "hibernation." They seek soybean fields in which to feed, mate, and lay eggs, so it is likely that the first generation will be diminished. On the other hand, it is very important for the growers who were able to plant soybeans relatively early to be alert for bean leaf beetles.

Bean leaf beetle feeding on newly emerged soybean seedling in 2003 (photo courtesy of Scott Stein).

Alfalfa weevil. Alfalfa weevil larvae have caused some economically threatening damage to alfalfa in several areas of the state, although there are indications that a fungal pathogen, Zoophthora phytonomi, may be suppressing alfalfa weevil populations in some fields. Cool, wet weather is amenable for the development of epizootics of this pathogen, and threatening levels of alfalfa weevil larvae can decline very rapidly. As you scout alfalfa fields, be on the alert for "not normally colored" and semi-moribund alfalfa weevil larvae. These larvae could be infected with Z. phytonomi; if they are, they will quickly die and turn brown. If a high percentage of larvae are infected, economic damage probably will be averted.

Alfalfa weevil larvae infected with and killed by the fungal pathogen Zoophthora phytonomi (photo courtesy of Matt Montgomery, University of Illinois Extension).

Even though the growing season has been fairly frustrating thus far, don't let impatience trigger haste in your insect control decisions. Assess all of the factors interacting within a field to make an informed plan.--Kevin Steffey

Kevin Steffey

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