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Issue No. 11, Article 2/June 6, 2008

All (Mostly) Quiet on the Insect Front

As has been true for several long, soggy weeks, the weather is creating most of the headlines related to crop production in Illinois and elsewhere in the Midwest. So commentary about insect management while crops in many areas are under water or not planted seems relatively insignificant. However, some folks continue to find cutworm activity in cornfields, and a few other insects have been observed, so a brief update is in order. We've speculated about as much as we can on the relationship among delayed crop development, wet weather, and insects, so on that topic we will remain mute for this issue of the Bulletin.

During a teleconference on June 2, several extension entomologists in the north-central region provided updates of the insect situation in their states. Although not all north-central states were represented, the general theme on insect issues in Illinois, Iowa, Kansas, Michigan, North Dakota, and Ohio was "not much is happening." Among the insect and mite issues mentioned were cutworms (black, dingy, sandhill, variegated, and winter [Michigan, North Dakota]), the appearance of bean leaf beetles, and scattered reports of armyworm, cereal leaf beetle, and wheat curl mite in wheat. Since then, we entomologists in Illinois have continued to receive reports about cutworm injury, mostly in a band across the central third of the state, especially the westernmost counties.

Dan Schaefer, with Illini FS in Tolono, and John Fulton, University of Illinois Extension director in Logan County, both reported some instances of economic and subeconomic infestations of black cutworms in eastern and central counties. John also noted the presence of dingy and variegated cutworms in a few fields. Loretta Ortiz-Ribbing, University of Illinois Extension specialist in Macomb, indicated that a noticeable number of cornfields in her region had been treated for control of black cutworms, and Robert Williamson, with Agrivest in Jacksonville, has reported that fields are being sprayed for control of black cutworms in his area. Loretta also noted the presence of dingy and sandhill cutworms.

In fields that have not been flooded and in which scouting is still possible, continued scouting for cutworms will be necessary for at least two more weeks, depending on the stage of corn growth. When the corn plant reaches the 4-leaf stage or larger, cutting by black cutworm larvae is less likely. However, black cutworm larvae also have a tendency to bore into the bases of larger corn plants, and such injury can kill the growing point. This injury, often referred to as "dead heart," results in corn plants with wilted center leaves. So until the progeny from black cutworm flights in May develop into pupae and moths, the threat from the larvae persists. (Please note that other pests, including stalk borer, can cause dead heart as well.)

"Dead heart" symptom caused by tunneling of black cutworm larvae (photo courtesy of Robert Bellm, University of Illinois Extension).

Most of our attention on cutworms is justifiably focused on black cutworms annually. However, the other species mentioned in this article are important, too, some because they might cause economic damage (e.g., sandhill cutworm) and others because they rarely cause economic damage (e.g., dingy cutworm) but may be misidentified as black cutworms.

People who want to learn more about the identification, life cycles, and distributions of cutworms that threaten corn should consider buying the Handbook of Corn Insects, published by the Entomological Society of America and available through APS Press. The cutworm species discussed on pages 68 to 77 in the handbook are black, bristly, bronzed, claybacked, dingy, glassy, pale western, redbacked, sandhill, spotted, and variegated. Most of the discussions are accompanied by excellent photographs from Marlin Rice (Iowa State University, Ames) and Roy Rings (Ohio State University, Columbus). (All other insect and mite pests of corn also are discussed in detail, by the way.) A quick source of discussion and photos of some of these cutworm species is the article "Correct Identification of Cutworm Species Is Important Each Spring" in the Bulletin back in 2001, issue No. 5 (April 27).

Although less than half of the soybeans planted in Illinois have emerged thus far, according to the June 2 issue of Illinois Weather & Crops, bean leaf beetles are finding some of the early planted soybeans. If early planted and emerging soybeans are few and far between, bean leaf beetles will accumulate in relatively few fields. Consequently, the injury could be significant. Direct your scouting energies to the most advanced soybean fields in a given area, and watch for the telltale chewing injury on cotyledons and newly emerged leaves. Under normal circumstances, bean leaf beetles have to be numerous (16 to 39 beetles per foot of row at stage V2+) to cause economic damage. However, with soybean seedlings struggling to grow and considering the current and future values of soybeans, lower rational thresholds can be considered.

Bean leaf beetle injury to a soybean seedling (University of Illinois Extension).

If an insecticide for control of bean leaf beetles in soybeans is justified, products suggested for their control in Illinois can be found in Chapter 1, Table 2, page 11, of the 2008 Illinois Agricultural Pest Management Handbook (Adobe PDF).

It seems trite to say that time will tell what effects delayed crop growth and future weather conditions will have on insect problems in field crops, but it's true. This spring has been one for some record books, so we should learn more than we already know.--Kevin Steffey

Kevin Steffey

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