No. 6 Article 1/May 4, 2007

Black Cutworms Could Pose a Threat to Corn Fields in Illinois in 2007

Everyone knows that forecasting infestations of black cutworms is based more on assumptions than on fully developed equations. We can use degree-day calculators to make educated guesses about the first occurrence of cutting damage, but that's about it. We can also compare captures of black cutworm moths in pheromone traps in the current year with captures during previous years, but such comparisons are relative and may not mean much in the long run. However, corn is being planted later this spring than what we have become accustomed to, and moths continue to fly into the state. So it's entirely possible that black cutworm larvae will have developed to third instars or larger (the cutting instars) by the time corn seedlings are most susceptible to cutting damage. If you intend to scout for cutworm injury, consider fields that had plenty of winter annual or perennial weeds before corn was planted as your first-priority fields for scouting.

Black cutworm larva and chickweed in 2002 (photo courtesy of Andy Knepp).

Over the past week, we received several reports of intense captures (9 or more moths captured in 1 or 2 days) of black cutworm moths in pheromone traps--from as far south as St. Clair County to as far north as Lee County. Several trap operators in Adams County reported intense captures, and we have received or read reports of evidence of pinhole-feeding injury by early instars. Ron Hines, seed agronomist for Growmark's southern region, observed pinholes in leaves of emerging corn in Massac County on April 30. Lisa Coorts of Maxi-Yield Consultant Service found a few small black cutworm larvae in a field in southwestern Christian County during the April 28 and 29 weekend.

Be prepared, especially in areas where corn has yet to be planted or has just gone into the ground. Populations of black cutworms will only increase as moths continue to fly into Illinois. But even in areas with corn emerging, be sure to check fields for black cutworms and symptoms of their feeding injury. As we have indicated many times, leaf-feeding injury is not economic, but it is a precursor to stand-reducing cutting damage.

Leaf-feeding injury caused by early instar black cutworms (University of Illinois).

Black cutworm larva and cut corn seedling (University of Illinois).

As you scout for black cutworms, always be aware that you may encounter other cutworm species. One of the more commonly observed cutworms in corn fields is the dingy cutworm, which usually does little cutting and poses little to no threat to corn stands. Because the dingy cutworm and black cutworm are similar in appearance, you need to be able to distinguish the two species. Whenever I need to see distinguishing characteristics on pest insects, I turn to Marlin Rice's excellent photos. In the May 17, 2004, issue of Iowa State University's Integrated Crop Newsletter, Marlin wrote about black and dingy cutworms and presented comparative photographs. The most distinguishing characteristic between the species is the size of tubercles on the top surface of the caterpillars' respective abdomens.

In last week's issue of the Bulletin (No. 5, April 27, 2007), I indicated that corn hybrids with Herculex I Insect Protection (with the Cry 1F Bt protein) are labeled for control of black cutworm larvae. However, I failed to mention that if Bt corn is planted for control of black cutworms, the refuge requirement is the same as it is for European corn borers. We talk about non-Bt corn refuges for insect resistance management associated with corn borers and corn rootworm all the time, but we usually fail to mention that the same rules apply for non-Bt corn refuges for other species indicated on the "labels" of transgenic Bt corn hybrids. In light of a rather continuous stream of reports that refuges are not going to be planted, it's important to reiterate the requirement that non-Bt corn refuges must be planted when a grower intends to plant Bt corn.

Several insecticides (Table 1) are labeled for control of black cutworm larvae when cutting injury reaches or exceeds the threshold of 3% to 5% of the plants cut with larvae still present. In general, all of these products are effective for control of black cutworm larvae, although the level of control may vary depending on soil conditions. Please read and follow all label directions and precautions for whatever insecticide is selected, if control is warranted.--Kevin Steffey

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