No. 19 Article 2/August 1, 2008

Numbers of Western Bean Cutworm Adults Captured in Pheromone Traps Have Increased

Several people in Illinois, as well as elsewhere in the Midwest, have recently reported marked increases in numbers of western bean cutworms in pheromone traps. The counts from individual traps can be viewed at the Iowa State University "Western Bean Cutworm Monitoring Network." Numbers of western bean cutworm moths captured in several Illinois traps are in triple digits. Some University of Illinois Extension educators have reported counting more western bean cutworm moths in traps this year than during any of the preceding three years that they have been monitoring this insect pest. Some of the captures in Illinois this year rival those being reported in Iowa. If we are at peak flight, the peaks are at least a couple of weeks later than during most years.

Lots of western bean cutworm moths captured in a pheromone trap near Lena, Illinois, in Stephenson County (photo courtesy of David Feltes, University of Illinois Extension).

Western bean cutworms also have continued their eastward momentum. Chris DiFonzo, extension entomologist at Michigan State University, reported that moths have been captured throughout Michigan eastward to counties near Lake Huron. The densities of egg masses and small larvae are greater than the 8% infestation threshold in some cornfields in northwestern Michigan; more than 30% of plants are infested in some fields. Chris also reported that some insecticides are being applied for control of western bean cutworms. Andy Michel and Ron Hammond, entomologists at Ohio State University, reported that as of July 28, 114 western bean cutworm moths had been captured in pheromone traps in Ohio this year, compared with a total of 6 captured in 2007. Moths have been captured as far east as Wooster, Ohio. Tracey Baute, entomologist with the Ontario Ministry of Agriculture, Food and Rural Affairs, indicated that western bean cutworm moths had been captured at all trap locations in Ontario, although the numbers were low.

I have stated many times that the western bean cutworm has not established itself as a real threat to corn production in Illinois. However, the numbers of adults being captured this year may alter my perception. I think it is important, especially in northwestern Illinois, to obtain an accurate assessment of the range and average infestations of larvae in corn ears this year. If you are making regular field calls, please let us know what you find if you search for western bean cutworms. Remember, finding the insects is important. Finding injured ears in the fall, after all of the potential culprits have developed beyond larval states, does not necessarily signify a western bean cutworm infestation. Corn earworms and fall armyworms both cause injury to corn ears that is similar to injury caused by western bean cutworm larvae. Accurate association of injury with the causative agent is essential if we hope to make reliable insect management plans for forthcoming years.

Western bean cutworm egg mass in a field near Lena, Illinois, in Stephenson County (photo courtesy of David Feltes, University of Illinois Extension).

When scouting for western bean cutworms (an activity that should be underway right now), look for egg masses and small larvae first on the upper leaves. The larvae eventually work their way down corn plants to the ears, after which control with insecticides is not effective. As I stated previously, the current economic threshold is 8% of plants infested with eggs and/or small larvae. Insecticides suggested for control of western bean cutworms in corn are presented in Table 1, page 9, in the 2008 Illinois Agricultural Pest Management Handbook. Please follow all label directions and precautions.--Kevin Steffey

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