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Issue No. 9, Article 1/May 23, 2008

First Reports of Black Cutworm Injury

At the end of the week of May 12 and beginning of the week of May 19, we received our first reports of signs of black cutworm larval activity. Robert Williamson with Agrivest in Jacksonville found very small black cutworm larvae associated with pinhole feeding injury to corn seedlings on May 14. On May 20, he reported slight cutting of seedlings in cornfields in northern Pike County (although not confirmed as black cutworms). On May 19, we received reports of cutworm larval feeding in cornfields in southern Champaign County (Dan Schaefer, Illini FS, Tolono) and in Hancock County (Mike Roegge, University of Illinois Extension, Quincy).

You may recall from previous articles in the Bulletin that from intense captures of adults in April (nine or more adults caught over two nights), projected dates for cutting activity (300 degree-days above 50°F, fourth instar cutworms) were in mid-May. Well, the predictions are only as good as the weather will allow, and the cool temperatures that have prevailed have slowed development of black cutworms. You can check it out for yourself with the degree-day calculator. On May 21, I clicked on "Black cutworm" and Perry on the map, then entered 04/15 when prompted for a date of intense capture. The actual total on May 21 was 296 degree-days, whereas the average (based on 11 years of weather data) was 358 degree-days. This simply illustrates that regardless of predicted dates for cutting activity in your area, it's important to be watchful for the early signs (e.g., pinhole leaf feeding), just to be aware of the possibilities. Make sure, however, that black cutworms are the culprits if you discover uneven emergence, which is not uncommon under the current circumstances. There's no reason to waste money to control something that isn't there.

Small instar black cutworms may chew on corn tissue as soon as the coleoptile pushes through the soil surface and the first leaf emerges. Small cutworms may feed on the shoot before the leaf unrolls, and the result is a row of small holes across the leaf when it unrolls. This type of feeding will continue until the cutworm larvae are large enough to cut plant tissue. Plants that are cut off early and above the growing point usually recover relatively well because the cutworm may move to another plant. Plants cut off below the growing point obviously don't recover.

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

Keep in mind that black cutworm adults have been flying into Illinois for well more than a month (based on captures in pheromone traps). Moths that flew into the state in late April and early May will give rise to larvae well into June, assuming the larvae survived all of the wet weather. So people will have to maintain vigilance for cutworm larvae for some time to come. It's also important to note that we continue to capture black cutworm adults, so replanted corn also will be subject to a cutworm threat.

Drs. Jon Tollefson and Marlin Rice, entomologists at Iowa State University, have proposed economic thresholds based on modern prices for corn and insecticides. On May 15, they published an article in Integrated Crop Management News. Before publication, they sent a draft of the article requesting feedback and input from other extension entomologists in the Midwest, and there was consensus about their revised approach to determining an economic threshold for black cutworms. The range of percentages of cut plants, depending on the size of the larvae and the corn seedlings, has always allowed for some flexibility in making decisions about controlling black cutworms. Simple math with modern numbers will offer the same flexibility. But keep in mind that economic thresholds on the lower end of the scale approach levels for which an insect injury-plant yield response cannot be measured easily.

In the Black Cutworm fact sheet on our IPM Web site, we provide a table of yield loss factors based on the level of soil moisture--adequate or inadequate--indicating that damage caused by black cutworms may be worse when there are dry soil conditions. I spent some time trying to find the original research from which the tables of yield loss factors were developed, but I was unable to locate it in time to include it here. I will continue searching and provide a follow-up as soon as possible.

Insecticides registered for control of black cutworms are presented in Table 1, page 5, of the 2008 Illinois Agricultural Pest Management Handbook (Adobe PDF). The threshold indicated is "3 to 5% or more of the plants are cut and larvae are present." Use the previously mentioned adjustment in economic thresholds if you intend to consider modern prices.--Kevin Steffey

Kevin Steffey

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