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Issue No. 3, Article 1/April 11, 2008

The Potential for Black Cutworm Problems Increases When Corn Planting Is Delayed

Crop producers and others involved in agriculture are keeping track of and bemoaning the continuing storm fronts that keep dropping lots of rain onto already sodden fields. Near-future forecasts suggest more rain is on the way. To add insult to injury, the storm fronts likely are carrying adult black cutworms into Illinois. As the females "drop out," they will seek attractive sites for laying eggs, and they should have no trouble finding plenty of fields in which weeds are growing. Winter annual and perennial weeds, in particular, will attract egg-laying black cutworm females. Consequently, as planting is delayed, black cutworm larvae will hatch and begin feeding on the weeds. As the weeds are killed, the cutworms will need additional food and will be large enough to cut corn plants as seedlings emerge from the soil.

In the past we have had an extensive network of black cutworm pheromone traps throughout Illinois. The captures of black cutworm adults allowed us to keep tabs on flights of black cutworms into the state and to detect "intense captures" (nine or more adults captured over 2 days). An intense capture is the biological trigger, or biofix, we use to begin accumulating degree-days (above 50°F) to predict development of the larvae (Table 1), the stages that cause considerable grief when numbers of black cutworms are large. However, due to a number of factors, we no longer have the network of traps on which to rely for timely and localized information. Instead we rely on reports from others who voluntarily monitor for black cutworm adults early in the spring. (This is a not-so-subtle request to send us information about your captures so that we can share the reports.)

Thus far I have heard from only one individual about captures of black cutworm adults. Doug Gucker, with University of Illinois Extension in Piatt County, caught two black cutworm adults on April 4. To interpret your own findings if you are monitoring for black cutworm adults in Illinois, refer to the degree-day calculator. After you select "black cutworm" and a location, you will be asked to enter the date of the first significant moth flight (i.e., intense capture). You will obtain the accumulated degree-days (above 50°F) from the date of the moth flight to the most recent date in the database, as well as 1-week and 2-week projections and the projected date when 300 degree-days (potential for cut plants) will accumulate for that site.

After the larvae hatch, the early (1st through 3rd) instars of black cutworms don't usually cause economic damage because they feed primarily on leaf tissue. However, by the time the larvae become 4th instars, they begin cutting plants off and dragging them into their "burrows" to feed. As the larvae grow through their 6th or 7th instars (the number depends on diet and temperature), they require increasingly more food, so the larger larvae may cut off several corn seedlings. Depending on temperature, larvae develop over 28 to 35 days. You can read much more about black cutworms and their biology on our IPM Web site.

Some concern has been expressed about the efficacy of low rates (0.25 mg of active ingredient per seed) of seed-applied insecticides (i.e., Cruiser, Poncho) against black cutworm larvae. Experience has shown that the low rate of these seed-applied insecticides may not provide satisfactory control of black cutworms if the larvae causing the injury are large (4th through 6th instars). This reduced efficacy also has been observed under the same conditions for soil insecticides and corn hybrids with Herculex I Insect Protection. The take-home message for corn producers is that early-season scouting for black cutworms is essential, even if products for black cutworm control have been used. Rescue treatments for black cutworms usually are very effective if the infestation is detected early enough.--Kevin Steffey

Author:
Kevin Steffey

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