Michigan Corn: Management Tips for Western Bean Cutworm

    Western Bean Cutworm larva feeding on corn ear.

    Western bean cutworm moth flight has ramped up in southern Michigan (the lower two tiers of counties) plus northern Indiana and Ohio. It is critical to scout pre-tassel and freshly tasseling fields for western bean cutworm egg masses in this region, given the overwintering population that was generated last year. For those of you in central Michigan, flight should be increasing next week.

    Scouting for this pest isn’t too difficult. Western bean cutworm egg masses are large, initially white and laid on the upper third of pre-tassel plants. With some practice, they are easy to spot, especially if you put the sun behind the corn leaves and look for their shadows.

    Scouting is important because infestations can differ a lot among fields. There is also a risk of developing pyrethroid-resistant western bean cutworm if blanket or insurance spraying is done across a wide area. We have already lost several Bt toxins against this insect, so insecticide resistance is a concern.

    Entomologists in the Great Lakes region have adopted a threshold of 5 percent of plants with egg masses. This is lower than the threshold used in the western states because of the high risk of grain quality problems (ear molds) due to favorable climate conditions near the lakes. Western bean cutworm does not impact yield in most cases, but it opens the ear up to fungi that produce mycotoxins.

    It is important to understand that this is cumulative threshold since fields can be attractive for egglaying for two or three weeks. In other words, if a field is not at threshold, scout a week later and accumulate percent infestation towards the 5 percent threshold (add percent of week one + percent of week two). Careful scouting is needed to detect low levels of egglaying that add up to a damaging population over several weeks.

    Scouting strategy for pre-tassel fields

    First, dress the part—long sleeve shirt and a face shield. Concentrate on the plants, not on the corn leaves cutting into your eye balls.

    Second, do a quick scan for a high or low infestation. Motor at a steady pace, scanning rows with the sun behind.

    • Heavily-infested fields (10 percent or more of the plants with an egg mass) are “no-brainers” and immediately apparent without more-tedious counts. These fields should be treated. Fields with high infestations tend to be sandy where overwintering is very successful (larvae get deeper in the soil) or out-of-synch—early- or late-planted compared to the surrounding landscape, acting essentially as a trap crop.
    • If you find no egg masses after scanning a few rows (hundreds of plants), the infestation is probably very low. (Although I would scan several parts of the field to be sure, and check the field again in a week if it’s still attractive.)
    • The trickier situation is when you see just a few egg masses. Move on to step 3.

    Third, if needed, do a careful scouting for infestations at or below the 5 percent threshold. Count out plants individually or by length of row, and examine them carefully for egg masses. Remember, the threshold is cumulative, so even a low infestation of 1-2 percent can be important when it is added together with low percent infestation from the next week.

    Tips for treating western bean cutworm

    • Use a long-lasting pyrethroid.
    • Time the spray for when eggs are hatching.
    • Under wet conditions, ear mold infection is also a concern.

    If Proline fungicide is to be sprayed with a pyrethroid for western bean cutworm, adjust the timing of the tank-mix to target fresh silks, which is the optimum fungicide timing to reduce infection. That timing is still adequate for western bean cutworm control.

    Ear mold management and fungicides

    The following is an important note from Michigan State University Extension field crops pathologist Marty Chilvers.

    If you are considering making a fungicide application for ear mold suppression, it is best to use a DMI-group fungicide. Currently, Proline is the only fungicide labelled for ear mold suppression in Michigan.

    The application timing concept is similar to that for managing head scab in wheat. Application timing needs to coincide with silking, as this is the primary infection point for Gibberella ear mold. Studies in Ontario demonstrated that an application at full silking gave the best suppression.

    Applications of the QoI or strobilurin group of fungicides, such as Headline, during flowering are demonstrated to increase DON levels (vomitoxin) in corn, just like it can in wheat. These fungicides should be applied prior to tassel/silk stages (i.e., not in tank-mix with an insecticide for western bean cutworm control).

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