MSU Extension

Wet conditions continue planting delays in many parts of Michigan. The one region that has seen favorable conditions for field work has been the southern part of the state. Planting progress, while not stellar, has been steady here, but as we work to complete planting, growers should also continue to keep a pair of “sometimes” pests on their radar screens. Purdue University reports in their “Pest and Crop Newsletter” that black cutworm and armyworm moth trap catches have been elevated in Indiana during the last three weeks (as of May 23, 2014).

While we have not seen large numbers of armyworm caught in the Michigan State University Wheat Watcher’s network, we also do not have that many traps set across the southern portion of the state. The number of black cutworm traps set up across the region is even lower. With or without good moth flight monitoring, the best thing to do is scout your fields for incidence of these insects.

It is important to understand that the flight paths of these insects are highly sporadic. These pests do not overwinter in Michigan. Their arrival in the state is driven by thunderstorm updrafts and deposition. Also, in order for a particular field to be susceptible for damage, green tissue that is attractive for the female moths to lay eggs on has to be present when the moths arrive. For black cutworms, this often occurs when we have fields where there is an abundance of winter annual weeds. For armyworms, fields planted to wheat or that have cereal cover crops or grass weed infestations before burn-down for herbicide applications occurred are the most susceptible to damage.

Corn and seed corn are the most significantly impacted crops for this pest, although cutworms can clip soybean plants off at the stem as well. Focus scouting efforts on fields that had abundant green materials on Mother’s Day weekend, May 10-11, and beyond. It takes around 300 growing degree days (GDD) base 50 from egg hatch to larvae large enough to cause clipping injury for black cutworms.

Growers should not apply insecticides as a preventative treatment for these pests. Treatment with an insecticide is not recommended by Michigan State University Extension unless fields are reaching threshold levels of damage. Even in “really heavy outbreak” years, field-to-field variability in pest population is so great that scouting for damage is needed to decide if treatment is warranted. Also, there are differences between the various Bt corns in their protection level against black cutworm and armyworm larval feeding. Some seed treatments also provide some protection. Bt hybrids that contain the Cry 1F (Herculex/Smartstax) and Viptera events provide a little more protection against cutworm larval feeding.

Keep in mind, however, that overwhelming numbers of cutworms can cause clipping injury in just about all Bt corns, with or without soil insecticides applied. Be sure to consider whether or not your hybrid is likely to control black cutworms, especially if you are seeing sporadic damage in the field and you planted a “refuge in the bag” variety. MSU entomologist Chris DiFonzo’s Handy Bt Trait Table can help you to determine if your varieties are susceptible.

Black cutworm feeding injury

Typical black cutworm feeding injury symptoms are plants that are clipped off near the soil line. Early instar larval feeding often looks like small shotholes in the leaf tissue in corn.

Threshold: Treat when 5 percent or more of plants show cutworm damage. Read more information on black cutworms and view photos.

Armyworm feeding injury

Corn: Armyworm feeding injury in small corn usually occurs on the leaf margins, leaving a ragged appearance to the impacted leaves. In high populations on slightly larger corn, the pest will eat the leaves down to the mid-rib.

Threshold: Treat when 25 percent of plants have two or more larvae per whorl, or 75 percent of plants have one larva. Treat only if caterpillars are less than 1.25 inches in length. You may be able to limit spray to the field edge if armyworms invade from another field or grassy border. 

Wheat: Look for ragged leaf edges in wheat. In badly infested fields, nearly all of the leaf tissue may be gone. The insects can also clip off the heads.

Threshold: Before heading, treat when there are four or more worms per square foot. At heading, to prevent head clipping, treat when there are two or more worms per square foot. 

Please note: The pesticide options listed in the links directly above have not been able to be updated since the 2010 growing season. There may be additional options available. Growers should visit with your agri-chemical suppliers and consult the label carefully to select and use products in an appropriate manner.