Black Cutworms Are Here

 

In the process of sampling fields for grubs this week, I came across black cutworm (BCW) larvae. It’s been a cold wet spring with delayed tillage and herbicide spraying. Winter annuals that were green in April were attractive for egg laying by moths that moved up from the south. Although I didn’t see a high number of larvae, I found them in every field we sampled in Southeast Michigan (Monroe) and St. Joseph (Southwest Michigan).

 

The larvae were ¾ - 1 inch, probably fourth or fifth instar, with 2 more weeks of feeding left. The fields I was sampling were not planted yet, but were recently burned down or about to be sprayed, thus killing the weedy food source for the larvae. Do not spray fields for cutworm unless it is there! Based on trap numbers in Indiana, I don’t expect a lot of colonization. Simply be aware and prepared that there could be hot spots out there, because big larvae will already be present in a field as a delayed corn crop emerges.

 

 

Moths migrate into Michigan each spring. Female moths lay eggs on low-growing plants in and around fields, especially on winter annuals. Favorable areas for egg-laying are fields with poor or delayed weed control, a later-terminated cover crop, reduced /no-till (generally more spring weeds) or  weedy field edges.

 

Tiny larvae are usually present by early to mid-May; initially they are very difficult to detect. Larvae feed above-ground on weeds or early-emerged crops, creating small pinholes or feeding on leaf margins. This damage is not obvious. Older larvae move from dying (sprayed) weeds or the field margin into the crop, usually corn but sometimes soybean and sugarbeet.  They feed below ground, cutting plants at the base so that they wilt or die. 

 

 

Examine rows (especially along field margins or in parts of the field that were weedy) for wilted plants. To determine if black cutworm is the culprit, dig at the base of wilted plants to find larvae. They are black to gray in color, and usually curl up when handled. If larvae are not present, move to the closest undamaged plant and dig again. 

 

Larvae hide during the day and feed at night, so the best time to look for them is in the morning. The threshold for corn, soybean, and beets is 5% of plants cut AND larvae less than 1 inch long.

 

Again, do not simply add an insecticide to a herbicide application. Simply be aware of the potential for larvae as you check fields later in May. Many pyrethroids are effective rescue options for this insect. Some fields may only require spot treatment of border rows or areas where weed control was poor. Apply sprays in the evening if possible, because larvae emerge at night to feed.