By Adam Sisson
The black cutworm (BCW) is a migratory pest that cuts and feeds on early vegetative-stage corn. Black cutworm moths arrive in Iowa with spring storms each year. These moths lay eggs in and around fields and the emerging BCW larvae cut seedling corn. The sporadic nature of this pest makes scouting essential to determine if management is needed. Scouting for BCW larvae helps to determine if an insecticide application will be cost effective.
When to scout for BCW caterpillars is based on the “peak flight” of moths and accumulating degree days after the peak flight. Degree days are a measure of temperature used to gauge the developmental progress of the insect. A peak flight for BCW is defined as capturing eight or more moths over two nights in a wing style trap baited with a pheromone lure.
To find out when moths arrive in Iowa, cooperators around the state monitor pheromone traps and report moth captures. Cooperators started checking traps in the beginning of April, and the first BCW moths captured were part of a peak flight in Woodbury County on March 28. Moth captures peaked in several parts of the state in mid-April, with several peak flights recorded. The peak flights recorded during this time period were in line with the predicted corn cutting dates in surrounding states.
The map (Fig. 1) shows predicted BCW cutting dates for the nine Iowa climate divisions, based on actual and historical degree day data and peak flights during late March and April. We continued to see a few peak flights after mid-April, indicating several populations of BCW moths moving to Iowa. However, adult moth trap captures do not necessarily mean there will be economically significant BCW infestations in a particular location. Field scouting is essential to determine if an economically damaging infestation exists.
Figure 1. Estimated black cutworm cutting dates for each Iowa climate division based on peak flights of moths occurring in 2016. *Dates in red are cutting estimates based on a late March peak flight observed in Woodbury County. Scouting should start at the earlier date in these areas.
Scouting
Poorly drained, low lying, or weedy fields, as well as those next to natural vegetation or with reduced tillage, may have higher risk of BCW injury. Those cornfields with poorly terminated cover crops may also be attractive to egg-laying females. Late-planted corn can be smaller and more vulnerable to larval feeding. Some Bt hybrids provide suppression of BCW (e.g., Vip3A, Cry1A.105, Cry2Ab2, and Cry1F proteins), but larvae can still cut young plants.
Scouts are encouraged to start looking for any activity during early season stand assessment, or at least several days before the estimated cutting dates. Early scouting is important because local larval development may be different due to weather variation within a climate division. Fields should be scouted for larvae weekly until corn reaches V5. Examine 50 corn plants in five areas in each field for wilting, leaf discoloration and damage, or those that are missing or cut (Figure 2). Flag areas with suspected feeding and return later to assess further injury. Larvae can be found by carefully excavating the soil around a damaged plant.
Figure 2. Black cutworm larval injury usually begins above the soil surface. Leaf feeding (left) may be observed. As larvae mature, they can severely damage or kill plant (right).
Identification
BCW larvae have grainy, light grey to black skin and four pairs of fleshy prolegs on the end of the abdomen (Fig. 3). There are pairs of dark tubercles, or bumps, along the side of the body. The pair of tubercles nearest the head is approximately 1/3 to 1/2 the size of the pair closest to the abdomen (Fig. 4). BCW larvae can be confused with other cutworms and armyworms. Certain characteristics can be used to tell species apart and are summarized in this article on cutworm identification.
Figure 3. Black cutworm larvae have grainy and light grey to black skin.
Figure 4. Black cutworms can be distinguished from other larvae by the dark tubercles on the middle of the back. On each segment, the tubercle closest to the head is about 1/3 the size of the tubercle closest to the rear, as shown above.
Thresholds
Common thresholds for seedling, V2, V3, and V4 stage corn plants are 2, 3, 5, and 7 plants cut out of 100, respectively. A dynamic threshold for BCW may be useful with corn price and input fluctuations. An Excel spreadsheet with calculations built in can be downloaded here and can be used to help with black cutworm management decisions.
Preventive BCW insecticide treatments applied as a tank-mix with herbicides are a questionable practice. BCW is a sporadic pest and every field should be scouted to determine insect presence before spraying insecticides.