2020 Scouting Recommendations for Black Cutworm

Black cutworm (BCW) is a migratory pest that arrives in Iowa with spring storms each year. BCW moths lay eggs in and near crop fields, and larvae can cut corn seedlings or feed on leaves. Even though crops were planted earlier this year than previous years, cold temperatures may slow growth and allow BCW larvae to coincide with early vegetative corn that is vulnerable to BCW injury.

BCW is a sporadic pest, making it essential to scout to determine whether management is required. When to scout for BCW larvae is based on when a “significant flight” of moths arrives in Iowa; accumulating degree days after this significant flight can predict when BCW eggs will hatch and larvae will be active. A flight is considered significant when eight or more BCW moths are captured in a wing-style pheromone trap over two consecutive nights.

39 BCW traps were established by 36 cooperators in 34 counties around the state this year. These cooperators monitor pheromone traps and report moth captures to track moth movement in Iowa. BCW moths were reported almost immediately after cooperators began checking traps April 1. Moth captures continued through April, with one peak flight occurring in Washington County (SE) on April 12 and subsequent peak flights in Washington County and Marshall County (C) on April 28. Even though only two counties reported significant flights, statewide BCW moth captures increased dramatically during the week of April 26.

Figure 1 shows the predicted cutting dates for BCW for each crop reporting district. These cutting dates are predicted using actual and historical degree day data combined with the occurrence of peak flights during April. Predictions are made using the most accurate data available to us. BCW trapping will continue throughout May, and any additional peak flights that occur will be included in our weekly ICM Blog updates.

Capturing BCW moths in a pheromone trap does not necessarily mean there will be economic infestations in a particular location. Scouting fields is the only way to determine if BCW are present and whether management is warranted.

Identification

BCW larvae have grainy, light grey to black skin and four pairs of fleshy prolegs at the end of the abdomen (Figure 2). There are pairs of dark tubercles, or bumps, along the sides of the body. The tubercles are used to distinguish BCW from other similar cutworm species, like dingy cutworm (Figure 3), that rarely cause economic injury in corn. On each body segment, the tubercle closest to the head is about 1/3 the size of the tubercle closest to the rear for BCW; the corresponding tubercles on each segment for dingy cutworm are roughly the same size. The Field Crop Insects publication from the ISU Extension Store can aid in identification of other cutworm species.

Scouting for black cutworm

Poorly drained, low lying, or weedy fields may have a higher risk of BCW injury. Additionally, fields near natural vegetation, fields with reduced tillage, or fields with cover crops may be at higher risk. Green cover crops are attractive to egg-laying females. Late-planted or slow-growing corn are typically smaller and more vulnerable to larval feeding. Some Bt hybrids provide BCW suppression (hybrids with Vip3A and Cry1F proteins; see The Handy Bt Trait Table), but larvae can still cut young plants.

Fields should be scouted weekly (every 7-10 days) until corn reaches V5. Begin scouting at least a few days before estimated cutting dates; early scouting is important because local larval development varies due to weather variation within a climate division, and additional significant flights could prolong the arrival of BCW larvae to a field.

Examine 10 corn plants in five areas of the field (50 total plants). Look for wilting, leaf discoloration or damage, and missing or cut plants (Figure 4). BCW larvae sometimes drag cut plants under soil clods to continue feeding during the day. Flag areas with suspected BCW feeding and return later to assess further injury. Take note of plant growth stage and the size of larvae in the field; this will help determine how long larvae may continue to injure plants. Larvae can be found by carefully excavating the soil around a damaged plant – BCW larvae are nocturnal feeders and will hide in the soil during the day. Remember to watch for indications of other early season pests such as wireworms, white grubs, and seedcorn maggot.

Thresholds and management

The generic economic threshold for BCW in corn is 2-3% of plants cut when larvae are less than ¾ inch long and 5% of plants cut when larvae are greater than ¾ inch long. However, this threshold can be adjusted depending on corn price, final corn stand, and input costs.  A dynamic threshold calculator for BCW may be used to aid management decisions.

Preventative insecticide treatments are a questionable practice. Rescue treatments are usually the most efficient and economic approach to managing BCW since it is a sporadic pest. Scout to determine that larvae are still present in the field before spraying insecticides for BCW.

Will BCW survive freezing temperatures?

We have experienced a lot of fluctuating temperatures, including some freezing temperatures after black cutworm arrived in the state. Young larvae, pupae, and adults are not expected to survive when exposed to subfreezing temperatures. Eggs and older larvae (4^th to 6^th instars) are the most cold-hardy stages; they may survive weeks below freezing. If eggs are laid but have not yet hatched, they will likely survive but development may be slowed and cutting may occur later than predicted.

Report larvae!

If you see any fields with BCW larvae or injury while scouting, please let us know by sending a message or photo to bugtraps@iastate.edu. This information will help us refine cutting predictions and scouting recommendations in the future.

To keep up-to-date on the Iowa Moth Trapping Network, check for weekly updates on the ICM Blog webpage or subscribe to receive email notifications.