- Many species of cutworms feed on corn and soybean if it’s available.
- Black cutworm is the most damaging species but is a migrant into the state, slow to develop in these cool temperatures.
- The dingy, variegated, and claybacked cutworm species all overwinter as partially grown larvae.
- The dingy and variegated cutworms are mainly leaf feeders, whereas the claybacked will also cut plants.
- Seed treatments are not effective against these other species, because of their large size when feeding.
We have been impressed with the number of black cutworm moths captured by our pheromone trap cooperators, see “Black Cutworm Adult Pheromone Trap Report.” Though we are wary of the potential impact these larvae could have on the slow emerging corn crop, we know that heat units for insect development have been slow in coming during this cool spring. Many cutworm species look alike and identification is often confusing. Historically, the black cutworm is our most common species damaging the crop, which is why most assume it the culprit when damage is found.
Black cutworm do not overwinter in the Midwest, which is why we monitor their arrival each spring with pheromone traps. Once they arrive in large numbers (also called “intensive captures”) we begin predicting their development and subsequent damage with heat unit accumulations. We received multiple intensive captures on, and around, April 8 and have begun tracking their development (see accompanying map). Those females looked for broadleaf weeds to lay eggs in early April and those larvae have hatched, but since then, not much has happened. There have not been sufficient heat units accumulated this spring for black cutworm to get 1/2 to 3/4 inches long – the size when they begin to cut plants (300 accumulated heat units). So, if you are finding cutworm damage on emerged corn (or any plant, for that matter) at this time, there is another cutworm species to blame…that being one of the dingy, variegated, and/or claybacked cutworms.
Cutworms are the larvae of noctuid moths, a huge family with over 11,000 species worldwide and some are important pests. The dingy, variegated, and claybacked cutworm species all overwinter in Indiana as partially-grown larvae. They are not specialist feeders, and actively feed on a wide range of plants. Late in the growing season, as cold weather moves in, the larvae cease feeding and become dormant under mats of plants (e.g., chickweed) during the winter months. As temperatures begin to increase in the early spring, they resume feeding. They can complete develop on these weeds, but often these plants are killed by spring herbicide applications, forcing them to move to alternate food sources. If available, this could include the emerging crop. Because these larvae are about ¾ to 1 inch in length at this time of year, they aren’t deterred much by the low concentrations of insecticidal seed treatments in above-ground plant tissues. Therefore, depending on density of worms and the rate of the crop’s growth and development, damage can be quite significant.
The dingy and variegated cutworms are primarily leaf feeders and will rarely cut plants, and if they do, the cutting is above ground level. Because a corn plant up to the 5-leaf stage can withstand severe defoliation without a yield loss, treatment for these cutworms is rarely justified. However, the claybacked cutworm’s damage is a mix of leaf feeding and plant cutting so black cutworm thresholds should be observed. To add to the confusion, other species of cutworms may be encountered feeding on crops as well. The sandhill cutworm, as its name implies, is found on sandy knolls. Sandhill and the glassy cutworms tend to be a perennial threat in specific environments, most producers that have experience with them are quite aware of their destructive abilities.
Identification of these cutworm species is a little tricky and requires a pretty good understanding of morphological characteristics of immature insects, a course taught in Entomology. In short, while using a 10X magnifying lens, carefully analyze the skin texture of the worm. If it is considerably “bumpy,” it is most likely a black cutworm. The other cutworm species have smooth skin. Species identification can usually be confirmed by sending us quality, in-focus pictures, especially of the dorsal, i.e., top, of the cutworms. Happy Scouting!
Black cutworm cut plants about 300 heat unit accumulations (base 50).
Comparison of black (left) and dingy (right) cutworm skin texture, about 10X magnification.
