Erin Hodgson, Department of Entomology and John Holmes, Extension Field Agronomist
A recent scouting adventure in northeast Iowa resulted in finding a few different caterpillars infesting corn. The most abundant plant damage was from black cutworm. They have been feeding for several weeks throughout Iowa, and are reaching near maturity (Fig. 1). Black cutworm treatment thresholds for caterpillars over ¾ inches in length is five percent of the field with cut plants. But nearly fully developed larvae will stop feeding soon and drop to the soil to pupate. A foliar treatment now may not be warranted.
Fig. 1. Black cutworms make an entry hole just above the soil surface and move up into the stalk to feed. Nearly full-grown caterpillars are 1 ½ inches long. Photo credits Erin Hodgson.
We also saw several "dead head" weeds along the field edge borders, indicating that stalk borers were potentially in the area. Dissecting grass stems revealed copious amounts of frass pellets from stalk borer (Fig. 2). Some stems were still infested, but others were empty. This matches up fairly well with our predicted cutting dates for stalk borer in northeast Iowa this year.
Fig. 2. Grasses with dead heads are an indicator of stalk borer infestations. Splitting open the grass stems will help determine if caterpillars are still feeding or starting to migrate. Photo credits Erin Hodgson.
By looking for shredded corn plants adjacent to grassy field edges, we were able to find small stalk borer larvae infesting corn (Fig. 3). Infested older plants (V4-V7) will be discolored, wilted or dead. Plants older than V7 are unlikely to be killed. A timely stalk borer application must be made to migrating caterpillars.
Fig. 3. Stalk borer larvae attack corn plants from the top or tunnel into the side of the stalk. Foliar treatments are not effective once the caterpillar is inside the stalk. Photo credits Erin Hodgson.
John found a single spotted fireworm caterpillar in corn (Fig. 4). Not much is known about these rare corn pests, and a treatment threshold has not been developed. You may find them rolled up in a leaf with webbing and frass pellets.
Fig. 4 Spotted fireworm caterpillars are rare corn pests in Iowa. The body is dark purple and covered with long hair and bumpy white spots. Photo credits Erin Hodgson.
Some fields in that area were suspected to have hop vine borer in corn (Fig. 5), although we did not find any in our search. These caterpillars can cause stand loss in corn, particularly in northeastern Iowa. Hop vine borers attack the corn from underground and hollow out the base of the stalk, and shouldn't be confused with cutworm or stalk borer damage. But like stalk borer, foliar treatments must be made before they enter the stalk.
Fig. 5. Hop vine borer caterpillars are creamy white with an orange head and dark patches along the abdomen. Photo credit Marlin E. Rice.
Erin Hodgson is an assistant professor of entomology with extension and research responsibilities; contact at firstname.lastname@example.org or phone 515-294-2847. John Holmes is an ISU Extension field agronomist serving north central Iowa. He can be reached at 515-532-3453 or by emailing email@example.com.