Diagnosing Early Season Corn Insect Damage
Bryan Jensen, IPM Program
A few calls have been coming in regarding insect damage in seedling corn as well as some confusion when making a diagnosis. Below are a few observations regarding insect injury symptoms. Keep in mind that corn growth stage can affect symptoms and that injury symptoms are not always “classic” appearing. Look at a range of plant symptoms and try to locate the insect causing the problem. Finally, look at injury patterns in the field and determine if these patterns match what you would expect.
Seedcorn Maggot: Larvae may injure the seed and cause poor emergence. Seedcorn maggots also feed on the emerging shoot which results in holes in the first or second leaf. Leaves emerging later are often symptomless. Injury is more severe to corn following a green manure, an application of livestock manure or if the soil is freshly tilled. Damage is usually uniform across a field.
Wireworms: Feeding is always below ground. Injury to the seed will cause poor germination while shoot feeding results in “wilted whorl” symptoms. If feeding is above the growing point holes will appear in the leaves above ground. Wireworms are somewhat easy to find around damaged plants. However, wireworms will migrate deeper in the soil profile when soils warm up. Damage is more common in corn planted after sod and distribution maybe spotty within a field.
True White Grub larvae: Damage is below ground. White grubs will not injure corn seed, however, larvae will feed on roots or underground stems causing stunting or wilted plants. Larvae require two growing seasons to complete their development; as a result both large and small larvae may be present. Damage is more common in corn planted after sod or after any crop that had grassy weed problems. Grubs can be relatively easy to find around most, but not all, damaged plants.
Black cutworm: Early instar larvae feed on corn leaves. Late instars may cut small corn plants or feed below ground on the shoot causing wilted whorl symptoms above ground. Larvae can be found in the soil around damaged plants. Damage may be spotty within a field and sometimes confined to low poorly drained areas of a field.
Dingy cutworm: Overwinter as partially grown larvae and injury if often found before black cutworm damage is anticipate. Dingy cutworms rarely cut plants. Feeding is above ground and usually considered cosmetic. Identification is often confused with black cutworms. Use a hand lens to look at the tubercles (small black dots) on their back. Dingy’s have four small, similar sized tubercles/segment. Black cutworms also have 4 black tubercles but are of two different sizes. The tubercles to the rear of each segment are approximately 50% larger than the front tubercles.
Sandhill cutworm: Overwinter as partially grown larvae and feed almost exclusively below ground and in sandy soils. Above ground symptoms are dead heart or dead plants. Sandhill cutworms are pale in color and are easily found around damaged plants. Rescue treatments for sandhill cutworms do not usually work.
Stalk borers: Are found in close association with perennial grasses, including grass waterways, fencerows and terraces. This distribution can be an excellent diagnostic tool for stalk borers as well as hop vine borers. Larvae migrate short distances to corn and feeding is exclusively above ground. Larvae will either feed on whorl leaves before burrowing down into the stalk or will enter into the stalk directly. Symptoms include holes in leaves and dead heart. Larvae are easily found in damaged plants by pulling the whorl leaves or by splitting stalks. Stalk borer larvae have longitudinal black and light colored stripes and a black band, often called a “saddle” behind the legs.
Hop vine borer: Life cycle is very similar to stalk borer, however, all feeding is below ground. Above ground symptoms are wilted whorls or completely dead plants. No holes in the leaves. Larvae are cream colored, have tan heads with a pair of dark-colored eye spots behind the head.
Billbug: Adults injure corn seedlings by feeding into the stem above the soil surface. As leaves emerge from the whorl, holes will be present in the newest emerging leaves. Injury is often associated with grassy weeds and/or yellow nutsedge. Feeding is usually cosmetic and not considered to have economic consequences unless corn is very small. Unfortunately, adults are very hard to find making verification of damage difficult.
Armyworm: Early season armyworm damage is easy to differentiate from most other insects mentioned above. Larvae are usually visible in the whorl and feeding is from the leaf margin in.
Slugs: Not an insect, but a pest favored by cool wet weather, crop residue and/or weed growth. Slugs are nocturnal feeders and difficult to locate during the day. However, their damage is unique compared to the other insects. Slug symptoms include longitudinal feeding scars. The leaf cuticle can remain intact at first and eventually may fall out.