It looks like we have finally turned the corner and are getting the warm weather we all hoped for. When the fields dry, a lot of us will be walking corn looking for early season problems. One thing is certain; you will find some insect injury. Some injury will be cosmetic, some will be economic. Some will require rescue treatments, some not. Some symptoms will be diagnostic of a single species; most symptoms are common to more than one. Some of the injury, however, can be confirmed by the presence of the insect. But…sometimes not.  It is difficult to keep all the clues sorted out. Below is a brief description of some early season insect injury, habits and other useful diagnostic clues. Hopefully this will be a timely refresher and offer some new hints to help with diagnosis.

Wireworms: Feeding is always below ground on either the seed or emerging shoot. Injury to the seed will cause poor emergence while shoot feeding can result in dead heart or holes in the leaf. Wireworms are somewhat easy to find around damaged seedlings. However, wireworms will move deeper in the soil profile when soils warm up and dry out. Damage is more common in corn planted after sod and because of their extended larval life cycle may also be common in the second, third (or more) years of corn after sod. Damage is usually spotty within a field. 

True White Grub: Damage is below ground; however, white grubs will not feed on the corn seed like seed corn maggot and wireworms. White grubs will feed on corn roots causing stunted plants or cause dead heart symptoms if they feed on the underground shoot near the growing point. Larvae require two growing seasons to complete their development so both large and small grubs may be present. Damage is more common in corn after crops with grassy weed problems or sod. Grubs can be relatively easy to find around most, but not all, damaged plants.

Black cutworm: Moths migrate to Wisconsin and are attracted to crop residue (especially soybean residue) and/or spring weed growth to lay eggs. Early instar larvae feed above ground at night and chew on  corn leaves which is an early indication that the cutting stage may follow. Late instar larvae may also feed below ground level causing dead heart symptoms. Larvae can be found in the soil around some of the damaged plants if the field is monitored prior to pupation. Damage may be spotty wit in a field and sometimes confined to low poorly drained areas of a field.

Overwintering Cutworm: The two most common overwintering corn cutworms in Wisconsin are the dingy and sandhill cutworms. These overwintering cutworms are not as common as black cutworms. However, it is important to determine which specie is present. Developing a field history for sandhill cutworm is important and allows growers to plan preventive treatments.  

Dingy cutworm: Rarely cut plants, most feeding is above ground and usually considered cosmetic. Dingy cutworm overwinter as small larvae so injury is often noticed in the field before black cutworms are anticipated. Identification is often confused with black cutworms. Use a hand lens to look at the tubercles (small black dots) on their back. Dingy 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: Overwinter as partially grown larvae, feed almost exclusively below ground and in sandy soils. Above ground symptoms are dead heart or dead plants. Sandhill cutworms are pale in color. Pulsating blood vessels can be seen along the backs of mature larvae. Sandhill cutworms are easily found around damaged plants prior to pupation. 

Stalk borers: Commonly found in close association with perennial grasses including grassy waterways, fencerows and terraces. This field distribution can be a very good diagnostic tool for both stalk borers and 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 the majority of damaged plants by pulling the whorl leaves or by splitting stalks. Stalk borers have longitudinal black and light colored stripes with a black band, often called a “saddle” behind the legs. 

Hop vine borer: Life cycle very similar to stalk borer, however, all feeding is below ground. Above ground symptoms are dead heart or 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 emerging leaves. Injury is often associated w/ grassy weeds and/or yellow nutsedge. Feeding is usually cosmetic and not considered to have economic consequences. Unfortunately adults are very hard to find making verification of damage difficult. 

Armyworm: Early season armyworm damage is easy to differential from most other species 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 feed nocturnally and are difficult to find during daylight hours. Fortunately their damage is unique. Look for narrow longitudinal feeding scars running the length of the leaf. Initially the cuticle will remain intact leaving a window pane effect but will later drop out leaving an opening.