Troubleshooting Hints for Common Early Season Insects (and slugs) in Corn

Bryan Jensen

IPM Program

Poor emergence is likely a result of either seedcorn maggot or wireworm feeding. Seedcorn maggots have a very short generation time and may not be present when you arrive in the field. However, there are a few other hints which can help. Adult flight periods are short and damage maybe confined to a small number of planting dates and more severe when germination and emergence is slow. Also, livestock or green manures may attract more adults into fields to lay eggs and increase the likelihood of damage. Leaf injury (small holes) is possible but infrequent and is usually limited to the first leaf.  Wireworms may also be a cause of poor emergence. If fields are scouted early, larvae can be found feeding on the seed or in the base of the corn plant below ground. Larvae are somewhat mobile and you may not find them on every damaged plant so dig up a number of damaged seeds or plants. Field history will also be a help with wireworm diagnosis. Damage is often more severe in corn after small grains, pastures or other forage crops.   Above ground symptoms include dead/dying plants and holes in the leaves but these symptoms are not exclusive to wireworm feeding but rather to any insect which feeds on the base of the corn plant. To confirm, dig up a number of corn plants and/or seeds and try to find the culprit. If none are present, rely on field histories and crop rotations to confirm. 

When above ground symptoms are evident (holes in leaves, wilted whorl, dying plants) the first order of business would be of course to find the insect. For help with identification go to the IPM Field Crops Scouting Manual which can be found on this website by clicking on publications and searching for the title. Insects are mobile and may not be found on every damaged plant so be sure do inspect a number of injured seedlings. Dig around the roots, look underground for feeding signs or an entrance hole, split the stalk, look for an above ground entrance hole and under crop residue. If you find nothing, it indicates that the insect you are looking for is very cryptic (slugs or billbugs) or is not present and therefore control measures are not necessary. However it is always a good idea to give the farmer a possible diagnosis so they know what didn’t cause the damage and so they can better prepare for next season. These subtle hints can help. 

Stalk borer damage is usually found next to fence rows, grassy waterways or other patches of perennial weeds, especially quackgrass and wirestem muhly.   Initially you will notice small feeding scars in the whorl when larvae are small and beginning their migration to corn. Eventually they will move farther down into the whorl or burrow into the stalk above ground level. These symptoms will appear as larger holes in the leaves, wilted whorl or dying plants.

Hopvine borer injury is often confined to the same field areas as stalk borer and often their populations can be mixed. Hopvine borer damage corn by tunneling into the plant below ground level. Above ground symptoms are dead/dying plants without holes in the leaves.

White grub damage to corn roots can lead to loss of vigor or if they tunnel into the base of the corn plant symptoms similar to hopvine borer. Although white grub damage may be in pockets within a field it is usually not confined to fencerows and waterways like hopvine borer. 

Billbugs are an occasional insect pest on corn and are very difficult to find. Rarely do they cause economic damage but the concern for this article is damage confused with insect pests of greater economic concern. Billbug feeding is concentrated along field edges and often in association w/ perennial grasses and yellow nutsedge. The adult weevil will chew a hole into the above ground stem leaving a feeding scar which results in leaves with holes that tend to be oblong. Rarely will you get dead or dying plants.

Slugs are 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. Another telltale sign of slug feeding is the slime trail left during the previous nights feeding expedition.