Distinguishing Among Insect Injury Types in Seedling Corn

Sometimes it can be hard to tell what insect pest has injured your seedling corn.  In addition to looking for the presence of the pest or evidence of the pest, you might be able to gather clues from the injury itself.  Below are some pictures of different types of injury and options of common insect pests in North Carolina that can cause this type of injury.

Dead seedling

Seedlings killed by billbugs.

Many seedling pests of corn can cause plant death, including billbugs, cutworm, grasshopper, Japanese beetle larvae, stink bugs, sugarcane beetle, white grubs, and wireworms.  If you can, try to see how the plant was killed.  Stink bugs or billbugs can kill the plant “heart”.  This can directly kill the plant or cause it to stunt or tiller out, essentially rendering it a weed.  A clean cut near or above the soil line could indicate that a cutworm or grasshopper is the culprit.  These can be problems in fields with lots of surface residue (i.e., no-till).  Below the surface, wireworms feed on the roots, but can tunnel up into the large lateral roots or into the plant stalk.  Both white grubs and Japanese beetle prune roots and are more of a problem in no-till fields.  Finally, sugarcane beetles will often leave a distinctive cavity from their feeding.  They can also entirely clip plants.

A sugarcane beetle adult chewed this cavity below the soil on a corn seedling. The larva of this insect does not damage corn.

Sugarcane beetle adults clipped these plants below the soil surface.

Whipped or Stunted Plant

Plant whipping due to billbug feeding.

Whipped (buggy whipped) or stunted plants can be caused by many of the pests listed above and others not mentioned.  If injury if not apparent above the surface, you should dig up the plant to see if you see some of the injury described for the dead plants.  Often, whipped plants are caused by wireworm feeding, but I sometimes see this associated with billbug or sugarcane beetle feeding.  When these pests feed just below the soil surface at the growing point of the plant, they can cause problems when the plant attempts to elongate and to unfurl its leaves.

Streaked Leaves

Some pests of seedling corn can disrupt the vascular tissue that carries water and nutrients from the plant roots to the leaf.  In serious cases, this can cause the plant to wilt and die, but often it results in distinct streaks on the leaves.  Pests that commonly cause this streaking symptom are billbugs and sugarcane beetle.

Leaf streaking from billbug feeding.

Leaf streaking from sugarcane beetle feeding.

Leaf Holes

Transverse holes and leaf streaking from billbug feeding.

Billbugs feed using chewing mouthparts near the base of the plant.  As a seedling a corn plant growth and development takes place about an inch below the soil line.  The outer leaves in the whorl are the oldest and most developed.  As a billbug begins to feed on these outer leaves, it can chew through to inner developing leaves.  Some of these inner leaves may be tightly furled.  When these leaves unfurl, the previous single billbug feeding spot appears as several transverse holes through the leaf.

Sometimes injury is not severe (for example, if a high rate of neonicotinoid seed treatment kills the billbug after a short feeding bout). The resulting injury can appear as a small "blotch" across the leaf, sometimes accompanied by a hole.

Sometimes billbug feeding will completely cleave the leaves, rather than leaving holes.

Stink bugs can also cause transverse holes through the leaves.  Generally stink bugs are not present in corn until V5 and are associated with movement from wheat after harvest.  Although there may be stink bugs present in corn when you find this sort of injury, remember that the injury happened some time previously (perhaps one to several weeks).  Stink bugs do not commonly injure corn seedlings in North Carolina, although they have the potential to injure corn seedlings.  Stink bugs feed using piercing sucking mouthparts.  These mouthparts can pierce directly through leaves, including developing unfurled leaves, resulting in transverse holes once the leaf has unfurled.

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Distinguishing Among Insect Injury Types in Seedling Corn — 2 Comments

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