Time to Check for Corn Seedling Insect Pests (Billbugs, Stink Bugs, White Grubs, Wireworms)
Almost no one scouts their corn until it is too late. While insecticide seed treatments work some of the time and for some insects, they can’t eliminate all problems. This article is to urge you to check your corn now. A quick trip to the field at this point in the season might save you money next season, by helping you know where you might be able to cut down or increase your rate of insecticidal seed treatment, or where you need to hone in with an in-furrow insecticide treatment.
Of the insects mentioned in the title (billbugs, stink bugs, white grubs, wireworms), the only one that you can control this season is stink bugs. See this article for tips on stink bug thresholds and scouting early season. In my opinion, it is nearly impossible to distinguish billbug and stink bug feeding. Your best bet is to go out, visit the field, and find the culprit. So what should you look for?
Many seedling pests of corn can cause plant death, including billbugs, cutworm, grasshopper, Japanese beetle larvae, stink bugs, sugarcane beetles, 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.
Whipped or Stunted Plant
Whipped (buggy whipped) or stunted plants can be caused by many of the pests listed above and others not mentioned. If injury is 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. Sometimes you’ll get lucky and find the insect feeding on the plant. 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 or Nutrient Deficient 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. Nutrient deficiencies can be caused by both grubs and wireworms.
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. Also note that sometimes billbug feeding will completely cleave the leaves, rather than leaving holes. Finally, 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
Stink bugs can also cause transverse holes through the leaves. 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 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.