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Troubleshooting Early Season Corn Insect Damage 101: What To Look For

By Bryan Jensen
UW Extension
Troubleshooting early season corn insect damage can be difficult. There are several potential insect pests and some cause similar symptoms. Also, there is the possibility that if environmental and field conditions match up, new or unexpected insect damage may occur. Getting a complete “picture” of the situation will improve your diagnosis or provide more answers to questions if you seek outside help. Sometimes the answers is obvious. Other times, your diagnosis may be complete when several, but not all, clues point to one (or more?) insect.
It goes without saying, but try to find the insect causing the damage. Sometimes that is easier said than done. Look in the soil around the root zone, in the soil immediately surrounding the seedling, on the soil surface, under crop reside and in soil cracks. Take your time and be methodical. Early instar black cutworms are small and difficult to locate because they are the same color as many soil types. If you find an unknown insects take samples (best option) and/or digital pictures. Bring them to your local county extension crops and soils agents for identification or verification. Agents have access to several resources including the UW Entomology Insect Diagnostic Lab. Don’t assume that if you find an insect in high numbers it is the culprit. There is a lot of “guilt by association” going on in the insect world.
Dissect the corn seedling. If an insect is found in the plant, make note of where it was located (above/below ground, in the whorl?). Is there an entry hole and it located below or above ground? This information can help separate below ground insects (wireworms, white grubs, hop vine borer) from some of the above ground insects (stalk borer, billbug). Look to see if the damage is current. That is, are the newly emerging leaves showing symptoms.
Determine if the damage is uniform across the field or if it is spotty. Very few insects will have a completely random distribution. However, damage patterns can tell a story. Hop vine and stalk borer damage can almost exclusively be found along ditches, grassy waterways, fence row and terraces. Some pests, like slugs, are more pronounced in areas with a lot of crop residue. Others may be found in association with broadleaf weeds (black cutworms), yellow nutsedge (billbug) and grassy weeds (armyworms, wireworms).
Injury symptoms can, at best, be used to sort insects into groups but are often not descriptive enough to choose one insect over the other. Rather, plant injury symptoms can be used to rule out certain insects or used to help support diagnosis of others. Feeding on the leaf margin can often point to armyworm, grasshoppers and early instar black cutworms. Plant injury symptoms described as “wilted whorl” or “dead-heart” can include white grub, black cutworm, hop vine borer, stalk borer and wireworm. If symptoms include holes in the newly emerging leaves it might lead you to a stalk borer or billbug diagnosis. One exception is slug injury. Slugs have a very distinctive injury symptom which includes elongated feeding scars that may or may not have the leaf cuticle intact.
One final point. We occasionally get insects, which for some reason, flare up on either a local or landscape level. Some may be known corn pests and others not. Remember the variegated cutworm outbreak in 2012? Very extreme weather fronts brought this insect to Wisconsin at unprecedented levels. On a local scale, insects such as sandhill, glassy, spotted and dingy cutworms can be found in individual fields or even isolated areas of fields. Bring unknown insects and damage in for diagnosis. The benefits are many and will include accurate diagnosis, appropriate management decisions and data for field histories. Additionally, it gives us (UW Extension) a better feel for trends and we can get the word out.

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