Diagnosing Early Season Below Ground Insect Damage

 

A lot of corn and soybeans has been planted over the past few weeks and emergence has begun. Now, and over the next few weeks, is a good time to evaluate these stand for emergence problems cause by early season below ground insects. Knowing the exact cause of poor emergence can be helpful to explain low yields and for preparing future pest management plans. Below are some troubleshooting observations.

 

Keep in mind that crop growth stage can affect damage symptoms and that fields symptoms are not always “classical” in appearance. Look at a range of plant symptoms and try to locate the insect causing the problem. Finally, look at injury patterns within the field and determine if these patterns match your diagnosis.

 

Seedcorn Maggot: Larvae are small, white, legless and cigar-shaped. Feeding occurs below ground and larvae can feed on either the seed and/or emerging shoot. Remember,

 

there are other similar looking saprophytic maggots which can feed on decaying seed previously killed by other insects and/or pathogens. These are not considered pests, but more precisely opportunistic organic matter feeders. If seedcorn maggots are causing the injury, you should also notice above ground symptoms that include small holes in either the first and/or second leaf in corn. Damage to soybean includes scarred cotyledons, holes in the unifoliate leaves or severed hypocotyls. Seedcorn maggot damage is usually uniform across fields and more severe under cool/wet growing conditions. Damage may also be more severe when corn/soybeans are planted into recently tilled fields, fields with a green manure or in fields with heavy applications of livestock manure. Still unsure? Try using degree days to “back calculate” if corn or soybeans were planted during a peak adult flight. There are several generations of seedcorn maggots, however, the first two adult flight periods cause the most damage. These peak flight periods occur at 360 and 1080 DD (Base 39o F). To calculate seed corn maggot degree day accumulations at the time of planting, navigate to the Degree Day Calculator at the UW Extension Ag Weather website http://agwx.soils.wisc.edu/uwex_agwx/thermal_models/degree_days  The calculator will ask you for your Latitude/Longitude, type of degree day model to use (sine), biofix date (January 1), end date (planting date), lower threshold (39 degrees) and upper threshold (84 degrees). Your last prompt is for the type of report you would like.

 

If the damage is severe and you are still in doubt, please send the maggots to the UW Extension Insect Diagnostic Laboratory, c/o PJ Liesch, Dept. of Entomology, 1630 Linden Dr, Madison, WI 53706.

 

Wireworms: Larvae feed below ground and injury can occur to the corn seed or to the emerging shoot. Injury to the seed will reduce plant stand. Shoot feeding results in the newly emerging leaves showing signs of wilting if the feeding site is at or below the growing point or as holes in the newly emerging leaves if feeding site is above the growing point. Wireworms are somewhat easy to find around damaged plants. However, larvae will migrate deeper into the soil profile as soils warm. Therefore a quick response to complaints is helpful.  Damage is more common in corn planted after sod and distribution maybe spotty. Injury may also correspond with soil types. Wireworms have an extended life cycle. Depending on species present, they may take 1-5 years to mature. This information is an important predictor of damage potential if corn is to be planted next year.

 

True White Grub: Damage by larvae is always below ground. White grubs will not injure corn seed, however, larvae feed on corn roots or underground stems causing stunted or wilted plants, respectively. Larvae require two growing seasons to complete their development. As a result, both large and small larvae may be present. Damage is usually clumped within a field and is more common in corn planted after sod or after any crop that had grassy weed problems. Grubs can be relatively easy to find around most, but not all, damaged plants.