University of Illinois

No. 7/May 9, 1997

Critters in the Soil

The cool soil temperatures are causing concern among growers who have had corn planted for a while. As the seeds continue to lie in cool soil and seedlings grow slowly, they are exposed to several potentially damaging insects for a longer period of time. Although we have focused much of our attention on black cutworms thus far, other early season insects (most notably seedcorn maggots, seedcorn beetles, and wireworms) may attack planted seeds or slowly growing seedlings.

On May 1, Randy McElroy, regional agronomist with DeKalb Genetics Corporation, visited some Lawrence County cornfields in which some seeds had been hollowed out. Although the culprit was not discovered, seedcorn maggots or seedcorn beetles could have been responsible. Both insects eat out the seeds, leaving empty seed coats as evidence.

The adult seedcorn maggot is a small, gray-brown fly (smaller than a house fly) that prefers to lay its eggs in soils with decaying vegetation. Fields to which manure is applied also have a greater potential for infestation by seedcorn maggots. After the eggs hatch, the maggots burrow into the planted corn seeds, usually destroying the germ. Many injured seeds fail to germinate, resulting in gaps in the stand. If injured seeds germinate, the seedlings usually are weak, and they may die. The maggots are glistening yellow-white, legless, and about 1/4-inch long. They are cylindrical in shape and tapered toward one end, often resembling the tips of roots. A seedcorn maggot larva does not have a discernable head but does have mouth hooks with which it feeds.

Two species of seedcorn beetles attack planted corn seeds in Illinois. The adults, not the larvae, cause the damage. The seedcorn beetle is light brown, with a black head and a broad black stripe on each wing cover. Shallow furrows are evident on the wing covers. The slender seedcorn beetle is dark, shiny red, and appears flattened and elongated. The wing covers are wider than the thorax and do not have the black stripes evident on the seedcorn beetle. Both species are about 1/4 inch or less in length. Both the seedcorn beetle and slender seedcorn beetle prey on other arthropods, but they also feed on germinating seeds of corn when prey is absent or in short supply. The most obvious above-ground symptom of damage is a reduced plant stand.

Wireworms also attack planted corn seeds; their presence is usually easy to detect. Wireworms were discussed in detail in issue no. 2 (April 4, 1997) of this Bulletin. Some comments about their appearance and their presence are worth repeating.

Wireworm larvae are cylindrical, obviously segmented, and wirelike in appearance. They range in size from 0.5 to 1.5 inches long and are usually hard-bodied and yellow to brown, although some species are soft-bodied and white. Their three pairs of legs are short, and the last abdominal segment is either notched or conelike. The larvae live in the soil for 2 to 6 years, depending upon the species. Generations of the long-lived larvae overlap, so wireworms of all sizes and ages can be found in the soil at the same time. Wireworms cause early season injury to corn by boring into seeds before or during germination, preventing germination or killing seedlings before they emerge. Larvae also tunnel into the base of plants below the soil surface, injuring or killing young plants.

We offer this information primarily for diagnostic reasons because there are no effective "rescue" treatments for any of these insects. If seedcorn maggots, seedcorn beetles, or wireworms are reducing plant populations significantly, the only recourse is replanting. At the time of replanting, a grower can consider using a registered seed treatment (diazinon + lindane) or soil insecticide.

Also, as you begin digging around in cornfields, you likely will encounter other critters in the soil that are not pests. Some fields that are diagnosed to have a problem with wireworms turn out, instead, to have a thriving population of ground beetle larvae. Ground beetle larvae are opportunistic feeders that prey on other insects. Some ground beetles also consume a variety of weed seeds. It shouldn't surprise anyone who troubleshoots in a field of corn or other crops to find ground beetle adults and larvae.

Take a few extra minutes when diagnosing a suspected problem to determine what insect is present. By avoiding an unnecessary insecticide treatment, this time could be well spent.

Kevin Steffey, and Mike Gray, Extension Entomology, (217)3336652