The Time of Year for "Odds-and-Ends" Insects in Corn

June 11, 1999
Highlights:

· Grape colaspis infestations in west and south central counties

· Some no-till cornfields infested with stink bugs, corn root aphid, and slugs

At this time in June almost every year, the weeks between the end of cutworm activity and the beginning of European corn borer and rootworm activity, people scouting cornfields find any number of insects and their relatives in some fields. During the past couple of weeks, we have received quite a few phone calls about the "odds-and-ends" insects that people have encountered, including, but not limited to, grape colaspis, corn root aphids, stink bugs, slugs, and southern corn leaf beetle. I won't list all of the folks who provided these reports, because the list would be long. But here's a thank-you for all of your input.

The hands-down favorite among the odds-and-ends insects in corn right now is the grape colaspis. Several fields in west and south central Illinois have healthy infestations of these insects. Historically, grape colaspis have infested corn planted after clover; however, in recent years, they occur frequently in corn planted after soybeans. The larvae resemble small, white grubs, although they are more comma-shaped than U-shaped. The larva is 1/8 to 1/6 inch long, and has a plump, white body with a tan head and plate just behind the head. Its three pairs of legs are short. Bunches of hairs arise from bumps on the underside of the abdomen. The larvae feed on root hairs and may eat narrow strips from the roots. Denuded roots cannot obtain moisture and nutrients efficiently. Injury symptoms above ground include stunting, wilting, purpling of the leaves and stem (indicating a phosphorous deficiency), and browning of the tips and edges of the leaves. Severe infestations may cause plant death and reduced plant populations. It is possible that you could find grape colaspis larvae causing injury to soybean roots, too.

The grape colaspis passes the winter as a small larva in the soil 8 to 10 inches deep. Larvae become active early in the spring, feed on the roots of host plants, and complete their development from mid-June to early July in the Corn Belt. The narrative in the classic Destructive and Useful Insects: Their Habits and Control, by C. L. Metcalf, W. P. Flint, and R. L. Metcalf, indicates that grape colaspis larvae become full grown and pupate by mid-June in central Illinois. Pupation occurs in an earthen cell 2 to 3 inches below the soil surface. Adults, which resemble tan and ridged northern corn rootworms, emerge from the soil in July in the Corn Belt. Females lay eggs in the soil near host plants, including patches of smartweed and bull nettle.

As with white grubs and wireworms, "rescue" treatments are neither recommended nor effective. The only management approach for a cornfield infested with grape colaspis larvae is to determine whether replanting is necessary. And we all know that it's getting late for corn planting.

Several pests have been observed in no-till cornfields, including stink bugs, slugs, and corn root aphid. You likely are familiar with the first two, but you may not have heard much about the corn root aphid. However, some entomologists have speculated that as more acres are devoted to no-till corn production, the corn root aphid might become more problematic.

Although stink bugs do not occur frequently in Illinois, the potential for injury to corn usually increases when winter weather is mild, wheat or rye is planted as a winter cover crop, and corn is planted without tillage. Both the brown and onespotted stink bugs, which are similar in appearance, can injure corn seedlings by feeding at the base of the plants. Stink bugs insert their piercing-sucking mouthparts into the stem and inject digestive enzymes and other compounds that can be phytotoxic or cause growth abnormalities. Symptoms include lines of tiny holes surrounded by yellow to necrotic tissue; twisted leaves or stalks; tillering, stunting, and wilting; and plant death.

Stink bugs overwinter as adults, and they become active relatively early in the year. Adults of both species are shield shaped, brown, and measure 2/5 to 3/5 inch long and about 1/3 inch wide. Male onespotted stink bugs have a prominent dark spot near the tip of the underside of the abdomen. Well-defined thresholds have not been developed, but thresholds developed for cutworms have been used in some instances.

Although the temperatures recently are not favorable for their development, slugs have caused some significant stand reductions in some no-till cornfields. Apparently the residue has provided suitable cover for these noninsect pests. Slugs are mollusks, and several species can cause injury to corn. The different species vary in size and color, but all share the shiny, slimy appearance of snails without shells. An indicator of slug activity on corn is the presence of slime trails on the foliage and soil surface.

Slugs feed on germinating seeds, seedlings, and foliage. Severe stand losses occur when slugs destroy seeds and seedlings in the furrow below the surface residue. Heavy defoliation of young whorl-stage corn plants may retard plant development and cause stand loss. If prewhorl defoliation is severe or replanting is required, application of a molluscicide bait (such as Deadline Bullets) may be warranted. The bait should be applied to coincide with peak slug activity, which occurs above ground at night, usually when conditions are cool and wet.

The corn root aphid has been the suspected culprit causing injury in some fields of no-till corn. Corn root aphids feed on corn roots and remove nutrients. Injury often results in dwarfing, yellowing, and reddening of young plants. However, the most noticeable sign of the presence of corn root aphids is the presence of lots and lots of ants. The cornfield ant and corn root aphid have a symbiotic relationship. Cornfield ants collect the aphid's eggs and nurture them through the winter. The eggs hatch in the spring, and the ants transfer the nymphs to roots of weeds and corn. During the summer, the sexual form of the aphid is not present, and females give birth to live nymphs. The ants continually tend the aphids, feeding on their honeydew. Winged aphids emerge from the soil during the summer, take flight, and, with the help of ants, colonize other cornfields.

The wingless adult corn root aphid is a small, spherical, soft-bodied insect about 1/16 inch long, and generally is blue-green to gray-green with a black head, thorax, and first abdominal segment. A white, waxy pubescence often makes them appear gray-green. The aphids usually can be found in colonies on corn roots in infested fields. No management guidelines have been developed for this subterranean pest.

Our old friend the southern corn leaf beetle has been found in some fields in western Illinois again this year. A few years ago, after doing some library research on this little-known critter, we found an article written in 1915 that told us plenty about this pest. The southern corn leaf beetle occurs most frequently in fields previously devoted to pasture or in fields that have not been cultivated for several years. However, the beetle also is prevalent in fields infested with cocklebur, apparently another host. The southern corn leaf beetle overwinters as an adult under debris and in clumps of some weed species. Adults emerge early in the spring to feed on young weed hosts, especially cocklebur, and early planted corn. The adults feed on the stems and chew out notches on the edges of leaves of corn seedlings (Figure 2); injured plants appear ragged. Sometimes the beetles feed in such large numbers that injured plants die.

Adult southern corn leaf beetles (Figure 3) are 3/16 inch long, dark brown, and often covered with bits of soil, rendering them difficult to find in the field. The shield just behind the head has three "teeth" on each lateral edge. Because this insect has been reported so infrequently in corn, economic thresholds have not been established. The economic thresholds established for black cutworms could be used as management guidelines. However, information about insecticide efficacy is lacking. Probably the best management strategy is to control the weeds (such as cocklebur) that southern corn leaf beetles have as hosts.

That's a wrap-up of some of the interesting insect problems occurring in corn around the state. If you encounter any of these pests, or others, don't hesitate to contact us.--Kevin Steffey

Author: Kevin Steffey