Many CRP acres will be planted to corn or soybean this spring. After 10 years of grass, and maybe some broadleaf weeds, these fields may harbor a variety of insect pests that can destroy corn or soybean. I anticipate well witness an abundance of early-season insect damage, especially to corn. This article discusses potential insect problems and suggests management options for corn and soybean production.
Wireworms and white grubs
Fields grown in a grass cover for several years may harbor populations of wireworms or white grubs. Both pests feed on the roots of grasses. When corn (which is a grass) is planted into a recent grassy area, the possibility of stand reduction from these insects may be very high.
Steve Lefko, graduate research assistant in the Department of Entomology, searched for wireworms and white grubs in CRP fields in Floyd, Lucas, Monona, OBrien, and Story counties last summer. He pulled 20 soil cores (4 wide and 10 deep) from 87 fields. He found wireworms in 26 (30 percent) of the fields and white grubs in about 20 (23 percent) of the fields.
So what does this mean? Either wireworms and white grubs are not as abundant as we might think, or pulling 20 soil cores from a field is not an efficient method of confirming the presence of soil-inhabiting insects. I believe we must assume the potential for stand loss in corn following CRP ground from wireworms and white grubs is a realistic possibility, and plan accordingly. In soybeans, wireworms and white grubs are not serious pests of seedling plants.
Species such as the glassy cutworm and bronzed cutworm occur in grassy areas and will cut seedling corn. Corn should be scouted at the spike stage through the five-leaf stage, even if a soil insecticide was used at planting.
This is not a common pest, but they are more likely to occur if broadleaf weeds were present in the CRP field. Stink bugs kill seedling plants (through the four-leaf stage) by injecting a toxin during feeding. An at-planting soil insecticide will not provide protection from stink bugs. Fields should be scouted at spike through the four-leaf stage for evidence of twisted, yellowed leaves with rows of repeating patterns of holes in the leaves. A rescue insecticide may be needed to prevent further damage.
This insect uses more than 176 species of grasses and broadleaf plants as hosts, but should not be a pest to corn if the CRP field was pure grass. Stalk borers need plants with large diameter stems, such as giant ragweed, pigweed, cocklebur, smartweed, and lambsquarters for the larvae to complete their life cycle. Brome grass, big bluestem, and switch grass are not large enough in diameter for the insects to complete their life cycle, therefore stalk borers are unlikely to occur in CRP fields of pure grass. If the CRP ground contained a mix of broadleaf weeds, stalk borers should be expected and the field should be scouted from the spike stage through five-leaf stage for larvae in the whorls. Stalk borers rarely damage soybean seedlings.
This diminutive insect can destroy both corn and soybean seeds. The adult flies are attracted to fields that contain live, plant material that is killed and either plowed or disked immediately prior to planting. The flies lay their eggs in the soil and the maggots feed on the germinating seeds and seedlings. If CRP cover is killed this spring, as opposed to last fall, the freshly decaying plant material can attract seedcorn maggot flies and result in stand loss to both corn and soybean.
Webworm injury is similar to that caused by cutworms. Corn damage should be expected following grass. Plants are cut off at or just below the soil surface and holes are chewed in the leaves. An at-planting soil insecticide may reduce sod webworm damage, but the field should still be scouted at the spike stage of corn growth.
Imported long-horned weevils
No, I'm not making up this one. Last year, I had confirmed reports of soybean defoliation from five counties. The fields had been planted into CRP ground that went with the early-out option. The weevils are very small (about the size of a BB pellet), but often occur in large numbers and can kill seedling plants. In 1994, I sprayed a soybean field with the minimum label rates of Lorsban 4E, Pounce 3.2EC, and Sevin XLR Plus for weevil control. These rates were not effective in controlling the population. If this insect is encountered this spring, the maximum label insecticide rates should be used. Soybean should be scouted through the V5 stage.
Here are several options, prioritized by insecticide use, to reduce the potential for insect damage on former CRP ground. The four options are listed in order from no insecticide use to the highest insecticide use.
Option 1. Plant soybean and dont use a soil insecticide. Wireworms, white grubs, cutworms, stalk borers, and stink bugs very rarely damage soybeans. This crop is a very poor host for these insects. Also, soybean are planted at high enough populations that if stand loss occurs from these pests, neighboring plants often compensate for the damage and prevent yield loss.
Option 2. Plant soybean and use a seed treatment if the ground cover was killed and plowed or tilled this spring. A seed treatment, such as Agrox D-L Plus or Kernel Guard, will protect the germinating seeds and seedlings from seedcorn maggots, which are attracted to the freshly decaying organic matter.
Option 3. Plant corn and use a seed treatment if the ground cover was killed and plowed or tilled this spring. A seed treatment will protect the germinating seeds and seedlings from seedcorn maggots. However, a seed treatment may not provide adequate protection from white grubs or wireworms, so Option 4, which uses more insecticide, would provide more protection against a greater variety of insects. The field should be scouted at crop emergence for cutworms and stink bugs.
Option 4. Plant corn and use a soil insecticide at planting to protect against wireworms, white grubs, sod webworms, and seedcorn maggots. Some insecticides, such as Dyfonate, Force, and Lorsban, also will provide protection against moderate infestations of cutworms. However, the corn still should be scouted for cutworm, stalk borer, and stink bug injury through the six-leaf stage.
Read and follow the label directions for exact rates and placement if you use an insecticide or seed treatment.
This article originally appeared on pages 21-23 of the IC-476 (4) -- April 8, 1996 issue.