Soybean Insects: Early-Season Pests (Emergence to V8)

Lesser Cornstalk Borer, Elasmopalpus lignosellus

Lesser cornstalk borer larvae (Fig. 8) are bluish with distinct abdominal segments. Full-grown larvae are about 1/2 to ¾ inch long. Damage occurs when larvae bore near ground level into the main stems of young soybeans, resulting in wilting, lodging (plants fall over), and plant death (Fig. 9). Affected plants are often grouped in the same or adjacent rows because individual larvae can attack more than one plant (Fig. 10). Sometimes dead plants appear teepee- shaped.Larvae inhabit silken tubes in the soil, which are attached to the plant. Since larvae can survive on decaying vegetation, early field preparation can help prevent damaging infestations.

Because damage usually occurs during hot, dry weather, irrigating soybeans can help prevent injury. Soybeans grown on sandy soils can be more vulnerable to attack than those grown on finer soils. Since soybeans should have no fewer than four plants per foot of row for profitable yields, sparse, non-uniform stands of soybeans can tolerate less damage than denser, uniform stands. Insecticides applied to the seed and also applied post-emergence are available. Careful scout- ing during early soybean growth can detect developing populations before injury occurs.

Cutworms (various species)

Cutworms are the larval stages of moths that feed below or near ground level on the roots and stems of seedling soybeans (Fig. 11). Damage results in soybeans being “cut-off.” Affected plants that remain standing can appear drought-stricken.

As with all caterpillars, the larger they are, the more damage they can do. Full-grown caterpillars are relatively large (1½ inches long), dark (blending well with dark soil), and often curl up into a “C” shape when disturbed.

During the heat of the day, these caterpillars remain in the soil feeding below ground level. To find the larvae, dig up seedlings along with their roots during daylight hours.

Figure 11. Black cutworm.

Damage can occur quickly; scout the crop from planting to well into the vegetative stages of development. Replanting may be neces- sary where cutworms reduce stands to less than four plants per foot of row.

Usually, you can find cutworms in finer soils (loamy/clay) during dry conditions that coincide with planting, emergence, and early seed- ling growth. The moths are dark and triangular-shaped with the wings held flat over the abdomen. Since cutworms tend to lay eggs in crop residue, planting into fields with plant residue (such as conservation tillage) can increase the chance of cutworm damage.