Organic Field Crop Production and Marketing in North Carolina

By: John Van Duyn, Entomology Extension Specialist, NCSU

Soybean is a crop of one plant species; however, differences caused by variety selection, planting date, cultural techniques, site, and season make the crop highly variable in it's attractiveness to insect pests. In other words, all soybean fields are not alike, as far as attracting and building-up of pest insects is concerned. If the organic soybean farmer recognizes these differences, he can actually plan to manage the crop for reduced insect pest numbers or, when this is not possible, he can predict which of his fields are attractive and may need more attention to prevent yield loss. The organic soybean grower can normally rely upon reducing soybean attractiveness to pests, as well as beneficial insects to reduce pest numbers, and the soybean plant’s natural ability to compensate for insect damage (tolerance). In instances where caterpillar pests are not avoidable, organically approved insecticides may be successfully used. Scouting and the use of thresholds will indicate which fields are at risk. Important tactics used to reduce insect damage include the following:

Site selection: Sites that have limiting pest, physical, or soil chemical problems should be avoided if these problems cannot be corrected.

Rotation: Several insect pests (e.g. grape colaspis) and pathogens of soybean (e.g. cyst nematode) build-up if soybeans are not rotated. Therefore, rotation helps insure reduced pest levels and often improves crop health. Avoiding pests through rotation with a non-legume crop, allows soybeans to tolerate the feeding of pests that later move into the field.

Soil fertility and pH maintenance: Thin plant stands often have more corn earworms, but good growth reduces attractiveness and enhances the plant's ability to compensate for damage. Reducing plant stress from low pH, poor fertility, or inadequate moisture will enable plants to better tolerate insect feeding.

Variety selection & early planting: High caterpillar populations can often be avoided by early planting of an early maturing variety (e.g. varieties from Maturity Groups III, IV, or V). These plantings will bloom and harden-off before the corn earworm moth flight from corn fields and, therefore, plants will be unattractive to the moths. Also, early maturity can greatly reduce soybean looper, velvetbean caterpillar, and late stink bug infestations. In rare situations stink bugs can be trap-cropped by early maturity fields leading to greater damage.

Narrow rows: A complete canopy allows a higher level of biological control by insect predators, parasites, and diseases. Also, narrow-row soybeans seem to be less attractive to egg laying corn earworm moths. Rows of minimal width, that still allow cultivation, will close rapidly and provide pest management benefits.

Remedial control: Group V or later maturing varieties that are planted after late May can become infested by corn earworm moths moving from corn. These moths produce pod feeding corn earworm larvae and high infestation may reduce yield by as much as ca. 50%. Also, populations of leaf feeding caterpillars (green cloverworm, soybean looper, and velvetbean caterpillar) may occasionally damage soybeans to above threshold levels. These worms are usually very late season pests. An organically approved insecticide labeled for use on soybeans can be used for several caterpillar species. NCSU has information on scouting procedures for corn earworm. Conditions for use of a pesticide must be documented in the organic system plan (NOP 2000)