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Managing Insect Pests in Organically Certified Soybean

Soybean is a crop of one plant species, however, differences caused by variety selection, planting date, cultural techniques, site, and season makes 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 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 Entrust® insecticide 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 improved crop health. Avoiding pests through rotation, with a non-legume crop, helps soybeans to tolerate the feeding of pests that later move into the field.

Fertility and pH maintenance:

Thin and short 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 tolerate insect feeding to a high degree.

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, 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 with Intrust®:

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. The organically approved insecticide Entrust® is labeled for use on soybean for several caterpillar species. Entrust® is not effective against stink bugs and most beetles.

Scouting for foliage feeding insects:

Scouting for foliage-feeding insects involves estimating the percentage of the total leaf surface that has been eaten. The method for taking a single sample is to move to the sample site, look up and down the row at nearby plants and leaves, and estimate the percentage defoliation on the plants observed. Record this estimate and repeat the procedure for all samples. After the last sample is taken, average the individual defoliation estimates over all the samples taken and compare this average estimate to the defoliation threshold.

Beginners often overestimate the extent of defoliation. One way to help “calibrate” the eye is: (a) look at a single plant and estimate the percentage of defoliation — record this estimate, (b) remove the plant from the ground and pull all the leaves from the plant, (c) estimate the percentage of area eaten from each leaf — folding the leaflets into quarters will help, and (d) calculate the average defoliation (add the individual defoliation estimates and divide by the number of leaves). The individual leaf estimate should be most accurate and whole plant estimate should be compared to the leaf based score to judge the accuracy of the first estimate. Use this procedure for practice until the scout becomes proficient at estimating whole plant defoliation.

When scouting fields, each field should be sampled at a minimum of three sites (one sample per two acres with a minimum of three samples and maximum of 10 samples). Extra large fields should be divided into units not larger than approximately 30 acres (e.g. a 45 acre field would be divided into two units of approximately 22 acres each). If the defoliation level is clearly well above the threshold, or if it is near zero, after the first three samples have been taken – and if casual observations between sampling sites confirm the sampling results – further sampling is usually unnecessary. When foliage injury is encountered, the scout should determine which insect(s) is responsible and if population will cause further damage if not treated.

Each field should have a significant portion of the area scouted. Patterns of scouting are usually dictated by the travel pattern between fields, access, and field size and shape. Many patterns can be successfully employed. A “U” or “Figure 8” are good patterns to use. Samples should be widely spaced within the pattern used.

Scouting for corn earworm:

The corn earworm often functions as a foliage-feeder and is described under foliage-feeding caterpillars for this situation. However, it prefers to eat blooms and fruit (pods). Therefore, when plants are in the reproductive growth stage this caterpillar is sampled as a pod feeder. The corn earworm is the most damaging insect found on North Carolina soybeans. It’s feeding can reduce yields and delay plant maturity.

In organically grown, wide-row soybeans (30” rows or greater) a shake cloth (a 3 ft. X 3 ft. cloth sometimes called a shake sheet or ground cloth) is used. The cloth is unfolded between two rows, and plants along the row for 3 feet on each side of the shake cloth (6 row feet) are struck downward with the hand and forearm to dislodge caterpillars onto the sheet. Care must be taken to prevent the plants from sweeping across the sheet and displacing insects during the plant shaking event. Earworms that fall onto the cloth are counted, making sure that the worms are properly identified and not confused with other commonly found species of caterpillars. The size of the larvae should be noted as this information is needed in making control decisions. NOTE: The corn earworm is more attracted to open areas in soybeans and to field edges. Therefore, make sure that samples are collected away from the borders and in average areas of the field. Calculate the average number of caterpillars per six foot sample and compare this number to the appropriate threshold given below.

The number of samples taken is dictated by field size. Take a minimum of three samples per field, and take an additional sample for every 3 acres in fields over 6 acres – for example, in a 6-acre field, take three samples; in a 9-acre field, four samples; in a 21 acre field, seven samples; and so on. If the threshold is greatly exceeded (e.g. 4 times higher), or if no caterpillars are found after three samples, no further sampling is needed.

Thresholds:

When scouting, some level of insects and their damage will always be observed. However, most situations will not result in yield loss since the insect numbers present are too low to produce injury beyond the plant’s compensation ability. Thresholds are levels of pest injury or insect numbers (that are assumed to cause injury) that will result in yield loss beyond the cost of a remedial action (usually spraying with an insecticide). Current thresholds for soybean insect pests in North Carolina are:

    1. For defoliation — 30% foliage loss up to two weeks prior to blooming or 15% foliage loss from two weeks prior to flowering and until pods have filled.
    2. Corn earworm thresholds (shake sheet, 30” to 38” rows):

Selling price / bushel

Larvae per 6 row feet

$5.00

12

$7.00

10

$9.00

8

$11.00

6

$13.00

4

Page Last Updated: 9 months ago
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