Soybean South

 - INSECT CONTROL -

Stay On Top
Of NC Insect Pests

This information about the corn earworm and the stink bug –
soybean insect pests – is excerpted from a report by John Van Duyn,
North Carolina State University entomologist.

 

In spite of the grower’s wish and best attempts at pest prevention, crops cannot always be fully protected from insects by cultural and biological management techniques. Fields may become infested and economic yield loss may occur. Scouting, thresholds and insecticides are used to discover and identify damaging insect infestations and to reduce the level of pests before yield loss has happened.

The corn earworm often functions as a foliagefeeder and is described under foliage-feeding caterpillars for this situation. However, it prefers to eat blooms and fruit (pods). 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.

Consider crop row width
The sampling method used for the corn earworm after bloom depends upon the crop row width. For rows greater than 30 inches, a shake cloth (a 3 ft. x 3 ft. cloth sometimes called a beat 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. 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 worms. The size of the larvae should be noted as this information is needed in making control decisions. Note that 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.

Scouting for stink bugs
Scout for stink bugs when checking for corn earworms in August, using the same procedures. However, most stink bug infestations will occur later than corn earworm, in late August through early October. The beat cloth (wide rows), rigid beat cloth (narrow rows),or sweep net (narrow rows) can be used.

Follow the guidelines for sampler use as described for corn earworm. However, due to the more varied distribution of stink bugs, a greater number of samples must be taken.

Take 10 shake sheet or rigid beat cloth samples per field or 12 sweep net samples per field. For fields below 5 acres, 50 percent fewer samples may be taken. Groups of small nymphs (less than 1/2 inch long) should be counted as one adult or large nymph. Large nymphs (longer than 1/2 inch) should be counted together with adults.

Insecticide selection
Treatment with insecticide should only be made when insect numbers or damage exceed the economic threshold, as determined by scouting. Be sure that the target insects are present when treatment is applied, since several pests (e.g. corn earworm) can develop and leave the plants in a relatively short period in hot summer weather.

Lower rates can be used with ground application and on lighter infestations. Mid rates to high rates may be warranted by air when infestation is high, and/or in periods of rainy weather. Consult a current copy of the North Carolina Agricultural Chemicals Manual for specific insecticide suggestions to use against insect pests on soybeans.

To view Van Duyn’s entire report, go to the following Web site: http://www.ces.ncsu.edu/plymouth/pubs/ent/soybeans5.html. The information and recommendations in these Notes were developed for North Carolina conditions and may not apply elsewhere.