NC State Extension

Routine Scouting of Soybean Fields and Proper Selection of Insecticide Reduces Yield Loss

A major pest of soybean production within Craven County North Carolina is the corn earworm. This pest can cause severe yield loss if left unchecked. Typically, the pest shows a slight increase about mid-July and peak moth flight occurs some time during the first two weeks of August. When fields are scouted routinely, a proper insecticide application can be made when the pest population exceeds the economic threshold. Normally, one application timed properly is sufficient. Historically, pyrethroid insecticides were used because these products were cheap, highly effective and pose low risk to the environment and workers. However, over the past few years, North Carolina State University has documented an increasing corn earworm population resistant to these pyrethroid materials. (Read more HERE). This resistance, while low, has been confirmed within fields. Growers reported the need for a second application to manage this pest within fields in 2014.

The remedy for proper management of this pest with potential pyrethroid resistance is to continue to scout fields and only treat when the number of corn earworms exceeds threshold levels with a product other than a pyrethroid. Diligence in scouting should be emphasized since corn earworm feeding on foliage and blooms normally does not cause economic loss. Treatments should only be made when the soybean plant is beginning to set small pods. Typically, the non-pyrethroid products are slightly more expensive but the additional cost is less expensive than the time and fuel costs required to make a second trip through the field.

In addition to the corn earworm, soybean loopers and the velvetbean caterpillar should be monitored. Both are normally considered late-season pests. Soybean loopers are first noticed around early August. This pest initial feeds on the bottom of the plant and thus often is unnoticed. Too, the number of products that provide satisfactory control are limited and more expensive than materials for the corn earworm (Read more HERE). Lastly, since initial feeding is deep within the soybean canopy, even a good choice of insecticide material may provide inconsistent results since adequate distribution of the product deep into the soybean canopy is required.

The velvetbean caterpillar is present in most years but seldom reaches levels high enough to warrant an insecticide treatment. However, this pest is a rapid feeder and can quickly defoliate fields. Scouting for this pest should any time during the growing season but thresholds, if met, normally occur from mid-August through mid-October. Threshold for any insecticidal treatment for pre-bloom soybeans is 30% leaf defoliation. After bloom, the threshold drops to 15% leaf defoliation. Most insecticide products control this pest easily.

Written By

Photo of Mike Carroll, N.C. Cooperative ExtensionMike CarrollArea Agent, Agriculture (252) 633-1477 (Office) mike_carroll@ncsu.eduCraven County, North Carolina
Page Last Updated: 3 months ago
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