Treating Soybeans to Prevent Egg Laying by Western Corn Rootworms: Here We Go Again

August 13, 1999
We continue to receive reports that some soybean fields are being treated to prevent egg laying by western corn rootworms. We'll say it again: This is an irresponsible approach to the management of this insect pest. If insecticides are just now being applied, one must ask: How much egg laying has already taken place? How long will egg laying continue in soybean fields? How much residual activity will the chosen insecticide provide on soybean foliage? Will a soil insecticide also be recommended next spring at planting? The study of egg-laying patterns of western corn rootworms in soybeans continues to be an active research project in several producers' fields; however, we do not have enough answers at this point to satisfactorily address these questions. Producers who treat their soybean fields now in an attempt to prevent egg laying will still be faced with the uncertainty next spring regarding whether or not a soil insecticide represents a sound economic investment. Two insecticide applications most assuredly represent a poor economic and environmental decision.

Recall that our economic threshold is based on the use of Pherocon AM traps properly deployed in soybean fields for a 4-week period. We recommend that 12 traps should be used throughout east-central Illinois. For producers in counties on the margins of this problem area, we suggest that four traps be deployed to check for the presence or absence of western corn rootworms in soybeans. After 4 weeks, if your average equals two beetles captured per trap per day, this suggests that root injury equal to a root rating of 3.0 (several roots pruned, less than one node) may occur the following season. An average of seven beetles per trap per day may result in a root rating of 4.0 (one node or the equivalent of one node of roots destroyed) the following year. If these thresholds are reached, producers are encouraged to consider the use of a soil insecticide on rotated corn next spring. These thresholds should not be used to trigger insecticide applications to soybean fields.

In our final August issue of the Bulletin, we will pass along the results of our on-farm root rating trials. Results from these efforts should enable us to "fine-tune" our economic threshold if so required.--Mike Gray

Author: Mike Gray