Insect versus Weed: Resistance Management
by Bob Hartzler

February 12, 2013 -  While herbicide resistant weeds have received the most publicity (at least from a weed scientist’s perspective), the same phenomenon jeopardizes the future value of the insecticides and fungicides used in agronomic cropping systems. The Insecticide Resistance Action Committee (IRAC) released a statement in 2012 related to the use of insecticide mixtures.  The primary point IRAC made is that while the use of two active ingredients often improves insect control, most mixtures are not intended for resistance management.  Therefore, insecticide tank mixes generally provide minimal benefits in terms of delaying the onset of insecticide resistance.

The use of multiple herbicide groups is the primary tactic being used in the US Cornbelt to manage herbicide resistance.  As stated by IRAC, managing resistance is much more complicated than simply combining products in a way that provides effective pest control.  The committee stated the following are required for mixtures to be effective in resistance management:

   1)  Individual components should be highly effective against target species at the rates they are used in the mixture;
   2)  Individual components should be from different insecticide groups (sites of action);
   3)  Mixtures are less effective if resistance is already developing to the components of the mixture; and
   4)  The components should provide similar levels of residual control.

Although there are significant differences in how insecticides and herbicides are used, the evolution of resistance to these pesticides involves the same ecological processes.  If the contribution of the individual components of commonly used herbicide programs is critically evaluated, they usually fail to meet the criteria outlined by IRAC.  The process of evaluating herbicide programs for their effectiveness in managing resistance was described in detail in an earlier article on this website.

 Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University

For more information contact:
Bob Hartzler
1126C Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1164

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Common chemical and trade names are used in this publication. The use of trade names is for clarity by the reader. Inclusion of a trade name does not imply endorsement of that particular brand of herbicide and exclusion does not imply nonapproval.