MSU Extension

Farming is like most other fields of human endeavor – when we find something that works, we stick with it. When selecting pest control options in corn and soybeans, for instance, if the cost is right and the treatment is effective, growers tend to stay with the control programs they know work well. With all the things that can change over the course of a growing season, selecting “tried and true” crop protection programs helps make production agriculture more predictable and manageable. But staying the course with crop protection chemicals can lead to unexpected challenges.

For many years now, we have known that repeatedly using the same herbicide program causes weed species shifts in the field. Plant species that can tolerate the herbicide program become the dominate weeds. Continuous use of herbicide programs selects for species that have an edge to survive. But in addition to species shifts, we can also see a development of pests that are resistant to the pesticide program.

Tolerance is different from resistance. Tolerant species are less responsive to a herbicide program; resistance occurs when a weed species that used to be controlled by a herbicide is suddenly no longer controlled by the chemical. Pesticide resistance can develop by various mechanisms. The most common occurs when a pest evolves the ability to detoxify the pesticide, effectively reducing the amount of the active ingredient that is available for control. The pest may also change the metabolic process that the crop protection chemical attacks, rendering the material harmless to the organism. Pest resistance is not limited to pesticide programs. Even crop rotation, one of the oldest non-chemical tools in the pest management toolbox, can be thwarted. Western corn rootworm beetles changed their behavior to laying eggs in soybean fields in the Corn Belt during the 1990s.

In managing pesticide resistance, field crop producers have been relatively lucky compared with fruit and vegetable growers. In Michigan, herbicides were the only pesticide in field crop production that was routinely used each season. Michigan fruit and vegetable producers make multiple pesticide applications each growing season to control disease and insect pests. They have been battling resistant pests such as fungicide-resistant apple scab, streptomycin-resistant fire blight, and insecticide-resistant coddling moth and Oriental fruit moth for many years in parts of southwestern Michigan. These pests are costly to manage, and now growers have few options for control in many areas where they are prevalent.

Resistance management as a component of integrated pest management.

IPM has always stressed the tactical approach of using pesticides only when and where they are needed. Taking the process the next step includes rotation of classes of herbicides, insecticides, fungicides and other pest control strategies, and a higher level of understanding to be able to select control options that will allow you to rotate pesticide modes of action to help to stave off resistance. Tools such as the "Corn and Soybean Herbicide Chart" printed by University of Wisconsin Extension make the tedious job of researching herbicides by mode of action as easy as sorting potential programs at a glance by color. FRAC codes listed on fungicides can help you to design programs that reduce reliance on a single mode of action.

The more challenging job of sorting out issues such as level of control for the weed species or insects that you have in your fields, maximum weed and crop heights, and potential for crop injury still needs to be done before pesticide program selections are made. Michigan State University Extension bulletin E-434, "Weed Control Guide for Field and Forage Crops," or E-1582, "Insect and Nematode Control in Field and Forage Crops," do an excellent job of putting this information in a format that makes it easy for growers to evaluate the options and develop pest management programs to help reduce the chance of pests developing pesticide resistance. For most growers, this means investing more time in broadening our understanding of potential pesticide programs and the crop management strategies used in our field crop operations. That can definitely take us out of our comfort zone, but with increasing incidences of pesticide resistance showing up across the country, managing to delay or prevent resistant pests may soon be one of the most cost-effective decisions we can make for our operations.

Bruce Mackellar is a Michigan State University Extension field crops educator. He can be contacted at mackella@msu.edu.

Timeline of Michigan Field Crop Pest Resistance

Triazine-resistant lambsquarters       1975

Common ragweed (atrazine)             1990

Rotation-resistant WCR beetles        1996

Common ragweed (ALS inhibitors)   1998

Waterhemp (ALS inhibitors)              2000

Lambsquarters (ALS inhibitors)         2001

Horseweed (ALS inhibitors)               2002

Velvetleaf (atrazine)                            2004

Giant foxtail (ALS inhibitors)              2006

Horseweed (glyphosate)                    2007

Palmer amaranth (Gly+ ALS)              2011

Source: WSSA International Survey of Herbicide Resistant Weeds