Mode of Action Helps Counter Resistance Issues
Published: Friday, March 16, 2018
The following is from Jeff Burbrink, Elkhart County Extension educator.
If you are in agriculture, chances are high that you spray something on your crop to manage a disease, insect or weed. Even organic growers use pesticides (products that control pests), although the list of products they can use is limited. Remember, the definition of a pesticide is a product to be applied to repel, kill or otherwise protect a crop or animal. The definition of pesticide, therefore, includes insecticides, herbicides, fungicides, rodenticides, miticides and many other categories.
In recent years, there has been growing concern about resistance to pesticides. There is no better example than glyphosate, which was used on nearly every Corn Belt and Cotton Belt acre for 15 years, leading to a now well-documented list of weeds that are resistant to it.
Pesticide resistance is not limited to herbicides. There are several diseases which have developed resistance to both synthetic and organic fungicides. Copper sprays, for instance, are now ineffective against some bacterial diseases. Likewise, several insects and mites have documented resistance to products that were sprayed on them routinely.
To combat pesticide resistance, experts have developed codes which they assign to each product. The idea behind these codes is to help users understand how the product attacks the pest. The more different options you have to attack a pest, the less likely the pest will develop resistance. These Mode Of Action (MOA) codes are often posted on the labels of products.
To avoid resistance build up in a pest population, the users are encouraged to rotate between the various MOA products. For example, Fusilade herbicide is a MOA Group 1. There are several other Group 1 herbicides, such as Acclaim, Envoy Plus, and Segment, and each product as a different active ingredient. Despite the fact they are different chemistries, they still attack the weed in the same way, so switching between one of these products to another is not going to help prevent resistance issues.
To make it a little more confusing, the industry has created slightly different names for this categories depending on if the product is a herbicide, fungicide, insect, etc. In the fungicide industry, the codes are called FRAC codes (fungicide resistance action committee). In the insecticide/miticides business, the codes are called IRAC codes (Insect Resistance Action Codes).
In the herbicide industry, the code system is a little more complicated. There is a category for Mode of Action (MOA), as well as a Site of Action (SOA) category.
For example, one MOA group is Amino Acid Synthesis inhibitors. It includes two subgroup SOA categories: ALS inhibitors (also known as SOA 2), and EPSP Synthase Inhibitors (also known as SOA 9). Both groups attack amino acid production in plants, but do so in slightly different ways, and therefore get their own distinct category. You can view a herbicide chart at https://goo.gl/czVPr3 to get a better understanding of the herbicide classification system.
Since many of the herbicides on the market have 2 or more active ingredients, you will often see that a product may have more than 2 SOA codes. For instance, Harness Xtra contains acetochlor (SOA 15), and atrazine (SOA 5). Degree Xtra contains the same two active ingredients, so it is a SOA 5 and 15 also. Another product, Cinch ATZ has a SOA of both 5 and 15, but it contains metolachlor and atrazine. Bullet herbicide contains alachlor and atrazine, and is also an SOA 5 and 15 product. Using any one of these four products is attacking the weeds at the same sites on the plant, so it would be unwise to rotate among these four products.
The bottom line is, these codes are going to be important to protect our crops from being devastated by diseases, insects and weeds in the long run. If you are like me, there is no way to memorize all this information. That is where the codes come in. Just realize that when you protect your crop, you should be using these codes to help you find multiple ways to attack the pests that are giving you headaches.
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