For Release June 13, 2000

AGRI-VIEWS
by Chuck Otte, Geary County Extension Agent

I was listening to a radio talk show the other day when the subject of pesticide resistance in weeds came up. It was apparent from the question, and the answer, that there is a lot of confusion about this subject. In fact, this is such a confusing subject, it’s going to take me two weeks to cover it all.

Ever since the first time that we used a chemical to disrupt the normal life processes of a weed, insect or disease pathogen, we have been faced with the problem of pesticide resistance. (Just as a reminder, due to additional confusion, a pesticide is any chemical that will kill or control an organism. This includes insecticides, herbicides, fungicides and many other "-cides".)

Most people think that pesticide resistance "develops" due to the regular use of pesticides. They feel that the constant application of the chemical to the control population creates a change. The truth isn’t quite that simple. An organism does not slowly adapt to the constant use of a pesticide. Rarely, very rarely, a point genetic mutation does occur that can create a resistant individual. But it is unlikely that this occurs due to the use of the pesticide.

In any population, whether it be a weed or an insect, there is a great deal of genetic variability. Just look at the faces of the people you meet on the street. Even though trees and weeds may look all alike to you, these populations also have a great deal of genetic variability. Even in hybrid crops, there is probably as much as 5% genetic variability. It just isn’t in traits that are easily seen.

In this broad pool of genetic information, you will find that there is probably at least one individual that is just enough different from all the others, that a certain pesticide will not work on them. If there are a lot of individuals with this same trait, the pesticide won’t work very well and will probably not be released for sale. But if there are very few individuals with the resistant trait, the pesticide will work very well and it has a good chance of seeing the market.

Now a lot of "ifs" have to occur. If the product is quickly adapted and used across a large area, and if it works really, really good, and if there aren’t very many individuals in the population that aren’t exposed to the pesticide, a problem will develop. The pesticide has destroyed virtually all of the susceptible population. A majority of the remaining individuals carry the gene(s) for resistance and they are the only ones left to reproduce. Instead of being a very rare occurrence in the natural population, the resistant gene has become the dominant gene in the population. By using the pesticide as much as we have, we have provided a very heavy selection pressure to cause a shift in the population genetics.

Since it is easier for insects to move around and find a mate, we would expect pesticide resistant insects to develop more quickly than pesticide resistant weeds, and that’s exactly what we have historically seen. But herbicide resistant weeds are occurring as have pesticide resistant plant diseases and even human pathogens. Ironically, the better a pesticide works, and the more broadly it is used, the quicker a problem develops. Resistance is much less likely to occur on lesser used products with very limited use where a small percentage of the population is exposed to the selection pressure.

As we start to see more "herbicide tolerant" crops and things like BT corn become more commonly planted, the management of the crop, and the pesticide, become more important to keep the technology viable and effective. Next week I’ll discuss how to reduce the risk of pesticide resistance for the farmer and the homeowner.

-30-

Return to Agri-Views Home Page

Return to Ag Home Page