The Drought Monitor shows thing essentially unchanged from the previous week. Barton and Pawnee Counties remain in good shape with most of Stafford County listed as abnormally dry. Today’s topic may seem a bit early, or too late, and involves pesticide resistance developing in weeds and insects, especially weeds. Even most people not involved in agriculture are aware of resistance issues that have developed over the last four or so decades with the advent and increased usage of pesticides. Why be concerned now? Several reasons including all the challenges with new chemistries and the restrictions being placed on them for one. And a larger reason is how to we prevent this in the future? First, let’s look at how it happened.
The best place to start is to define a pesticide, something that kills a pest whether a disease, insect, or weed. For simplicity, let’s consider herbicides. Modern herbicides came into their own after WWII. All have one thing in common, they have a particular mode of action – how they kill the plant. Some are quite selective and only kill specific species or types of plants while others are nonselective and simply kill plants. There are many modes of action. Some target photosynthesis, some cellular respiration, while others affect cell division. And they target a particular aspect of a given biological process. They are grouped into different classes based on how they act.
Now let’s look at a selective herbicide, one that only targets certain species or types of plants – say a broadleaf herbicide such as dicamba. When sprayed on a field of corn it damages and hopefully kills broadleaf weeds like pigweed species. It is also sprayed on the corn but doesn’t harm the corn even though the corn takes it in. The corn is able to metabolize, or breakdown, the chemical so it doesn’t damage the crop.
Roundup ® or glyphosate was originally a nonselective herbicide and was effective on most, not all, plants. It didn’t matter if it was a broadleaf or grass. Even before the problems of today, glyphosate wasn’t equally effective on all plants. Some were harmed but not killed while others were resistant or able to metabolize the chemistry. By the mid-1990s Monsanto had developed and released Roundup Ready soybean and then corn cultivars which now include crops like alfalfa and canola. They were able to genetically engineer these crops to metabolize glyphosate.
Now what does this have to do with weed resistance? Say an average pigweed plant flowers. It is possible to produce well over one hundred thousand seeds and these are produced sexually so all the seeds are not identical genetically. So if you spray them with an herbicide they are susceptible to most will be affected and die. But out of say 100,000 seeds some will be different enough genetically they will not be killed or only damaged and able to produce seed and pass on that trait. A very few over time are going to be different enough that they are able to metabolize the chemistry and will essentially be resistant and they reproduce. So if a producer uses the same chemistry all the time without variation, the result is a shift in the plant population form susceptible to non-susceptible plants. The plants have evolved. This is what has happened.
Next week – Part II