The evolution of resistance to pesticides is increasingly problematic for pest control in agriculture. For example, in 1970, there were no known weeds with herbicide resistance, while now there are over 273 herbicide-resistant weeds known. (An "herbicide-resistant weed" is not necessarily resistant to all herbicides, but is resistant to at least some.) It is estimated that, across all kinds of agricultural pests and diseases, approximately 1000 species are resistant to at least one pesticide. In fact, some pests are now resistant to multiple pesticides, with some being resistant to all pesticides that are legally approved for use on them. By as early as 1999, pest resistance to pesticides was estimated to cost US agriculture about $1.5 billion in increased pesticide costs and decreased yields.

Resistance increases because of:

(1) High rates of pesticide use (intense selective pressure) and also

(2) The changing nature of pesticides themselves. Early generation pesticides attacked multiple sites in their targets, and it was more difficult for pests to develop resistance to these. By contrast, newer pesticides are often specific for one biochemical pathway, which allows faster development of resistance. (Fewer genes are involved.)

The greater specificity of modern pesticides is an attempt to make them less broad-spectrum (i.e. to make them more pest specific), and thus to minimize their ecological effects. Again we have an example of the concept that, often, the attempt to solve one problem creates another; the effort to make pesticides less broad-spectrum makes it easier for pests to evolve resistance to them.

Farmers and pesticides producers have locked themselves into a race with rapid pest evolution; an "EVOLUTIONARY ARMS RACE." Crops in many areas of the world are actually viewed as being "at risk" from pesticide-resistance problems, including corn in the US and potatoes in the NE US.

In the face of increasing pest resistance, the strategy of growers often is to apply more and more of the pesticide in hopes of overcoming the resistance. The problem is that this increases the strength of selection for resistant pests (and exacerbates ecological problems associated with pesticides).

Growers also have the option of switching to other kinds of pesticides, but this option is limited as well. The development of a new pesticide takes years and years for all the testing and licensing, and switching to a new compound is a short term solution in any case, because resistance arises again so rapidly!! Development of a new pesticide takes, on average, $80 million, while the typical time before a pest develops resistance is only 10 - 25 years, after which time the pesticide's utility decreases.

Basically, pest susceptibility to pesticides is a valuable resource that has been overexploited. There is concern that the planting of crops that produce pesticides themselves, as a result of genetic engineering, will further speed the evolution of pest resistance to pesticides -- again, exploiting pest susceptibility.

Chemicals designed to enhance and stabilize agricultural production (that is, pesticides) have in some cases done just the opposite -- have caused yield decreases (see case studies , below). This has occurred because the natural systems of control have been interfered with and the old pesticide tricks don't work as well anymore because of resistance problems.

Later in the term, we'll discuss alternatives to reliance on pesticides for pest control in agriculture (sustainable).

Click on "pesticide resistance " for additional information on this topic (from earlier in these notes), ">>" at the bottom of the page to move to a discussion of "secondary pests," and "Navigate " for reminders on how to move within and among these pages.

Page maintained by Patricia Muir at Oregon State University; last updated Dec. '08.