Posted 29 May 2012. Crop Management.
Gambling With Resistance
Source: University of Illinois at Urbana-Champaign Press Release. aces.illinois.edu
Urbana-Champaign, Illinois (May 21, 2012)--Anyone raising crops and/or produce knows that doing so is a gamble. Will rain come when the crop needs it or will the season prove to be a drought? Will you keep ahead of weeds or will they get ahead of you? Will the market offer a good price or will prices take a tumble? However, each year growers also face another gamble – the resistance gamble. This article will address some different manifestations of that gamble.
• Bad gamble number one: betting that resistance will develop in an economically insignificant pest: Production lore is populated with some interesting examples of resistance development in economically insignificant pests. In such tales, the pest was minor, a once effective product ceased to work, and yet the resistance impact was muted because there was little-to-no dollar impact. However, growers should remember that currently minor pests are not necessarily resigned to insignificance. Previously referenced production lore is also populated with interesting examples of minor pests that became major pests. Resistance development in an otherwise minor pest should always be viewed as a resistance "shot across the bow."
• Bad gamble number two: betting the resistant pest will revert back to normal: Pest populations occasionally become susceptible to a pesticide when they have not been exposed to it for a while. However, there are three problems with "betting on" this approach to resistance. First, this approach willingly discards pest management tools for a period of time. Our pest management goal should always be "lots of tools in a big pest management tool box" not "periodic cannibalization of the pest management tool box." Second, this approach counts on resistant offspring being less fit than susceptible offspring once pesticide exposure is eliminated. Yet, pest management history is littered with examples to the contrary. Finally, those making this argument often contradict themselves. Resistance development (a population-wide genetic /biological shift) is often deemed nearly impossible by individuals who say that "reversion to normal" is commonplace. Yet, the latter is also a population-wide genetic/biological shift. Can we really have it both ways?
• Bad gamble number three: betting that a complicated mode of action/low-to-moderate resistance rating will ward off problems: Some products attack a narrow site of action. Pests may rapidly (and often do) develop resistance to such "simple" products. However, this does not mean that "complicated" products are devoid of resistance concern. For example, DMI fungicides (the family to which triazole fungicides belong) are typically deemed to possess a low-to-moderate resistance risk because they have a very complicated site of action. Yet, a brief survey of fruit tree literature to our south and turf management literature to our north will detect evidence of DMI resistance despite the "complicated nature" of such products. Furthermore, such "complicated" products may manifest resistance as a gradual loss in efficacy rather than a sudden loss in efficacy. This season-by-season reduction in effectiveness, this covert loss of efficacy, may be a more troubling scenario because it often avoids detection until the resistance scenario has become dire.
• Bad gamble number four: betting on "another product" if resistance develops: Rotating between products and/or rotating sites of action (modes of action) can be components of a comprehensive resistance management program. However, such an approach does not eliminate resistance risk. Waterhemp (a pigweed relative) provides the best example of this. Products in the ALS family no longer work against most Illinois waterhemp populations, and as resistance developed, our industry shifted to other products. ALS resistance is now accompanied by glyphosate resistance, PPO resistance, and HPPD resistance in some populations. Addressing resistance by merely shifting to a different product runs the risk of removing many pesticide options simultaneously.
The previously mentioned "bad gambles" should remind growers that their best bet is to use IPM (Integrated Pest Management). IPM programs detect real problems, count on adaptive pests, and attack real problems with various techniques that sometimes take the place of chemicals or various techniques that sometimes accompany the use of chemicals. In many ways, IPM programs "bet" on resistance and that "bet" is pretty reliable.