November 2010

Managing Pesticide Resistance By Steve Larson

While seemingly unpredictable, resistance is manageable once its principles are better grasped and preventative best practices are put in place. For growers, understanding resistance realities improves the health of plants and maximizes pesticide investments.

Pest Pressure and Resistance Realities

Healthy plants can be difficult to sustain for extended periods of time even under the best conditions. The fact is the threat of pests is ever-present. Pests, including diseases and insects, constantly threaten plants. The classic Disease Triangle illustrates a host plant growing in a suitable environment along with an infectious pest is likely to result in a disease infection. The triangle refers to disease but its principle also explains weed and insect infestations. The Disease Pyramid (page 30) expands on that principle by adding environmental and human-induced stresses, such as tight spacing in growing operations, altered fertility (excessive or lean), or plant growth regulator use, which subject plants to even more pest pressure. All of these factors play an integral part in managing pests and pest resistance.

It is important to remember that while pesticides are used to control pests, pesticides do not change the pest, only the pest population structure.

Resistance does not stem from a mutation induced by pesticide applications, but rather is naturally inherent in each disease pathogen's biology. Pesticide resistance can be monogenic, conferred by one gene (example, benomyl), or polygenic conferred by more genes (example, DMI fungicides). The level of naturally occurring pest resistance varies among any population of fungi, bacteria and mold species.

That means not every individual within a specific population structure can be controlled. Different individuals within the population contain different levels of tolerance or susceptibility. Resistant characteristics surface when the repeated, uninterrupted use of the same pesticide mode-of-action eradicates a portion of the pest population whose genetic traits were susceptible to that particular mode-of-action. Since not every pest within a given population can be controlled, some pests with resistant (or tolerant) characteristics survive.

Surviving pests that carry resistant genes reproduce, bringing to life a new population that exhibits genetic traits not affected by the chemistry that controlled its predecessor. This "survival of the fittest" biology (above) creates pest populations that can evolve beyond the control of available pesticide chemistries. So if the same mode-of-action is used on a population of pests repeatedly, it will eventually lose effectiveness because the susceptible pests it once controlled are no longer predominant in the population. What we see here is essentially a genetic shift from susceptibility to resistance.

Low resistance levels can occur without complete loss of efficacy making it difficult to predict an onslaught of resistant pests. Resistant subpopulations must reach a sufficient level of economic importance in order to compromise performance and become immune to a mode-of-action. This performance compromise or genetic shift usually takes place over a long period of time — often years. But, unfortunately, once resistance develops to a pesticide, all products with the same mode-of-action are affected.

Resistance Risks

The last thing growers want to do is fuel the creation of an impervious pest population in their nursery or greenhouse operation. Pesticides need to be selected and applied responsibly in order to avoid resistance and sustain efficacy.

The risk of overusing current pesticide products is compounded because only a limited number of basic manufacturers are investing in new chemistries that control pests using new modes-of-action. Pesticide developments are dwindling due to the high cost of research and development and intense registration and regulatory processes.

In fact, a recent CropLife America study cites a nearly 40 percent increase in investment expenditure over the past decade in the discovery, development, and registration of new pest and disease control products in the U.S. and Europe. Bringing a new product to market reached an average of $256 million per product in 2005-2008 and the number of products actually making it through the research and development stages to market introduction declined from four in 1995 to only 1.3 in 2005-2008.

Product stewardship is critical to sustaining a mode-of-action's vitality in the market. Chemical manufacturers and growers must work together to implement best practices that prevent resistance and support investments in future pest control products that serve growers' evolving plant protection needs.

Pesticide Selection and Rotation

Growers should work to use pesticides as a last line of defense against pests. It is important to implement an integrated pest management plan that brings pesticides into play only when the viable threat or presence of pests reaches an economically important threshold. When controlling disease, for example, knowing the benefits and effects of a diverse range of fungicide products is important, because relying on a single product or type of fungicide can spur the development of areas that resist fungicides — an increasingly common trait among fungal diseases.

When using pesticides, follow these resistance management tips:

• Instead of applying pesticides on a calendar schedule, monitor growing conditions to determine when chemical controls are imperative.
• Identify chemistries proven to be "at risk" for resistance in your pesticide shed to avoid any confusion between similar chemistries with different product names. For example, fungicides containing benzimidazole, dicarboximide, phenylamide and strobilurins, utilize just one mode-of-action and are considered "at risk" for resistance.
• When possible, limit the use of "at-risk-for-resistance" pesticide chemistries or consider tank mixing them with a "low-risk-for-resistance" product. Combination products containing two or more modes-of-action are good alternatives to products with a singular chemistry that may have proven successful in past rotations.
• Rotate pesticides' modes-of-action after one to two consecutive applications to avoid prolonged disease exposure to one mode-of-action. Rotation exposes the infectious pest population to multiple, unique modes-of-action and reduces the chance for resistance development to any one specific mode-of-action.
• Maintain a log of the effectiveness of pesticide use against prevalent pests you face. If resistance is suspected, alert your extension office or the manufacturer's technical specialists.