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38 Potato Grower | MAY 2014 Potato farming is an inherently variable business, affected by a great variety of factors ranging from the amount of rain locally to political stability in the Middle East. One unfortunate constant in this sea of continuous change is the threat of significant crop losses due to the damage inflicted by the Colorado potato beetle. Ever since the first major outbreak observed in 1859 in Nebraska, this insect remains a formidable foe of profitable potato farming. High fecundity, healthy appetite at both adult and larval stages, propensity to fly over considerable distances when necessary, and ability to wait out unfavorable conditions by digging into the soil make the Colorado potato beetle a difficult problem to manage. Paris Green was the first insecticide proven to be effective against the Colorado potato beetle in 1871. Since then, a plethora of other compounds have appeared on the market. Unfortunately, the beetle turned out to be extremely adaptable to a wide range of chemicals, with eventual failures reported for virtually every compound used against this pest. Of course, not every population is resistant to every insecticide, but simultaneous resistance to multiple chemicals is fairly common. In some cases, resistance was reported within one or two years of use; however, a lifespan of 10 to 15 years is more common for most products. Development of insecticide resistance is an evolutionary process based on survival of the fittest individuals. Random mutations result in the appearance of beetles that are capable of breaking up poisons in their guts, flushing them out of their systems, or avoiding ingesting poisons on the first place. Under normal circumstances, resistant mutants do not do very well. Their survivorship and reproductive success is usually lower than that of the susceptible beetles. Therefore, relatively few resistant beetles persist in a population that is not exposed to a particular insecticide to which they have developed resistance. When such an insecticide is being applied, however, susceptible beetles perish. Resistant mutants, on the other hand, survive and thrive in the absence of competition. As a result, their numbers increase to the densities sufficient for causing economically significant damage. One of the reasons Colorado potato beetles adapt so readily to a variety of poisons is their original diet. Plants in the nightshade family, including potatoes, contain a fair amount of toxins in their foliage. These protect them from most herbivores. However, the beetles had evolved physiological traits necessary for dealing with natural plant poisons. With some relatively little changes, such traits are also effective in dealing with human-made insecticides. Another reason is that high beetle fecundity (on average, about 600 eggs per female) increases the probability that one of the numerous offspring mutates, and that the numbers of surviving mutants grow at a fast pace. Yet another contributing factor is extensive insecticide use on the majority of commercial potato farms. This ensures that only resistant beetles can survive, while their susceptible competitors die off. Historically, potato growers in the western United States experienced few or no problems with insecticide-resistant Colorado potato beetles compared to their peers in other areas of the country, especially in the Northeast. Exact reasons for this are unclear. It is possible that Western beetles are genetically different. Also, longer crop rotations and larger separation between fields planted in potatoes from one year to the next make it more difficult for Western beetles to find potatoes in the spring, thus increasing the probability that resistant mutants perish on the way to a new host habitat. Nevertheless, Western growers cannot safely assume that insecticide resistance will never develop in the Colorado potato beetle populations on their fields. Evidence from the rest of the country (and the world) suggests otherwise. Preventing resistance development is essential for ensuring long-term success in chemical pest control. So far, the crop protection industry has been able to come up with new chemicals to replace failing compounds. However, the era of abundant and cheap broad-spectrum insecticides—opened with the discovery of insecticidal properties of DDT in 1939—in all likelihood has come to an end. Development and registration of new insecticides is an increasingly involved and expensive process, while older chemistries are being continuously lost to resistance or removed from the market because of environmental concerns. As a result, the existing compounds are becoming more Colorado potato beetle larva. Photo courtesy Erik J. Wenninger, University of Idaho diggin' in PESTICIDE RESISTANCE / by Andrei Alyokhin Breaking Down Resistance Insecticide resistance in Colorado potato beetles