   Understanding Pesticide Resistance - January 11, 2012 Jeff Schalau, Agent, Agriculture & Natural Resources University of Arizona Cooperative Extension, Yavapai County For millennia, human beings have protected their crops, stored food, domestic animals, clothing, and living quarters from various pest organisms. These pests include insects and other arthropods, plant pathogens and wood decomposers, weeds, rodents, and other vertebrate pests. The industrial revolution increased food production and transport, but only within the last 60 to 70 years did we see the widespread use synthetic pesticides. These included insecticides, herbicides, fungicides, rodenticides, and algicides. No doubt these products have increased crop production and our ability to store food and fiber. However, pesticides have also caused unforeseen consequences. One of these is the increasing resistance of pest species to pesticides we use to manage them. To better understand pesticide resistance, let’s review the process of natural selection. Natural selection is the nonrandom process by which biological traits become either more or less common in a population as a function of differential reproduction of their bearers (i.e. survival of the fittest). With respect to pesticide resistance, organisms that are exposed to a given pesticide and die are unable to pass on their biological traits to the next generation. Conversely, survivors can breed with other survivors and increase their biological traits within the exposed population. Continued exposure to a given pesticide can cause subsequent pest generations to increase their resistance to that pesticide. DDT was the first big breakthrough pesticide. It was used to control mosquitoes, fleas, bedbugs, flies, and many other insect pests. In 1947, wallpaper manufacturers were adding DDT to their product so that it would kill flies on contact. And federal scientists had begun evaluating DDT’s safety in the paper used by stores to wrap groceries. In 1949, DDT was used as a treatment for rivers: just two quarts were needed to deal with fly- and mosquito-infested regions up to 25 miles downstream. The scientific community once stated that thanks to DDT (and good sanitation), families could plan on soon kissing their flyswatters goodbye. However, within only a few years, researchers were starting to see resistance to DDT in mosquitoes, houseflies, bedbugs, and many other insect pests. DDT also had other issues – it was very resistant to decomposition and accumulated as it went up the food chain, especially in birds. Environmental author, Rachel Carson, wrote the book Silent Spring in 1962 which focused on environmental issues related to the widespread use of DDT. Many of us know the end of this story – in short, during the 1970’s and 1980’s DDT was banned from agricultural use in most developed countries. Over the last 20 years, we have seen the introduction and widespread use of transgenic crops – specifically, crop plants that have had their genetic makeup artificially altered to have resistance to certain herbicides and/or produce insecticidal compounds. Herbicide resistance allows farmers to spray specific herbicides on the entire field. The intent is for crop plants to survive and weeds to be killed. However, weeds are now becoming increasingly resistant to the specific herbicides. Bacillus thuringiensis (Bt) is a bacteria that has insecticidal properties. It is used widely by home gardeners and organic growers to manage caterpillars and other pests. Plant breeders genetically modified some crop plants (corn, soybeans, cotton, and others) to have the ability to produce the Bt toxin within the plant. In these plants, most insects feeding on them die, but a few survive - again leading to greater Bt resistance in the pest population. We know that when entire fields are planted with these genetically modified crops, the insect and weed pests are more likely to become resistant. To combat resistance, growers were given guidelines to plant some non-transgenic crops in refuge areas to maintain populations of insects and weeds that were not exposed to Bt or herbicides. Still, we are observing increased pesticide resistance in many insect populations. What’s the solution? As usual, common sense should prevail. Any pest control strategy that relies on one method of control is doomed to fail. The tried and true practice of Integrated Pest Management (IPM) tells us that we should use all tools in our toolbox to manage pests. These tools can include prevention, cultural practices, sanitation, crop rotation, trapping, barriers, monitoring, and pesticides (if necessary). Human beings continue to learn (and relearn) through experience. At this point, I usual remind myself: if it sounds too good to be true, it probably is! If you have other gardening questions, call the Master Gardener line in the Camp Verde office at 928-554-8999 Ext. 3 or e-mail us at cottonwoodmg@yahoo.com and be sure to include your name, address and phone number. Find past Backyard Gardener columns or provide feedback at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/.  |