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Pesticide resistance

Posted 10/22/2015

There have been so many inventions and discoveries over the past hundred years, more or less, that it can be hard to imagine life without them. Our essentials, like phones, cars, and computers have changed all our lives, mainly for the better. For agriculture and horticulture, pesticides have had an impact as profound as many of these other discoveries.

Although some people hear the word “pesticides” and think only of chemicals to kill insects, the term also covers products that we rely on to control weeds, plant diseases, even those small, cute furry creatures that invade our gardens, eating vegetables and landscape plants.

Conventional and organic growers have their own strong opinions about the benefits and risks. Still, no one can dispute that those chemicals have been tremendously important to modern farming. Much of the world’s food production depends on using herbicides, insecticides and fungicides to protect crops and insure yields. Unless we exclusively eat organic foods, most fruits, vegetables and grains we consume received treatments with some kind of agricultural chemical. Fortunately, the US Department of Agriculture (USDA) closely regulates pesticide residues on food crops, so that is much less of a worry than it might have been without that governmental intervention.

As a nation, we spend large amounts of money on all sorts of pest control. USDA figures from 2007 indicate that we spent about 12.5 billion dollars on chemical control, with a large portion being for herbicides (weed control). We spent nearly the same amount on insecticides. This works out to about 500,000,000 pounds of herbicides and 99,000,000 pounds of insecticides applied to public and private American lands. Modern agriculture uses much of these, although ornamental horticulture in parks and golf courses also relies on them.

Pesticides have been so successful that we often take them for granted. If there is an insect pest, we spray it! If weeds have invaded the garden, spray them! This has been occurring for years, but we have reached a point where we need to reconsider that approach to pest control.

We use several methods when applying these chemicals, and they may have helped to increase a problem called “pesticide resistance.” Some of our most valuable pest control products have lost much of their usefulness, as more and more problem creatures develop the ability to survive despite being treated with very potent products.

This difficulty with resistance can develop in several ways. Most commonly, it happens when the same chemical gets used repeatedly, without other interventions. Originally, people would saturate an area with the largest dose of the longest lasting product, as often as possible. The idea was to kill all the pests. This was successful, at least to a degree, but it has had several side effects. For one thing, when we were killing the pests, other, non-target organisms also succumbed. Many researchers now believe that one of the contributing factors in the decline of important pollinators such as bees and butterflies has been overuse of insecticides.

There is no way to kill every pest, no matter how attentive we are or how potent the pesticide is. Resistant insects, weeds, and fungi are becoming more and more widespread. When we used large quantities of these chemicals in our attempts to wipe out a population, we succeeded in killing most of the vulnerable individuals but left behind resistant survivors.

Sometimes the problem is not that the chemical itself has stopped working, but the person applying the product made a mistake. It may sound curious, but if we are going to use a pesticide, we must never use too little. It should not be too dilute, since that will only destroy the most susceptible creatures while the resistant ones go on to reproduce. Their offspring would probably also be resistant.

Applying these chemicals at the wrong time or in the wrong weather can also contribute to loss of their effectiveness. If part of what was sprayed simply drifts off-target in high winds or temperatures, it is the same as using too little product. Only the most vulnerable will perish.

The results of this resistance are alarming. Currently, 17 species of insects are not controlled by most, if not all, insecticides. Houseflies do not succumb to most insecticides. By now, over 500 species are resistant to at least one. It is not simply a bug problem either. Some of our worst weeds can withstand our pesticides.

Ragweed, the main cause of hay fever, can survive four different types of weed killers. There is a serious problem weed called “barnyard grass” that resists nine different types.

I choose to think of pesticides as the last weapon in an arsenal. Try to make the environment less hospitable to the pest, by growing resistant plants. Scout the landscape to find insects before they do damage. Keep the garden aerated to limit disease.

Email or call Angela O’Callaghan, Social Horticulture Specialist, at 702-257-5581.

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