Pest resistance to pesticides develops

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By Susan Brown, County Extension

I receive several calls a week from residents wanting to know how to get rid of pests (moles, weeds, aphids, scale, mole crickets, etc.) in their yards. Humans have been attempting to control insects, weeds, rodents and other pests for thousands of years.
In the last 50 years, significant progress has been made in the development of synthetic (man-made) pesticides, including insecticides, herbicides, fungicides, rodenticides and algicides. Pesticides are important pest management tools. Many pesticides have gradually lost their effectiveness due to the development of resistance by pests they once controlled.
Resistance occurs when a pest population (insects, for instance) is exposed to a pesticide. When this happens, not all insects are killed. Those individuals that survive have done so because they are genetically predisposed to be resistant to the pesticide.
Repeated applications and higher rates of the insecticide will kill increasing numbers of individuals, but some resistant insects will survive. These offspring, many of which will inherit the ability to survive the exposure to pesticide, will rapidly reproduce. A new generation of insects can take place in a few weeks.
The more times a population is exposed to a broad spectrum pesticide, the quicker resistance will develop. Because many generations of some pests can develop in a single year, it is easy to see how resistance can develop quickly in so many pest species.
Studies indicate there are now more than 500 species of insects and mites resistant to pesticides. More than 270 weed species, more than 150 plant pathogens and about a half dozen species of rats are resistant to pesticides that once controlled them.
One pest control strategy is rotating pesticides and/or using tank mixtures or premixes with different mode/target sites of action. This will delay the onset of resistance, as well as slow the development and subsequent buildup of resistance, without resorting to increased rates and frequency of application, and ultimately, will prolong the useful life of many pesticides.
Regardless of the pesticide, remember that knowing the pest you are treating for and its life cycle are crucial for proper control. Most pesticides do not kill the eggs of an insect. Timing is the key to success. Remember that the label is the law and we should always exercise proper pesticide stewardship.
Several products, such as Bacillus thuringiensis, a natural pesticide, have the problem of pest resistance and it is almost unavoidable. Precaution must be taken and research must be done to understand and help prevent resistance with Bt.
The active ingredient, spinosad, is derived from a naturally occurring soil dwelling microorganism. This product was used to control the western flower thrip, which is a problem in greenhouse settings.
Now thrip has developed a resistance to this pesticide. Growers can help delay the development of resistance by applying pesticides only when they are needed, by rotating between different chemical classes, and by using rates of pesticides within the labeled range. Integrating non-chemical approaches such as pheromone mating disruption and cultural controls can also help delay resistance.
Susan Brown is a horticulture agent with the Brunswick County Extension Service. Call 253-2610 or e-mail susan_brown@ncsu.edu.