I receive several calls each week from residents wanting to know how to get rid of pests such as moles, weeds, aphids, scale, and mole crickets in their yards. Humans have been attempting to control insects, weeds, rodents and other pests for thousands of years. In the last 50 years synthetic, or man-made, pesticides have become important pest management tools. Over time, many pesticides have lost their effectiveness due to the development of resistance in the target pests.
How a pest becomes resistant
Resistance occurs when a pest population, insects for example, is exposed to a pesticide. When this happens, not all of the insects are killed. Some individuals survive because they are genetically predisposed to be resistant to the pesticide. The few remaining resistant insects reproduce rapidly, passing on their ability to survive exposure to the pesticide to their offspring.
The more times a population is exposed to a pesticide the quicker resistance will develop. Because pests can reproduce many generations in a single year, it is easy to see how resistance can quickly develop in pest species. Studies indicate that there are now over 500 species of insects and mites resistant to pesticides. In addition, over 270 weed species and over 150 plant pathogens are resistant to the pesticides that once controlled them. In many agricultural systems pest resistance has become a serious challenge to food production.
What might help?
Pesticides include products that control weeds, insects, and plant diseases. In agriculture and commercial horticulture, growers do not spray the same pesticide over and over again. Instead, pesticides with different modes of action are rotated to minimize the development of resistance in pests. This is also a good practice for home gardeners.
Pest resistance develops for both synthetic and organic pesticides. Reducing the amount of pesticides used is an important strategy for limiting pesticide resistance. Integrated pest management, known as IPM, makes use of many different control strategies to manage pest problems, with pesticides used as a last resort.
IPM practices for weeds include hand weeding, mulching to reduce weed problems, and spacing plants so they shade out weeds. IPM practices for insect and disease control include growing varieties resistant to specific pests, keeping plants healthy by preparing the soil before planting, and reducing stress by maintaining proper moisture and nutrient levels.
When pesticides are necessary, having the pest you are treating properly identified and knowing its life cycle are crucial for effective and sustainable control. Timing is the key to success because many pests are only susceptible to pesticides during a certain stage of their development. For example, most insecticides do not kill insect eggs. Spraying when only eggs are present will result in failed pest control, wasted product, and unnecessary exposure of people, pets, and wildlife to pesticides. When applying any pesticide, remember to read and follow all label directions.
Many older products are no longer effective for controlling the broad range of pests they once controlled. For example, products containing carbaryl (commonly sold as Sevin) are no longer effective for controlling insect pests such as Colorado potato beetle. Some types of caterpillars and worms have become resistant to B.t. (Bacillus thuringiensis), a natural pesticide often used by organic gardeners. Insect control products containing the active ingredient spinosad, derived from a naturally occurring soil dwelling microorganism, are no longer effective for controlling western flower thrips. Many weeds are now resistant to glyphosate, the active ingredient in Roundup and many other broad spectrum weed killers. This has caused challenges for both farmers and home gardeners.
Gardeners can help delay the development of pest resistance by applying pesticides only when they are needed, rotating products with different modes of action, and by using rates of pesticides within the labeled range. Integrating non-chemical approaches such as resistant varieties and physical removal of pest infested leaves or plants can also help reduce chemical use and delay the development of pest resistance.
Pesticide modes of action are listed in the North Carolina Agricultural Chemicals Manual, available online at ipm.ncsu.edu/agchem/agchem.html. To learn more about managing garden pests, visit ces.ncsu.edu where you can submit questions via the ‘Ask an Expert’ link, or contact your local Cooperative Extension center by phone: If you live in Pender County, call 910-259-1238; in New Hanover County, call 910-798-7660; in Brunswick County call 910-253-2610.
Susan Brown is the horticulture agent with the New Hanover County Cooperative Extension of NC State University, College of Agriculture & Life Sciences.