Pesticide Resistance

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Q: I sprayed for weeds in my yard and aphids on my roses, and neither treatment seemed to work. Do you think my two biggest pest problems have developed resistance to pesticides?

A: Pesticide resistance is a serious worldwide problem, but there are other possible reasons your sprays did not work. It would take genetic testing of your pests to be sure. Is it possible that you didn’t follow the label directions and applied the products incorrectly?

Some products are less effective if the pH of the water you use is not correct, and they can become even less effective the longer they sit in the water before being applied. For instance, the fungicide Captan has a half-life of 32 hours in water at pH 5, but only 10 minutes at pH 8. My house’s water has a pH of 9, so I don’t use Captan.

All plants, bacteria, viruses, and animals are chemical factories. Insects and plants are in constant chemical battles as plants try to create toxic chemicals that stop insects and insects create methods to bypass the plant’s defenses.

People use plant chemicals such as neem oil and pyrethrin and create new formulations of pesticides based on plant and insect chemicals. It should not be a surprise when plants, bacteria, viruses, and animals adapt to resist pesticides just like they adapt to the other chemical toxins in their environments.

Pesticide tolerance is the natural ability of a species to survive and reproduce after pesticide treatment at a normal use rate because the species is already naturally tolerant. Because of genetic differences, tolerance to a pesticide can exist in individuals of a population of pests prior to the application of the pest controls.

Pesticide resistance is the inherited ability of individuals to survive a pesticide application that would kill a normal population of the same species.

Pesticides kill susceptible individuals and not resistant individuals. Resistant individuals then reproduce and may or may not pass on the ability to resist the pesticide. As more resistant individuals survive, the majority of the population becomes resistant to that pesticide. The genetic variability of the susceptible individuals is lost, meaning that the chemically resistant individuals may be weaker in other areas, such as cold tolerance. Cases of resistance have been found in all types of pests.

Genetic changes that create resistance are not just to manmade chemicals. Some insects are now resistant to the natural bacterial insecticide Bt (Bacillus thuringiensis). Other insects have changed their behavior. Mosquitoes in some places now stay outside buildings during the day. Corn rootworms in corn to soybean crop rotations spend an extra year in diapause during the soybean year.

In each general category of chemical treatment, such as herbicide, rodenticide, etc., there are groups of chemicals that work in different ways. Switching between pesticide groups with different modes of action can delay resistance. Manufacturers often recommend only a few consecutive applications of a pesticide before moving to a different pesticide group.

When pests develop resistance to a pesticide in a group, the resistance will probably be to all pesticides in the group. Some groups are close enough in mode of action that the resistance is to multiple groups.

A pest species in one area of the country may become resistant to one group of pesticides, and the same species in a different area may develop a resistance to a different group.

To help prevent resistance, it is wise to regularly change the type of controls used on a pest. Following an integrated pest management approach provides a balanced approach that minimizes resistance. Don’t use chemical controls whenever possible.

This is not just a weed or insect problem. Antibacterial soap is a pesticide against bacteria. There are now bacteria resistant to the chemicals in these soaps.