Concerns about insecticide resistance

Insects become resistant to chemical insecticides very rapidly. This can happen in as few as five generations – natural selection at work.

The problem is that an insecticide never kills all of its intended victims. If even a few insects survive, they will reproduce. They will produce two types of young - those that are resistant to the spray, and those that are not.

The non-resistant insects will be killed in the next spraying, but those that are left reproduce. At each generation, the number of naturally resistant insects in the population increases.

An individual insect does not become resistant during its lifetime. It is born either resistant or non-resistant, and it is the population as a whole that gradually becomes resistant to the pesticide over time. The Bt toxins become ineffective, and the benefits of using them (less toxicity to non-target species) disappear.

As this occurs, a new pesticide must be developed. Over time, populations of insects can become resistant to more and more pesticides. As a result, humans need to make different pesticides that are generally stronger.

Organic farmers have used Bt on their crops for a number of years. They are concerned that the increased use of the Bt toxin could speed up the development of resistant insect populations.

Entomologists know that controlled, laboratory experiments with generations of insects can not be easily reproduced in the field. How the resistant insects breed with refuge insects, and over what time frames, will determine the success of this technology.

These concerns are balanced by concerns that existing pesticide practices can be much more dangerous for non-target insect species than insect-resistant crops. Conventional non-selective pesticides kill many non-target insects. By reducing the number of sprays needed, insect-resistant crops help to preserve beneficial predator insects and simplify management decisions.