CEU Series: Learning About Pesticide Resistance Is Anything But Futile

Pesticides are used to control various types of pests, such as weeds, harmful insects, and diseases caused by bacteria and fungi.

Pesticides are often the most effective and efficient and sometimes the only pest control tools available.

Resistance to pesticides is a serious, and growing, problem. Globally, there more than 600 species of pests that have developed some level of pesticide resistance. If resistance to a particular pesticide or “family” of pesticides evolves, these products are no longer effective in controlling the target pest reducing the options available for pest management. With few new pesticides (new modes of action) in the development pipeline, pesticide users must do all they can to extend the useful life of the products currently available.

Resistance to insecticides was first documented in the early 1900s when scale insects demonstrated resistance to an inorganic insecticide. Between 1914 and 1946, 11 additional cases of resistance to inorganic insecticides were recorded. Following the introduction of synthetic organic insecticides in the 1940s, such as DDT, it was not long before the first cases of resistance were detected and by 1947, resistance to DDT was confirmed in houseflies. Since then with every new insecticide introduction, organophosphates, carbamates, formamidines, pyrethroids, Bacillus thuringiensis, spinosyns and neonicotinoids, cases of resistance appeared some 2 to 20 years after their introduction in a number of key pest species.

This phenomenon has been described as the ‘pesticide treadmill’, and the sequence is familiar. As a result of continued applications over time the pest evolves resistance to the pesticide and the resistant strain becomes increasingly difficult to control at the labeled rate and frequency. This in turn has often led to more frequent applications of chemicals. The intensity of the resistance and the frequency of pesticide-resistant individuals in the population both increase still further and problems of control continue to worsen as yet more product is applied.

The development of pest resistance has not been restricted to insecticides but has been documented in all types of organisms including bacteria, fungi, vertebrate pests and more recently weeds.

• Colorado potato beetle has developed resistance to 52 different compounds belonging to all major insecticide classes. Resistance levels vary greatly among different populations and between beetle life stages, but in some cases can be very high (up to 2,000-fold).
• In Hawaii and Japan, the diamondback moth developed a resistance to Bacillus thuringiensis about three years after it began to be used heavily.
• In England, rats in certain areas have developed such a strong resistance to rat poison that they can consume up to five times as much of it as normal rats without dying.
• DDT is no longer effective in preventing malaria in some places as the mosquitoes which transmit the disease have become resistant to the chemical, a fact which has contributed to a resurgence of the disease.
• In the southern U.S., the weed Amaranthus palmeri, which interferes with production of cotton, has developed widespread resistance to the herbicide Roundup.

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