The Evolution of Resistance to Pesticides

A retrospective search for the causes of the pesticide dilemma inescap­ ably leads to the question of resistance; had it not been for the phenomenal development of resistant populations, insect control could doubtless have been accomplished with fewer applications at far lower dosages. Against the walnut aphid in California, parathion was originally recommended at a dos­ age of 0.25 lb per tOo gal of water. Seven years later the required dosage was 1-H2 Ib per tOO gal (173). In northern Nigeria, treatments of the river Ni­ ger with DDT for control of Simulium damnosum had to be increased from 0.033 ppm in 1961 to 0.077 ppm in 1968 (195). In Armenia, one applica­ tion of BHC at 30 kg/ha was sufficient to control Phytonomus variabilis on alfalfa in 1949. By 1961 six times as much BHC was needed for adequate control (115). Entomological literature abounds with similar examples aptly illustrating the considerable genetic plasticity of insect populations. However, the problem is not simply that a higher dose is needed to bring about the desired effect. Because of their generally wide spectrum of toxicity, insecticides also suppress predatory and parasitic species. Since these are de­ pendent on host density, their capability for developing resistance is severely hindered and often annulled by the abrupt reduction in host density following the application of an insecticide. Of more than 225 species of arthropods in which resistant strains have been documented (20, 43), only four are natural enemies of pests. Suppression of natural enemies contributes further to the unleashing of resistant pest populations. This necessitates even more frequent applications of insecticides, often of greater variety, since a number of pest species, including some formerly secondary pests, may now be involved. In extreme but not unusual cases, insecticides must be applied at weekly inter­ vals or more frequently, to afford protection. In Central America, as many as 30 applications on cotton during the 6-month growing season are common