Status of Boll Weevil Resistance to Insecticides in Louisiana During 19611

Expansion of the areas in Louisiana infested with chlorinated hydrocarbon resistant boll weevils (Anthonomus grandis Boheman) has occurred yearly since 1955, with the result that all cotton-producing areas of the State are now affected. Considerable reversion to susceptibility to the chlorinated hydrocarbon insecticides has occurred in field populations, but resistance remains at a level sufficiently great to prevent effective control with these insecticides. After 5 to 6 years of intensive use of organic phosphorus insecticides or toxaphene-DDT mixtures in certain areas, there has been no measurable change in response of the boll weevil to these insecticides. Under laboratory conditions, a 10- to 12-fold increase in the LD-50 of a boll weevil culture to toxaphene-DDT mixture was obtained in 22 generations.