Decreased Susceptibility of Blackfly larvae to p,p'-DDT in New York State and Eastern Canada

In laboratory tests using jars or buckets, blackfly larvae (Simuliidae: Prosimulium and Simulium spp.) from streams with a history of DDT treatment were 4- to 10- fold less susceptible to DDT than those from untreated streams. Studies comparing the influence of test variables on susceptibility indicated that mortalities were not significantly different if alcohol or acetone were used as solvents for the DDT, if the exposure period was 20 or 30 minutes, or if the larvae were exposed in glass jars containing 3000 ml of water and toxicant or in waxed paper cups with 130 ml of water and toxicant. Aeration of water and DDT during the exposure period increased its toxicity about 10-fold. In all tests the larvae were confined in white organdy bags to facilitate transfer and counting.