The biochemical basis of tolerance to malathion in Rhodnius prolixus.

1. LC50 of malathion, fenitrothion and lindane were determined in R. prolixus and T. infestans. R. prolixus was shown to be tolerant to malathion. 2. The penetration rate of (14C)-malathion into R. prolixus and T. infestans was similar. 3. Acetylcholinesterase from R. prolixus heads was 3.3-fold less sensitive to inhibition by malaoxon than the similar enzyme of T. infestans. 4. R. prolixus showed more activity of GSH-S-transferases against DCNB than T. infestans. 5. The in vitro degradation of (14C)-malathion demonstrated that R. prolixus is more active than T. infestans in carboxyester splitting to give alpha and beta monoacids. 6. The synergism of TPP and TOCP on malathion toxicity was higher in R. prolixus than in T. infestans. 7. Esterase activity against alpha and beta naphthyl acetates proved to be much lower in R. prolixus homogenates than in T. infestans homogenates. An inverse result was observed when PTA was the substrate.