The change from larval to adult mosquito control in the Gezira area of Sudan resulted in a decrease in the prevalence of
malaria in this region. House spraying with
malathion began in 1975 and resistance to this compound was first detected in 1978. Laboratory tests showed that adult Anopheles arabiensis Patton were resistant to
malathion and
phenthoate but susceptible to all other
organophosphates tested. The larvae of this strain were susceptible to
malathion. The
malathion resistance in the adults was synergized by
triphenyl phosphate, but not by
piperonyl butoxide. This suggests that a
carboxylesterase enzyme may be the basis of
malathion resistance in this strain. Analysis of general
esterase levels to alpha- and beta-
naphthyl acetate showed that there was no quantitative change in the amount of
carboxylesterase enzyme present in the resistant strain as compared to the susceptible. The absence of larval resistance suggests that house spraying rather than agricultural spraying is the major source of selection pressure. The presence of a high level of adult
malathion resistance in A. arabiensis may decrease the efficacy of
malathion for
malaria control in Sudan. The lack of cross-resistance to
organophosphates which do not contain a carboxylester bond means that
insecticides such as
fenitrothion are still practical alternatives.