RESISTANCE MANAGEMENT STRATEGIES IN MALARIA VECTOR MOSQUITO-CONTROL -BASE-LINE DATA FOR A LARGE-SCALE FIELD TRIAL AGAINST ANOPHELES-ALBIMANUS IN MEXICO

A high level of DDT resistance and low levels of resistance to organop hosphorus, carbamate and pyrethroid insecticides were detected by disc riminating dose assays in field populations of Anopheles albimanus in Chiapas, southern Mexico, prior to a large-scale resistance management project described by Hemingway et al. (1997). Biochemical assays show ed that the DDT resistance was caused by elevated levels of glutathion e S-transferase (GST) activity leading to increased rates of metabolis m of DDT to DDE. The numbers of individuals with elevated GST and DDT resistance were well correlated, suggesting that this is the only majo r DDT resistance mechanism in this population. The carbamate resistanc e in this population is conferred by an altered acetylcholinesterase ( AChE)-based resistance mechanism. The level of resistance observed in the bioassays correlates with the frequency of individuals homozygous for the altered AChE allele. This suggests that the level of resistanc e conferred by this mechanism in its heterozygous state is below the l evel of detection by the WHO carbamate discriminating dosage bioassay. The low levels of organophosphate (OP) and pyrethroid resistance coul d be conferred by either the elevated esterase or monooxygenase enzyme s. The esterases were elevated only with the substrate pNPA, and are u nlikely to be causing broad spectrum OP resistance. The altered AChE m echanism may also be contributing to the OP but not the pyrethroid res istance. Significant differences in resistance gene frequencies were o btained from the Fi mosquitoes resulting from adults obtained by diffe rent collection methods. This may be caused by different insecticide s election pressures on the insects immediately prior to collection, or may be an indication that the indoor- and outdoor-resting A. albimanus collections are not from a randomly mating single population. The und erlying genetic variability of the populations is currently being inve stigated by molecular methods.