Can pesticides and larval competition alter susceptibility of Aedes mosquitoes to arbovirus infection?

Worldwide distribution of Aedes aegypti and recent expansion of Aedes albopictus represents major health concerns because these mosquitoes are the major vectors of dengue viruses. In general, mosquito larvae develop in aquatic habitats that may be exposed to pesticides directly through vector control activities or indirectly through agricultural runoffs. Larval exposure to these pesticides usually occurs in the presence of biotic and abiotic stressors. We examined how intraspecific competition among larvae and low concentrations of malathion alters Ae. aegypti and Ae. albopictus adult life history traits and competence for Sindbis virus (SINV). We manipulated two larval densities (low, high) and two malathion concentrations (0 and 0.04 parts per million) and exposed the resulting females to an infectious blood meal. For both species, competition and the presence of malathion reduced survival to adulthood. The presence of malathion eliminated the negative effects of competition which resulted in lengthened development time and smaller sized adults. For Ae. aegypti, but not Ae. albopictus, high competition conditions and the presence of malathion independently led to twofold increases in virus dissemination from the midgut to other tissues. Our results suggest that larval competition and chemical contaminants may influence disease transmission directly by altering adult mosquito traits and indirectly by altering vector interactions with arboviruses.