Standardised bioassays reveal that mosquitoes learn to avoid compounds used in chemical vector control after a single sub-lethal exposure.

Vector-borne diseases are worldwide public health issues. Despite research focused on vectorial capacity determinants in pathogen transmitting mosquitoes, their behavioural plasticity remains poorly understood. Memory and associative learning have been linked to behavioural changes in several insect species, but their relevance in behavioural responses to pesticide vector control has been largely overlooked. In this study, female Aedes aegypti and Culex quinquefasciastus were exposed to sub-lethal doses of 5 pesticide compounds using modified World Health Organization (WHO) tube bioassays. Conditioned females, subsequently exposed to the same pesticides in WHO tunnel assays, exhibited behavioural avoidance by forgoing blood-feeding to ensure survival. Standardized resting site choice tests showed that pre-exposed females avoided the pesticides smell and choose to rest in a pesticide-free compartment. These results showed that, following a single exposure, mosquitoes can associate the olfactory stimulus of pesticides with their detrimental effects and subsequently avoid pesticide contact. Findings highlight the importance of mosquito cognition as determinants of pesticide resistance in mosquito populations targeted by chemical control.

Molecular cloning and xenobiotic induction of seven novel cytochrome P450 monooxygenases in Aedes albopictus.

Cytochrome P450 monooxygenase (P450) is a superfamily of enzymes that is important in metabolism of endogenous and exogenous compounds. In insects, these enzymes confer resistance to insecticides through its metabolic activities. Members of P450 from family 6 in insects are known to play a role in such function. In this study, we have isolated seven novel family 6 P450 from Aedes albopictus (Skuse) (Diptera: Culicidae), a vector of dengue and chikungunya fever. Induction profile of these seven genes was studied using several insecticides and xenobiotics. It was found that deltamethrin and permethrin did not induce expression of any genes. Another insecticide, temephos, inhibited expression of CYP6P15 for fivefold and twofold for CYP6N29, CYP6Y7, and CYP6Z18. In addition, copper II sulfate induced expression of CYP6M17 and CYP6N28 for up to sixfold. Benzothiazole (BZT), a tire leachate induced the expression of CYP6M17 by fourfold, CYP6N28 by sevenfold, but inhibited the expression of CYP6P15 for threefold and CYP6Y7 for twofold. Meanwhile, piperonyl butoxide (PBO) induced the expression CYP6N28 (twofold), while it inhibited the expression of CYP6P15 (fivefold) and CYP6Y7 (twofold). Remarkably, all seven genes were induced two- to eightfold by acetone in larval stage, but not adult stage. Expression of CYP6N28 was twofold higher, while expression of CYP6P15 was 15-fold lower in adult than larva. The other five P450s were not differentially expressed between the larvae and adult. This finding showed that acetone can be a good inducer of P450 in Ae. albopictus. On the other hand, temephos can act as good suppressor of P450, which may affect its own bioefficacy because it needs to be bioactivated by P450. To the best of our knowledge, this is the first report on acetone-inducible P450 in insects. Further study is needed to characterize the mechanisms involved in acetone induction in P450.

Permethrin resistance in Aedes albopictus (Diptera: Culicidae) and associated fitness costs.

Insecticide resistance has become a serious issue in vector management programs. Information on insecticidal resistance and its associated mechanisms is important for successful insecticide resistance management. The selection of a colony of permethrin-resistant Aedes albopictus (Skuse) (Diptera: Culicidae), originating from Penang Island, Malaysia, yielded high larval-specific resistance to permethrin and cross-resistance to deltamethrin. Synergism assays showed that the major mechanism underlying this resistance involves cytochrome P450 monooxygenase. The resistance is autosomal, polygenically inherited and incompletely dominant (D = 0.26). Resistant larvae were reared under different conditions to assess the fitness costs. Under high larval density, larval development time of the resistant SGI strain was significantly longer than the susceptible VCRU strain. In both high- and low-density conditions SGI showed a lower rate of emergence and survival compared with the VCRU strain. Resistant larvae were more susceptible to predation by Toxorhynchites splendens (Wiedemann) (Diptera: Culicidae) larvae. The body size of SGI females reared under high-density conditions was larger compared with females of the susceptible strain. SGI females survived longer when starved than did VCRU females. The energy reserve upon eclosion was positively correlated with the size of the adults.