The primary objective of the current study was to quantify the effects of exposure of Ae. aegypti to spatial repellent compounds on catch rates from a validated adult mosquito trap – the Biogents Sentinel™ (BGS) . The purpose was to generate critical information regarding how a spatial repellent may interfere with the efficacy of the BGS when the two tools are used in combination as a push-pull strategy. We specifically sought to determine: (i) if Ae. aegypti females exposed to spatial repellent chemicals (DDT, metofluthrin and transfluthrin) have a reduced likelihood of being captured with BGS traps (i.e., effect host-attraction) and (ii) if such an effect, should it occur, is immediate but short-lived or potentially latent.
Metofluthrin has been previously evaluated for repellency against Ae. aegypti[46, 47], Culex quinquefasciatus[48, 49] and An. balabacensis[48, 49] and metofluthrin coils have been reported to significantly reduce landing counts of Ae. aegypti. Transfluthrin is a fast acting insecticide that exhibits high volatility and knock down activity at high concentrations . It is used in household products against various pest insects, such as mosquitoes, flies and moths. Evaluations have been conducted using transfluthrin to repel Cx. quinquefasciatus[51, 52], An. arabiensis and Ae. albopictus[54, 56]. DDT has spatial repellent qualities as indicated in previous experimental hut studies [17, 42, 43].
The BGS trap has previously been validated as an effective tool for the monitoring and surveillance of the dengue virus vector Ae. aegypti[26–31]. However, the efficacy of the BGS trap to attract and catch (or pull) chemically-repelled or insecticide-exposed mosquitoes is not known. Many chemicals can elicit repellent behavioral responses in Ae. aegypti at doses well below those required for toxic outcomes [1, 16, 17], but the effects of such exposures on the mosquito’s host-seeking behavior are poorly understood. For another important dengue virus vector, Ae. albopictus, changes in both host-seeking and blood-feeding behaviors upon exposure to plant volatiles under laboratory conditions have been described . Aedes albopictus females surviving exposure to geraniol, citral, eugenol, or anisaldehyde for 24 and 48 h all showed different degrees of reduction in host-seeking ability (e.g., increased times to reach a target location and to search for a suitable feeding site and insert the stylet). After 48 h of exposure to 0.250 μg/cm3 of anisaldehyde, 100% of the mosquitoes showed loss of host-seeking ability, through impacts on the time to host-seeking activation, orientation, probing and engorgement compared to unexposed controls. In another study, Ae. aegypti females were exposed to sublethal levels (LD25) of pyrethroid insecticides to evaluate the effects of the neurotoxicants 24 h post-exposure. A significant reduction in time of activation to flight was observed in mosquitoes exposed to deltamethrin and permethrin . Similarly, excito-repellency studies using lower concentrations of deltamethrin showed that mated Ae. aegypti exhibited significant differences in escape responses with and without hosts present . This type of knowledge is critical to define the expected efficacy of a BGS trap in a repellent focused push-pull strategy.
Holding female Ae. aegypti in experimental huts with DDT-treated fabric did not significantly impact subsequent BGS capture compared to non-exposed females, regardless of whether the exposed females were evaluated immediately following DDT exposure or following a 12 h recovery period in a repellent–free setting. Thus, there was no immediate or latent negative impact on host-seeking ability, as estimated by BGS trap catches, using this DDT exposure route. Similarly, mosquitoes exposed to metofluthrin coils with high (0.0065%) or low (0.003%) concentrations were as likely to be captured with the BGS traps as non-exposed control mosquitoes. Previous exposure to transfluthrin at 0.125 and 0.062 FAR resulted in significantly lower trap catches, compared to control mosquitoes, for mosquitoes released immediately following exposure but not for those allowed to recover for 12 h before BGS trap evaluation.
The comparison of results for mosquitoes released immediately following exposure versus those allowed to recover for 12 h before being released into the environment with the BGS traps indicate that this is a temporary phenomenon. This is suggestive of effects on sensory pathways used to detect host cues that resolve following the 12 h holding period. Similarly, Hao et al. noted that a reduction in host-seeking ability in Ae. albopictus in the laboratory was reversible following recovery times that were dependent on chemical and concentration specific exposure conditions. However, we cannot rule out the possibility that the observed increase in host-seeking activity/trap catch rates for the delayed release females resulted, in part, from that they were only supplied with water during the recovery phase and thus were more motivated to locate a food source at the end of the recovery period.
Repellents have been shown to induce changes in responses of olfactory receptor neurons of female mosquitoes , specifically involving the grooved peg sensilla and sensilla trichodea, which are located on the mosquito antennae [61–64]. This neuronal activation disrupts the mosquito’s ability to detect host-seeking kairomones, components of human sweat and presumably also the BGS trap lure (BG Lure). The decline in sensitivity of Ae. aegypti to human odor as a result of repellent exposure might be a mechanism for the temporary suppression of host-seeking behavior , as seen for the IR mosquitoes exposed to transfluthrin. Most evidence published to date on the basis of action of repellent compounds (e.g., DEET) are clearly conflicting and support either hypotheses indicating that repellents mask odors by blocking their receptors or act as true odorants that seem to be avoided by pests. However, none of these studies refer to insecticides similar to those tested in the present study. Several insecticides have been described that induce hyperactivity at sublethal doses and even promote the avoidance of impregnated areas. However, no clear evidence exists to date to link these effects to those of known repellents acting on insect chemoreceptors. The specific mechanism of action behind the observed change in BGS recapture rates over time following exposure to transfluthrin, could not be addressed in the current study, but highlights the need to integrate laboratory and field evaluations as a model for translational research.
There are several study design biases that could have influenced our results. This includes innate differences in spatial repellent actives, such as volatility, and the fact that trials were performed independently at various times of year under varying temperatures that could also affect chemical volatility. Although the two treatment formats varied (treated fabric vs. coils), each format represented the typical exposure method that target vector mosquitoes would experience under operational implementation for these interventions. This allowed for a more accurate assessment of exposure effects of the spatial repellents as would be expected under natural conditions. In addition, the exposure methods used – where mosquitoes were held in screened cages placed inside experimental huts containing spatial repellent treatments (treated fabrics or coils) mimicked expected exposure routes for airborne repellent molecules where direct contact with treated surfaces does not occur. Despite these potential study design biases, it is clear that mosquito behavior is an area of research that will continue to be of high importance, especially as development of novel vector control tools are necessary in order to combat diseases such as malaria and dengue .