Insecticide susceptibility status in Mexican populations of Stegomyia aegypti (= Aedes aegypti): a nationwide assessment

In Mexico, mosquito vector-borne diseases are of public health concern as a result of their impact on human morbidity and mortality. The use of insecticides against adult mosquitoes is one of the most common ways of controlling mosquito population densities. However, the use of these compounds has resulted in the development of insecticide resistance. The aim of this study was to estimate susceptibility to six pyrethroids, two carbamates and two organophosphates in Mexican populations of Stegomyia aegypti (Linnaeus, 1762) (= Aedes aegypti) (Diptera: Culicidae) mosquitoes. Bottle insecticide susceptibility tests, with 1 h exposure, were performed on adult mosquitoes from 75 localities across 28 states. At 30 min of exposure, the proportion of fallen mosquitoes was recorded. After 60 min of exposure, mosquitoes were recovered in non-treated containers and mortality was determined at 24 h after the set-up of the experiment. In general, the carbamate insecticides represented the most effective group in terms of the proportion of mosquitoes fallen at 30 min (72-100%) and 24-h mortality (97-100%). High and widespread resistance to pyrethroids Types I and II and, to a lesser extent, to organophosphates was observed. Insecticide susceptibility among and within states was highly variable.

Sustained efficacy of the novel topical repellent TT-4302 against mosquitoes and ticks

Mosquitoes and ticks are blood-feeding pests of humans and animals that can vector many important aetiological agents of disease. Previous research demonstrated that TT-4302 (Guardian(®) Wilderness) containing 5% geraniol applied to human subjects gave 5-6 h of repellency against mosquitoes (depending on species) and was repellent to ticks in vitro. This study was conducted to obtain an independent third-party evaluation of TT-4302 against Stegomyia aegypti (= Aedes aegypti) (Diptera: Culicidae) mosquitoes and to test the efficacy of the product in the field against ticks. TT-4302 provided an average of 6.5 h of repellency of ≥ 95% [Weibull mean protection time: 7.4 h, 95% confidence interval (CI) 5.8-11.3 h] for St. aegypti, whereas a 15% DEET formulation provided 4.7 h of repellency (Weibull mean protection time: 5.2 h, 95% CI 3.7-6.9 h). In tick field trials, the efficacy of TT-4302 did not differ significantly from that of a 25% DEET formulation against Amblyomma americanum (Ixodida: Ixodidae). TT-4302 was 81.3% repellent at 2.5 h after application, whereas DEET was 77.2% repellent at the same time-point. Results at 3.5 h after application were 71.4% for TT-4302 and 72.9% for DEET.

Experience- and age-mediated oviposition behaviour in the yellow fever mosquito Stegomyia aegypti (=Aedes aegypti)

In repeated behaviours such as those of feeding and reproduction, past experiences can inform future behaviour. By altering their behaviour in response to environmental stimuli, insects in highly variable landscapes can tailor their behaviour to their particular environment. In particular, female mosquitoes may benefit from plasticity in their choice of egg-laying site as these sites are often temporally variable and clustered. The opportunity to adapt egg-laying behaviour to past experience also exists for mosquito populations as females typically lay eggs multiple times throughout their lives. Whether experience and age affect egg-laying (or oviposition) behaviour in the mosquito Stegomyia aegypti (=Aedes aegypti) (Diptera: Culicidae) was assessed using a wind tunnel. Initially, gravid mosquitoes were provided with a cup containing either repellent or well water. After ovipositing in these cups, the mosquitoes were blood-fed and introduced into a wind tunnel. In this wind tunnel, an oviposition cup containing repellent was placed in the immediate vicinity of the gravid mosquitoes. A cup containing well water was placed at the opposite end of the tunnel so that if the females flew across the chamber, they encountered the well water cup, in which they readily laid eggs. Mosquitoes previously exposed to repellent cups became significantly more likely to later lay eggs in repellent cups, suggesting that previous experience with suboptimal oviposition sites informs mosquitoes of the characteristics of nearby oviposition sites. These results provide further evidence that mosquitoes modify behaviour in response to environmental information and are demonstrated in a vector species in which behavioural plasticity may be ecologically and epidemiologically meaningful.

An in vitro bioassay for the quantitative evaluation of mosquito repellents against Stegomyia aegypti (=Aedes aegypti) mosquitoes using a novel cocktail meal

To assess the efficacy of new insect repellents, an efficient and safe in vitro bioassay system using a multiple-membrane blood-feeding device and a cocktail meal was developed. The multiple-membrane blood-feeding device facilitates the identification of new insect repellents by the high-throughput screening of candidate chemicals. A cocktail meal was developed as a replacement for blood for feeding females of Stegomyia aegypti (=Aedes aegypti) (L.) (Diptera: Culicidae). The cocktail meal consisted of a mixture of salt, albumin and dextrose, to which adenosine triphosphate was added to induce engorging. Feeding rates of St. aegypti on the cocktail meal and pig blood, respectively, did not differ significantly, but were significantly higher than the feeding rate on citrate phosphate dextrose-adenine 1 (CPDA-1) solutions, which had been used to replace bloodmeals in previous repellent assays. Dose-dependent biting inhibition rates were analysed using probit analysis. The RD(50) (the dose producing 50% repellence of mosquito feeding) values of DEET, citronella, carvacrol, geraniol, eugenol and thymol were 1.62, 14.40, 22.51, 23.29, 23.83 and 68.05 µg/cm(2), respectively.

Pre-treatment of Stegomyia aegypti mosquitoes with a sublethal dose of imidacloprid impairs behavioural avoidance induced by lemon oil and DEET

The present study was conducted to determine whether imidacloprid can impair the avoidance behaviour of the mosquito Stegomyia aegypti. Laboratory investigations using a T-maze apparatus showed that St. aegypti mosquitoes present long term avoidance behaviour when they are exposed to repetitive trials with lemon oil and DEET. The present study tested the effect of a sublethal dose of imidacloprid on the avoidance behaviour of St. aegypti mosquitoes over a 48 h period. Data suggest that 0.5 ng of imidacloprid/mosquito reduces the avoidance behaviour of mosquitoes exposed to lemon oil, on the first day of exposure, after the second trial; whereas imidacloprid affected DEET repellency only the first day of exposure, after the second trial. Imidacloprid was toxic against St. aegypti mosquitoes, and at sublethal doses was able to impair the repellency induced by lemon oil and DEET. The present data were consistent with the finding that St. aegypti mosquitoes exhibit long term avoidance behaviour, and treatment of mosquitoes with a sublethal dose of imidacloprid under DEET application can affect the repellency of DEET against St. aegypti.

Invasion of Wolbachia at the residential block level is associated with local abundance of Stegomyia aegypti, yellow fever mosquito, populations and property attributes

Wolbachia can suppress dengue and control mosquito populations and this depends on the successful invasion of Wolbachia-infected mosquitoes into local populations. Ovitrap data collected during the recent invasion of wMel-infected Stegomyia aegypti (Diptera: Culicidae) (Linnaeus) into Gordonvale near Cairns, Australia, were used to identify variables that help predict the success of localized invasion. Based on the variance in Wolbachia frequencies across Gordonvale as well as at another release site at Yorkeys Knob in comparison to simulations, it was estimated that on average 2-4 females contributed eggs to an ovitrap. By collating ovitrap data from two collection periods at the start of the release from residential blocks, it was found that uninfected mosquitoes had a patchy distribution across the release site. Residential blocks with relatively high uninfected mosquito numbers were less easily invaded by Wolbachia than blocks with low numbers. The numbers of uninfected mosquitoes in ovitraps were negatively correlated with the proportion of brick houses in a residential block, whereas local Wolbachia frequencies were correlated positively with this variable as well as negatively with the amount of shading in a yard and availability of breeding sites. These findings point to proxy measures for predicting the ease of localized invasion of Wolbachia.