Effects of irritant chemicals on Aedes aegypti resting behavior: is there a simple shift to untreated "safe sites"?

BACKGROUND: Previous studies have identified the behavioral responses of Aedes aegypti to irritant and repellent chemicals that can be exploited to reduce man-vector contact. Maximum efficacy of interventions based on irritant chemical actions will, however, require full knowledge of variables that influence vector resting behavior and how untreated "safe sites" contribute to overall impact. METHODS: Using a laboratory box assay, resting patterns of two population strains of female Ae. aegypti (THAI and PERU) were evaluated against two material types (cotton and polyester) at various dark:light surface area coverage (SAC) ratio and contrast configuration (horizontal and vertical) under chemical-free and treated conditions. Chemicals evaluated were alphacypermethrin and DDT at varying concentrations. RESULTS: Under chemical-free conditions, dark material had significantly higher resting counts compared to light material at all SAC, and significantly increased when material was in horizontal configuration. Cotton elicited stronger response than polyester. Within the treatment assays, significantly higher resting counts were observed on chemical-treated dark material compared to untreated light fabric. However, compared to matched controls, significantly less resting observations were made on chemical-treated dark material overall. Most importantly, resting observations on untreated light material (or "safe sites") in the treatment assay did not significantly increase for many of the tests, even at 25% SAC. Knockdown rates were ≤5% for all assays. Significantly more observations of flying mosquitoes were made in test assays under chemical-treatment conditions as compared to controls. CONCLUSIONS/SIGNIFICANCE: When preferred Ae. aegypti resting sites are treated with chemicals, even at reduced treatment coverage area, mosquitoes do not simply move to safe sites (untreated areas) following contact with the treated material. Instead, they become agitated, using increased flight as a proxy indicator. It is this contact irritant response that may elicit escape behavior from a treated space and is a focus of exploitation for reducing man-vector contact inside homes.

Space optimizing methods for laboratory rearing of Aedes aegypti.

Methods used to rear Aedes aegypti larvae were modified to maximize production under limited space requirements. Traditionally, 150 larvae were reared in 25 x 42 x 5-cm pans containing 1 liter of water. It was noticed, however, that the larvae congregated during development and did not utilize the entire rearing environment. Two new rearing methods were investigated to partition larval populations within the rearing space and thus increase rearing efficiency. The 2 methods separated groups of 50 larvae into each of either six 500-ml cylindrical cups or twelve 400-ml rectangular bins, each containing approximately 450 or 400 ml of water, respectively, and occupying approximately the same amount of shelf space as the 25 x 42 cm pans. The cup and bin protocols multiplied rearing potential by 2 and 4, respectively, under the same space constraints. A preliminary experiment was conducted to compare larval development and adult body size (using wing length as a surrogate) of mosquitoes reared in partitioned containers with those reared in the traditional open pan method. Results indicated that the pan and bin partition methods produced adults that were significantly larger than pan-reared adults, yet develop to 4th stage at a significantly slower and less synchronized rate.