Absence of close-range excitorepellent effects in malaria mosquitoes exposed to deltamethrin-treated bed nets

Abstract

Flight behavior of insecticide-resistant and susceptible malaria mosquitoes approaching deltamethrin-treated nets was examined using a wind tunnel. Behavior was linked to resulting health status (dead or alive) using comparisons between outcomes from free-flight assays and standard World Health Organization (WHO) bioassays. There was no difference in response time, latency time to reach the net, or spatial distribution in the wind tunnel between treatments. Unaffected resistant mosquitoes spent less time close to (< 30 cm) treated nets. Nettings that caused high knockdown or mortality in standard WHO assays evoked significantly less mortality in the wind tunnel; there was no excitorepellent effect in mosquitoes making contact with the nettings in free flight. This study shows a new approach to understanding mosquito behavior near insecticidal nets. The methodology links free-flight behavior to mosquito health status on exposure to nets. The results suggest that behavioral assays can provide important insights for evaluation of insecticidal effects on disease vectors.

Figure 1.

Schematic diagram of the wind tunnel. Air inlet (AI), lamination screen (LS), glass funnel containing heat element (F), mesh screen (S), framed net (FN), release cup (RC), cameras (C1 and C2), and infrared lights types 1 (IR1) and 2 (IR2) are shown.

Figure 2.

(A and B) Percentage of mosquitoes that was recorded knocked down after 1 hour after testing in the wind tunnel or after 3 minutes of exposure in a cylinder tube or WHO cone. C and D show the death rate in percentage after 24 hours. Numbers above bars represent numbers of individuals recaptured in the wind tunnel and total numbers tested (in groups of five) for the other bioassays.

Figure 3.

Mean (±SEM) response time in seconds for each treatment in which mosquitoes took off for upwind flight within 3 minutes after being released at the downwind side of the wind tunnel.

Figure 4.

One-hour health status of both mosquito strains in relation to the duration spent on the net (x axis) and immobile on the net (y axis) in seconds.

Figure 5.

Example of tracking results for individual mosquitoes of the susceptible strain approaching three different netting samples. The odor source is located on the upwind side, just behind the netting. The mosquito that contacted the untreated netting was unaffected. The other two mosquitoes showed a 1-hour knockdown response and died within 24 hours after recapture.