Absence of Close-Range Excitorepellent Effects in Malaria Mosquitoes Exposed to Deltamethrin-Treated Bed Nets

Abstract

Abstract. Flight behavior of insecticide-resistant and susceptible malaria mosquitoes approaching deltamethrintreated 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. BACKGROUND

Quantifying late-stage host-seeking behaviour of Anopheles gambiae at the insecticidal net interface using a baited-box bioassay

“…Fundamental questions include clarification of the extent to which ITNs can or cannot repel host seeking mosquitoes. Laboratory studies have produced conflicting results, reporting that close to the ITN surface, repellency does [16][17][18] or does not [19,20] occur. Clearly the net material, the insecticide used, the method of loading the net with active ingredient(s) and the age of the net are all important.…”

Infrared video tracking of Anopheles gambiae at insecticide-treated bed nets reveals rapid decisive impact after brief localised net contact

“…Some studies reported that insecticide residues repelled mosquitoes prior to contact 13 14 , which would reduce or eliminate the chance of mosquitoes receiving an effective dose and potentially divert them to unprotected hosts 15 . Others found no evidence for such repellency 16 17 18 19 indicating that LLINs attract and impact on mosquitoes by direct contact.…”

“…Informative studies to date have been limited to wind-tunnel or small-scale laboratory tests, potentially restricting mosquito flight. Frequently, tests use artificial or incomplete attractants such as human breath or limited body parts, carbon dioxide, single attractant chemicals or simple odour blends 9 18 28 , rather than an entire human host. Experimental huts 29 30 31 , electrocution grids 32 33 , taxis boxes 34 and other methods overcome some of these obstacles but are unsuitable for detailed exploration of behavioural sequences.…”

Determination of the amount of insecticide picked up by mosquitoes from treated nets

“…Mosquito -LLIN interactions have been characterized using infrared video tracking, showing that susceptible mosquitoes made between 11.0 and 57.1 seconds of contact with a LLIN during the initial 10-minute period of most intense mosquito activity around the net [8]. Mean time spent on deltamethrintreated net by a susceptible mosquito, causing knockdown and death, was measured to be 70.1 seconds, with the minimum required to cause knockdown just 0.4 seconds [24]. The exposure time in our experiments was chosen accordingly, starting at 30 seconds, whereas the longest exposure time (3 minutes) was the same as that used in WHO standard method for LLIN evaluation [47].…”

“…Developing new vector control tools, including the next-generation LLINs, will require a more thorough understanding of how they function in terms of their physiological mode of action and mosquito behavior around them [8]. The minimum duration of LLIN contact necessary to deliver an effective insecticide dose is not known, but mosquito-LLIN interactions have been described and average contact times measured (<2 min contact time for a single mosquito) [8,24].…”