Effects of vegetation densities on the performance of attractive targeted sugar baits (ATSBs) for malaria vector control: a semi-field study

Abstract

Abstract Background Attractive targeted sugar baits (ATSBs) control sugar-feeding mosquitoes with oral toxicants, and may effectively complement core malaria interventions such as insecticide-treated nets even where pyrethroid-resistance is widespread. The technology is particularly efficacious in arid areas. However, their performance remains poorly-understood in tropical areas with year-round transmission, and where the abundant vegetation constitutes competitive sugar sources for mosquitoes. This study compared the efficacies of ATSBs (active ingredient: 2% boric acid) in controlled settings with different vegetation densities. Methods Potted mosquito-friendly plants were introduced inside semi-field chambers (9.6m by 9.6m each) to simulate densely-vegetated, sparsely-vegetated, and bare sites without any vegetation (two chambers/category). All chambers had volunteer-occupied huts. Laboratory-reared Anopheles arabiensis females were released nightly (200/chamber) and host-seeking mosquitoes recaptured using human landing catches outdoors (8.00pm – 9.00pm) and CDC-light traps indoors (9.00pm – 6.00am). Additionally, resting mosquitoes were collected indoors and outdoors each morning using Prokopack aspirators. The tests included a “before-and-after” set-up (with pre-ATSBs, ATSB and post-ATSB phases per chamber), and a “treatment vs. control” set-up (where similar chambers had ATSBs or no ATSBs). All tests lasted 84 trap-nights. Results In the initial tests when all chambers had no vegetation, the ATSBs reduced outdoor-biting by 69.7%, indoor-biting by 79.8% and the resting mosquitoes by 92.8%. In tests evaluating impact of vegetation, the efficacy of ATSBs against host-seeking mosquitoes was high in bare chambers (outdoors: 64.1% reduction; indoors: 46.8%) but modest or low in sparsely-vegetated (outdoors: 34.5%; indoors: 26.2%) and densely-vegetated chambers (outdoors: 25.4%; indoors: 16.1%). Against resting mosquitoes, the ATSBs performed modestly across settings (non-vegetated: 37.5% outdoors and 38.7% indoors; sparsely-vegetated: 42.9% outdoors and 37.5% indoors; densely-vegetated: 45.5% outdoors and 37.5% indoors). Vegetation significantly reduced the efficacy of ATSBs against outdoor-biting and indoor-biting (P < 0.001), but not resting mosquitoes. Conclusion While vegetation can influence the performance of ATSBs, the devices remain modestly efficacious in sparsely-vegetated and densely-vegetated settings. Higher efficacies may occur in places with minimal or completely no vegetation, but such environments are naturally unlikely to sustain Anopheles populations nor malaria transmission in the first place. Field studies therefore remain necessary to validate the efficacies of ATSBs in the tropics.