Development and assessment of plant-based synthetic odor baits for surveillance and control of malaria vectors

Abstract

Background: Recent malaria vector control measures have considerably reduced indoor biting mosquito populations. However, reducing the outdoor biting populations remains a challenge because of the unavailability of appropriate lures to achieve this. This study sought to test the efficacy of plant-based synthetic odor baits in trapping outdoor populations of malaria vectors.

Methodology and principal finding: Three plant-based lures ((E)-linalool oxide [LO], (E)-linalool oxide and (E)-β-ocimene [LO + OC], and a six-component blend comprising (E)-linalool oxide, (E)-β-ocimene, hexanal, β-pinene, limonene, and (E)-β-farnesene [Blend C]), were tested alongside an animal/human-based synthetic lure (comprising heptanal, octanal, nonanal, and decanal [Blend F]) and worn socks in a malaria endemic zone in the western part of Kenya. Mosquito Magnet-X (MM-X) and lightless Centre for Disease Control (CDC) light traps were used. Odor-baited traps were compared with traps baited with either solvent alone or solvent + carbon dioxide (controls) for 18 days in a series of randomized incomplete-block designs of days × sites × treatments. The interactive effect of plant and animal/human odor was also tested by combining LO with either Blend F or worn socks. Our results show that irrespective of trap type, traps baited with synthetic plant odors compared favorably to the same traps baited with synthetic animal odors and worn socks in trapping malaria vectors, relative to the controls. Combining LO and worn socks enhanced trap captures of Anopheles species while LO + Blend F recorded reduced trap capture. Carbon dioxide enhanced total trap capture of both plant- and animal/human-derived odors. However, significantly higher proportions of male and engorged female Anopheles gambiae s.l. were caught when the odor treatments did not include carbon dioxide.

Conclusion and significance: The results highlight the potential of plant-based odors and specifically linalool oxide, with or without carbon dioxide, for surveillance and mass trapping of malaria vectors.

Figure 1. Trapping efficacies of carbon dioxide and linalool oxide for An. gambiae s.l. and An. funestus s.l.

. Number of replicates  = 18; bars capped with asterisks are significantly different from their respective controls as determined by general linear model with negative-binomial error structure and log link in R 2.15.1 software; **  = P<0.01, ***  = P<0.001.

Figure 2. Proportions of male and engorged female An. gambiae s.l. caught by different odor treatments.

The bars show the proportions of male and engorged (blood-fed + semi-gravid/gravid) female An. gambiae s.l.; numbers embedded in the bars are the total of mosquitoes caught by each of the odor treatment; LO  =  (E)-linalool oxide; OC  =  β-ocimene; the different treatments were compared to socks (control); bars capped with asterisks are significantly different from their respective controls as determined by chi square test of proportions in R 2.15.1 software *  = P<0.05, **  = P<0.01, ***  = P<0.001.

Figure 3. Proportions of male and engorged female An. funestus s.l. caught by different odor treatments.

The bars show the proportions of male and engorged (blood-fed + semi-gravid/gravid) female An. funestus s.l.; numbers embedded in the bars are the total of mosquitoes caught by each of the odor treatment; LO  =  (E)-linalool oxide; OC  =  -β-ocimene; the different treatments were compared to socks (control); bars capped with asterisks are significantly different from their respective controls as determined by chi square test of proportions using R 2.15.1 software *  = P<0.05, **  = P<0.01, ***  = P<0.001.