Molasses as a source of carbon dioxide for attracting the malaria mosquitoes Anopheles gambiae and Anopheles funestus

Abstract

Background: Most odour baits for haematophagous arthropods contain carbon dioxide (CO2). The CO2 is sourced artificially from the fermentation of refined sugar (sucrose), dry ice, pressurized gas cylinders or propane. These sources of CO2 are neither cost-effective nor sustainable for use in remote areas of sub-Saharan Africa. In this study, molasses was evaluated as a potential substrate for producing CO2 used as bait for malaria mosquitoes.

Methods: The attraction of laboratory-reared and wild Anopheles gambiae complex mosquitoes to CO2 generated from yeast-fermentation of molasses was assessed under semi-field and field conditions in western Kenya. In the field, responses of wild Anopheles funestus were also assessed. Attraction of the mosquitoes to a synthetic mosquito attractant, Mbita blend (comprising ammonia, L-lactic acid, tetradecanoic acid and 3-methyl-1-butanol) when augmented with CO2 generated from yeast fermentation of either molasses or sucrose was also investigated.

Results: In semi-field, the release rate of CO2 and proportion of An. gambiae mosquitoes attracted increased in tandem with an increase in the quantity of yeast-fermented molasses up to an optimal ratio of molasses and dry yeast. More An. gambiae mosquitoes were attracted to a combination of the Mbita blend plus CO2 produced from fermenting molasses than the Mbita blend plus CO2 from yeast-fermented sucrose. In the field, significantly more female An. gambiae sensu lato mosquitoes were attracted to the Mbita blend augmented with CO2 produced by fermenting 500 g of molasses compared to 250 g of sucrose or 250 g of molasses. Similarly, significantly more An. funestus, Culex and other anopheline mosquito species were attracted to the Mbita blend augmented with CO2 produced from fermenting molasses than the Mbita blend with CO2 produced from sucrose. Augmenting the Mbita blend with CO2 produced from molasses was associated with high catches of blood-fed An. gambiae s.l. and An. funestus mosquitoes.

Conclusion: Molasses is a suitable ingredient for the replacement of sucrose as a substrate for the production of CO2 for sampling of African malaria vectors and other mosquito species. The finding of blood-fed malaria vectors in traps baited with the Mbita blend and CO2 derived from molasses provides a unique opportunity for the study of host-vector interactions.

Figure 1

A picture of an outdoor trapping system baited with the Mbita odour blend augmented with carbon dioxide produced by fermentation of 250 g molasses using 17.5 g dry yeast mixed with 2 L water. Carbon dioxide was produced in a plastic container and delivered into the MM-X trap through silicon tubing.

Figure 2

Average release rate (ml/min) of carbon dioxide produced by 2 L water mixed with different quantities of yeast and 125 g (panel A), 250 g (panel B), or 500 g (panel C) molasses compared to a combination of 250 g refined sugar, 17.5 g dry yeast and 2 L water (reference treatment).

Figure 3

Effect of adding the Mbita odour blend to carbon dioxide produced by fermentation of 250 g (panel A) and 500 g (panel B) molasses on the mean number (±SE) of Anopheles gambiae collected for four nights. A total of 200 female An. gambiae were released per night within a screen-walled greenhouse. Bars with different letters in the same graph differ significantly (P <0.05). Error bars represent the standard error of the mean number of mosquitoes collected.

Figure 4

Mean number (±SE) of female mosquitoes caught overnight outdoors in MM-X trap without odour, baited with the Mbita blend (MB) augmented with carbon dioxide produced by fermentation of either 250 g refined sugar or 250 g and 500 g molasses. Mean values within the same mosquito type with different letters are significantly (P < 0.05) different.

Figure 5

Mean number (±SE) of outdoor-biting Anopheles gambiae s.l. (panel A) and Anopheles funestus (panel B) mosquitoes of different abdominal conditions (unfed, blood-fed and gravid). The mosquitoes were collected overnight in a MM-X trap without odour, baited with the Mbita blend (MB) augmented with carbon dioxide released from 250 g refined sugar, 250 g or 500 g molasses for 20 nights. Treatments are shown in the legend of panel A. Mean values within the same mosquito type with different letters are significantly (P <0.05) different.