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

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.

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.

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.

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.