Exploring phytoplankton management for controlling the malaria vector Anopheles gambiae in Benin

The relationship between different species of phytoplankton and Anopheles gambiae mosquito larvae was studied in order to improve malaria control. Anopheles gambiae larval habitat was surveyed by sampling phytoplankton at 51 sites. The deleterious effect of Scenedesmus quadricauda on third instar larvae was evaluated in the laboratory. 27 phytoplankton species were encountered in the field survey. Anopheles gambiae larvae had a positive association with high densities of Spirogyra (Pearson chi-square = 25.56, P < 0.0001) and a negative association with high densities of green algae (Spirogyra, Elakotothrix, Coelastrum, Ankystrodesmus, Chlorella, Kirchneriella, Selanastrum, and Scenedesmus) and cyanobacteria (Oscillatoria, and Nostoc). Laboratory bioassays revealed that S. quadricauda is indigestible and toxic to An. gambiae larvae that ingest it. The larval mortality was 100% when larvae ingested only S. quadricauda and 85.6% if larvae were exposed to a mixture of S. quadricauda and cat food. The findings of this study indicate that phytoplankton management with S. quadricauda or other green algae offers a possible way to eliminate An. gambiae production. Complementary studies are needed to investigate the deleterious effect of these algae on An. gambiae under semi-natural and natural conditions.