Does mosquito control byBtispraying affect the phytoplankton community?A 5-year study in Camargue temporary wetlands (France)

Mosquitoes are both vectors of disease and a hindrance to outdoor activities. Since its discovery in 1976, the larvicide Bacillus thuringiensis var. israelensis (Bti) has proven its effectiveness in controlling mosquito populations, with negligible environmental impact. We performed an integrated, 5-year study on the potential ecological direct and indirect effects of mosquito control agent Bacillus thuringiensis var. israelensis (Bti applied as VectoBac® WG) on phytoplankton communities in the Camargue temporary wetlands of the French Mediterranean coast. Mosquito larvae are considered major algae predators, so a distinct reduction in mosquito larva density (>80%) in natural wetlands due to Bti-treatment could be expected to indirectly affect these phytoplankton communities. Physical parameters and phytoplankton were sampled in the water of three temporary oligohaline pools between 2006 and 2011 in the following order: T0=1 day before treatment (control) – T2=2 days after treatment – T5=5 days after treatment – T11=11 days after treatment. No negative effects on phytoplankton densities and diversity could be attributed to Bti-treatment: no plytoplankton proliferation was observed in these treated pools. However, changes in the density of phytoplankton taxonomical groups were observed: the Diatoms group was reduced and replaced by other algal groups typical of temporary wetlands and able to tolerate drought. These results suggest that the phytoplankton community's evolution in total density, density of phytoplankton taxonomical groups and community structure is largely driven by natural environmental factors and by the ecological complexity of these temporary wetlands.