Impact of mosquito control operations on waterbirds in a Camargue nature reserve

The Domaine de la Palissade is a 700-ha nature reserve located at the mouth of the Rhône river in southern France. Since 2006, the tidal wetlands have been sprayed with Bacillus thuringiensis israelensis (Bti) and water level fluctuations minimized to reduce mosquito nuisance. Aerial spraying of VectoBac® 12AS (32 ITU/ha) is carried out 10 to 23 times a year, whenever Ochlerotatus caspius and Oc. detritus larvae develop, covering between 611 and 1937 cumulated hectares. Long-term trends (2000–2013) in monthly abundance of 50 bird species were compared with two nearby nature reserves that served as controls. A significant decrease in abundance of eight species (from 1958 to 941 monthly counts) was attributed to mosquito control operations and more specifically to: (1) aircraft disturbance caused by aerial spraying in a proportion of 36%, affecting mostly ducks; (2) stabilization of water levels in a proportion of 34%, affecting mostly grebes; and/or (3) reduced food resources caused by Bti effects in a proportion of 30%, affecting mostly waders. The European coot, affected negatively by all three factors, suffered the highest decline. Alternatively, the lower water levels and higher salinity resulting from the reduced exchanges with the Rhône River had a positive impact on five species, mostly gull and shorebirds (from 201 to 280 monthly observations). Overall, this study suggests a 23% significant decrease in waterbird abundance due to mosquito control, in spite of the collaboration between site managers and the operator of mosquito control to minimize negative effects on the non-target fauna.