Indirect effects of mosquito control using Bti on dragonflies and damselflies (Odonata) in the Camargue

Abstract

Abstract Bacillus thuringiensis var. israelensis (Bti) has become the most commonly used larvicide to control mosquitoes worldwide. Bti is considered non‐toxic to most organisms, except some Diptera such as chironomids, which are a major prey in wetland food webs. Although Odonata are important predators of mosquitoes and chironomids at the larval and adult stages, no study has ever considered the potential indirect effects of Bti on Odonata abundance through trophic interactions. We addressed this topic in the Camargue where 2500 of the 25 000 ha of mosquito larval biotopes are Bti‐sprayed (aqueous solution of VectoBac 12AS at 2.5 L ha−1) whenever mosquito larvae appear in water bodies (i.e. 30–50 aerial treatments overall annually). Adult Odonata were surveyed along a 100‐m line transect in spring, summer and autumn at three control and three treated sites over a 5‐year period. Mean number of species (9.9 vs. 5.2) and of individuals (100 vs. 50) detected per year were significantly higher in control areas compared to Bti‐sprayed areas. Bti treatment contributed to 87.3% of the explained variance in Odonata richness, compared to 2.9% for site, 6.8% for year and 3.0% for salinity effects. These results are coherent with other studies carried out in the same area and time period highlighting a lower abundance of chironomids, and a lower intake of odonates by breeding birds in treated areas. We conclude that mosquito control using Bti should be acknowledged as a potential threat to Odonata.