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

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.

Mosquito Traps: An Innovative, Environmentally Friendly Technique to Control Mosquitoes

“…Testing this new approach to mosquito control in Camargue was motivated by the significant impacts revealed by Bti spraying on natural predators of mosquitoes and chironomids [2,3,4,5]. In contrast to larvicide spraying of natural areas, the environmental impact of traps is expected to be negligible, being mostly limited to the impoverished fauna found in urbanized areas where the traps are located.…”

“…However, its sustained use in wetland-dominated areas has revealed strong indirect impacts on animal species that depend on small dipterans and/or their predators for breeding and survival [2]. In Camargue (Rhône delta, southern France), the spraying of 2500 out of 25,000 ha of mosquito larval biotopes with Bti has led to a significant 30%—60% decrease in the breeding success of house martins [3], in the richness and abundance of odonates [4], as well as the invertebrate prey available to reed passerines [5]. Mosquito control in Camargue was initiated 50 years after its implementation on the French Mediterranean coast, on the assumption that Bti use would permit, in contrast to chemical insecticides, a reconciliation of nature protection with human comfort.…”

Using DNA metabarcoding for assessing chironomid diversity and community change in mosquito controlled temporary wetlands

“…So far, field studies have yielded ambiguous data on possible side effects of Bti on chironomid abundances. These range from positive effects on chironomid larvae richness possibly due to reduced mosquito competition (Lundström et al 2010), over no effect on chironomid abundance (Lagadic et al 2016), to a 35-80% reduction of chironomids abundances (Rodcharoen et al 1991, Hershey et al 1995, Vaughan et al 2008, Poulin et al 2010, Jakob and Poulin 2016.…”

Mosquito Traps: An Innovative, Environmental Friendly Technique to Control Mosquitoes

“…Testing this new approach to mosquito control in the Camargue was motivated by the significant impacts revealed by Bti spraying on natural predators of mosquitoes and chironomids [2][3][4][5]. In contrast to larvicide spraying of natural areas, the environmental impact of traps is expected to be negligible, being mostly limited to the impoverished fauna found in urbanized areas where the traps are located.…”

“…Yet, its sustained use in wetland-dominated areas has revealed strong indirect impacts on animal species that depend on small dipteran and/or their predators for breeding and survival [2]. In the Camargue (Rhône delta, southern France), the spraying of 2500 out of 25 000 ha of mosquito larval biotopes with Bti has led to a significant 30-60% decrease in breeding success of House martins [3], in richness and abundance of odonates [4], as well as invertebrate-prey available to reed passerines [5]. Mosquito control in the Camargue was initiated 50 years after its implementation on the French Mediterranean coast, on the assumption that Bti use would permit, in contrast to chemical insecticides, to conciliate nature protection with human comfort.…”