Pesticides that kill mosquitoes could have indirect effects on other species, according to researchers. The study indicated that a well-known mosquito insecticide led to reduced numbers of eggs and chick survival for house martin birds.
Microbial insecticides infect insects with bacteria, viruses, amoebas or fungi. As they usually have no effect on non-target animals or plants, they are considered more environmentally friendly than traditional chemical pesticides. A well known example is Bacillus thuringiensis var. israelensis, serotype H14 (Bti), a bacterium used to control mosquitoes, fungus gnats and blackflies. Its direct effects on non-target species are largely limited to non-biting midges.
The study investigated the possible effects on breeding house martins of spraying Bti in the Camargue area of Southern France during the nesting season. It compared the diet, foraging activity and breeding of house martins in an area that was treated with Bti with an area where Bti was not used. Six colonies of house martins located at least 5 km apart were studied.
At the untreated sites midges and mosquitoes were eaten in a greater amount by house martins, as were spiders and dragonflies, which are predators of mosquitoes and midges. At the treated sites, flying ants were more common in the house martin diet. The use of Bti also affected the size of the prey with smaller prey eaten at the treated sites and larger prey eaten at the untreated sites.
Furthermore, the number of eggs and the number of young that actually fledged were both smaller at the treated sites, with an average of two chicks fledged per nest at the treated sites compared to three at the untreated sites.
The impact of Bti on the house martins' diet is not necessarily surprising. What is more insightful are the indirect impacts on the wider food chain, which then lead to knock on effects on the house martins. For example, there were also fewer spiders and dragonflies in the diet of birds in the treated area, most probably because these also feed on mosquitoes and midges. This is likely to have influenced the decrease in the size of prey detected in the diet. Smaller prey tends to contain less energy, which in turn could have influenced the lower number of eggs and chicks at treated sites.
It seemed that the diet of the house martins in the Bti treated sites allowed breeding but possibly limited the number of eggs in a brood. The decrease in the number of chicks that fledged at treated sites was most likely due to starvation.
The study highlights the importance of cascade effects caused by repeated application of Bti , despite it being the most selective and least toxic mosquito pesticide. Other animal species are certainly at risk considering the role of midges in wetland food webs. Mosquitoes are a pest for humans but the researchers suggested that mosquito-control programmes should balance social needs with measures that consider the natural value of midges and mosquitoes to ecosystems. Such measures could include reducing areas and periods of Bti spraying, using alternatives such as mosquito traps and avoiding mosquito control in environmentally sensitive areas.
Source: Poulin, B., Lefebvre, G. & Paz, L. (2010) Red flag for green spray: adverse trophic effects of Bti on breeding birds. Journal of Applied Ecology. 47:884-889.