Experimental mosquito control using Bti has been carried out in the Camargue since 2006. Researchers from the Tour du Valat and other laboratories conducted a five-year study on the impact of this bioinsecticide on non-target fauna. The effects of Bti on biodiversity have now been proven, yet they are not widely known or taken into consideration. An overview.
Is Bti really an alternative to chemical insecticides?
Bti is a bacterium that is naturally present in soils. Its toxicity towards Nematocera (a group of diptera that includes mosquitoes and chironomids) was discovered in 1977. Since that time, it has become the most widely-used bio-insecticide in the world. Considered to be non-toxic for mammals, birds, plants and most aquatic organisms, Bti seemed to be a promising option for protecting our natural heritage while diminishing the annoyance caused by mosquitoes. The Camargue wasnotsubjected to mosquito control operations implemented along the Mediterranean coast since 1965; however, experimental mosquito control operations were started in 2006 in the Camargue, for a five-year test period. The control was experimental because it was not applied to the entire delta and included impact studies on non-target fauna.
Evaluation of the effects of Bti on non-target fauna in the Camargue
Does the significant reduction in Nematocera stemming from the use of Bti have repercussions on the fauna that are dependent on them? The Tour du Valat performed two case studies on: (1) passerines nesting in reeds, the abundance of which is linked to that of their invertebrate-prey, which in turn is dependent on how long the marshes are flooded; (2) breeding house martin colonies in inhabited areas and feeding on small flying insects caught near their nests. So that the varying productivity of the wetlands according to season and year would not interfere with the evaluation of the effects of Bti, the study was based on the comparison of several sprayed sites and (unsprayed) control sites. Passerine monitoring involved sampling the invertebrates in the reeds during the reproduction season (May) using a net, and measuring water levels on a monthly basis. Making use of results from a 1998-99 study, the monitoring then consisted in comparing the observed values of invertebrate numbers (net captures) with predicted values (in function of how long the marshes were flooded) in order to determine whether the difference between the two varied when sites were sprayed with Bti. House martin monitoring focused on the size of colonies, food availability, feeding rates, the diet of the chicks and reproductive success.
Indirect effects, but with important repercussions on biodiversity
The overall results from these five years of monitoring show a 39% drop in the numbers of invertebrates consumed by reed passerines, with in particular a 58% decrease in the number of spiders. Spiders are important predators of mosquitoes and chironomids, and also a prey greatly appreciated by passerines because of their high protein content. They are the favourite prey of the Aquatic warbler, the only reed-nesting passerine considered to be vulnerable in Europe.
House martin monitoring revealed a significant modification in the diet of chicks in the zones sprayed. Fewer Nematocera are consumed there, which is also true of their predators such as spiders, Neuroptera and dragonflies. More flying ants are consumed, but these hard-bodied insects are not easy to digest, and were often found intact in the 700 faeces analysed. This dietary deficiency resulted in a 33% decrease in the number of chicks that reach adult age in the colonies surrounded by sprayed marshes. Probably combined with the lower survival rate of adults and fledglings, the size of the colonies on the sprayed sites has dropped from 240 to 92 nests since 2009, whereas numbers have remained stable on the control sites (from 198 to 200 nests) over the same period of time.
This is the first demonstration of the considerable effect of Bti on non-target fauna. These effects are even more significant than those generally reported for chemical insecticides, which are nevertheless more toxic and less selective than Bti. Even though it does not affect birds directly, the decrease in the numbers of prey species, which are essential for their reproduction, has significant repercussions on their numbers and survival rates. The literature on the use of Bti is still generally limited to its effectiveness and direct effects on non-target fauna. We hope that the results of this study will motivate the scientific community to try to better quantify its indirect impacts, which are likely to be important for other protected sites as well.
Does the Camargue not deserve to be treated differently?
In spite of the negative impacts reported in this study, a governmental decree was issued to implement Bti mosquito control operations in the same way as before. However, other solutions do exist for decreasing the annoyance caused by mosquitoes in the Camargue, at lower ecological and economic cost, which deserve to be tested out ‘experimentally’. Two possible strategies are described in this study: 1) acting on the ecosystems that produce mosquitoes, particularly through water and habitat management operations; and/or 2) taking action in the inhabited areas by installing selective traps.
Poulin, B., Indirect effects of bioinsecticides on the nontarget fauna: The Camargue experiment calls for future research, Acta Oecologica (2012), doi:10.1016/j.actao.2011.11.005