A widely used microbicide may not be as environmentally friendly as previously thought.

The bacteria Bacillus thuringiensis israelensis (Bti) is now the most commonly used microbicide to control mosquitoes worldwide and is considered to be the least toxic alternative to chemical pesticides. But a new study has revealed adverse effects on the reproductive success of birds.

When ingested by water-inhabiting mosquito larvae, toxic proteins produced by Bti cause pores to form in the guts of the larvae, destroying their digestive tract and eventually killing them. The microbicide has been in use for more than 25 years and is the favoured method of mosquito control in West Africa, the United States and Europe. The handful of previous field studies on its toxicity to vertebrate populations have not found significant adverse impacts.

But work1 – by Brigitte Poulin, a bird ecologist at the Tour du Valat research centre in Arles, France, and her colleagues – in the Journal of Applied Ecology provides evidence that mosquito control has effects further up the food chain. The team shows that the breeding success of house martins (Delichon urbicum) in Bti-treated areas in a national park in the Camargue, France, dropped dramatically compared with that of birds living in untreated sites. The fall in reproductive success was due to the loss of mosquitoes – the birds’ preferred food source.

“We demonstrated that Bti clearly has an impact on house martins,” says Poulin.