July 28, 2010, 9:19 a.m.
House martin numbers hit by 'environmentally friendly' insect control.
Spraying Bti to control mosquitoes may hit bird populations.WHO/TDR/Pasteur Institute
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.
Before the Bti spraying in 2006, the researchers found no difference in type and size of prey eaten by the birds in the control sites and the areas which were to be treated.
Over the next three years, the group recorded 9,051 flights to obtain food and 14,857 prey items collected by the birds across three control sites and three treated sites. They found that birds inhabiting the control sites predominately ate mosquitoes and midges. But flying ants accounted for a larger proportion of the birds' diet in the treated areas.
They found that 58% of the differences in food source and 63% of the variation in the size of prey taken by birds in the control and treatment sites is due to Bti application. In contrast, differences in these factors as a result of naturally occurring seasonal or yearly variations, or because of differences in site or nest quality ranged from 5–15%.
The overall breeding success of birds at sites sprayed with Bti was lower than those at untreated sites. The former produced on average two fledging chicks per nest in 2009. In contrast, birds at the control sites achieved an average of three fledging chicks.
Pounlin says she hopes that the results will stop a proposed expansion of Bti spraying in the Camargue, where the aim is to reduce mosquito numbers for human comfort rather than for disease control.
Pierre Mineau, a pesticide ecotoxicologist at Carleton University in Ottawa, Canada, says that the results could help to explain the drop in numbers of insectivorous birds occurring in North America.
"As a group, these birds are not doing well but we are at a loss to explain it," he says.
But he urges caution in assuming that the results apply to other species. "It is just one species in one context. The impact is likely to be context dependent, including whether suitable replacement food items are available," he says.
Poulin says she is addressing this concern in a second study on the impacts on invertebrates living in reeds beds in the same national park.
Despite the effects seen in the recent study, Bti is still much safer than chemical pesticides, says Mineau. "The message that should not be taken away from this study is that people should go back to using chemical pesticide," he says.