Behind mass die-offs, pesticides lurk as culprit

“The companies believe this stuff is safe,” says US Department of Agriculture (USDA) entomologist Jeff Pettis. “It is used at lower levels, and is a boon for farmers,” since neonicotinoids don’t require repeated application, nor wide broadcasting into the environment, he explains. Plus, years of research have shown that only very low levels of the chemicals are exuded from the pollen and nectar of treated plants.

But University of Padua entomologist Vincenzo Girolami believes he may have discovered an unexpected mechanism by which neonicotinoids - despite their novel mode of application - do in fact kill bees. In the spring, neonicotinoid-coated seeds are planted using seeding machines, which kick up clouds of insecticide into the air. “The cloud is 20 metres wide, sometimes 50 metres, and the machines go up and down and up and down,” he says. “Bees that cross the fields, making a trip every ten minutes, have a high probability of encountering this cloud. If they make a trip every five minutes, it is certain that they will encounter this cloud.”

And the result could be immediately devastating. In as-yet-unpublished research, Girolami has found concentrations of insecticide in clouds above seeding machines 1,000 times the dose lethal to bees. In the spring, when the seed machines are working, says Girolami, “I think that 90 per cent or more of deaths of bees is due to direct pesticide poisoning”.

Girolami has also found lethal levels of neonicotinoids in other, unexpected - and usually untested - places, such as the drops of liquid that treated crops secrete along their leaf margins, which bees and other insects drink. (The scientific community has yet to weigh in on Girolami’s new, still-to-be-published research, but Pettis, who has heard of the work, calls it “a good and plausible explanation.”)

Two years after the honeybees started disappearing, so, too, did bats. The corpses of hibernating bats were first found blanketing caves in the northeastern United States in 2006. The disease that killed them, caused by a cold-loving fungus called Geomyces destructans - and dubbed White-nose Syndrome for the tell-tale white fuzz it leaves on bats’ ears and noses - has since destroyed at least one million bats. University of Florida wildlife ecologist John Hayes calls it “the most precipitous wildlife decline in the past century in North America”.

Like the mysterious Batrachochytrium dendrobatidis fungus infesting amphibians, Geomyces could be a novel pathogen, newly preying upon defenseless bat species. But scientists have also started to investigate whether pesticide exposure might be playing a role.

Bats are especially vulnerable to chemical pollution. They’re small - the little brown bat weighs just 8 grams - and can live for up to three decades. “That’s lots of time to accumulate pesticides and contaminants,” points out Boston University bat researcher and PhD candidate Marianne Moore, who is studying whether environmental contaminants suppress bats’ immune function. “We know they are exposed to and accumulate organochlorines, mercury, arsenic, lead, dioxins,” she says, “but we don’t understand the effects”.

Which, in the end, is the central dilemma facing pesticide-reliant societies. Proving, with statistical certainty, that low-level pesticide exposure makes living things more vulnerable to disease is notoriously difficult. There are too many different pesticides, lurking in too many complex, poorly understood habitats to build definitively damning indictments. The evidence is subtle, suggestive. But with the rapid decimation of amphibians, bees, and bats, it is accumulating, fast.