Photo: Stephen Ausmus (USDA)
For once, the mystery behind mass die-offs in honeybee colonies might be getting simpler. At least that’s what Harvard researchers are saying after half of the hives exposed to neonicotinoid pesticides collapsed over the course of a single season.
A 13-week delay between exposure and death is significant, because the bees left their hives behind and mimicked colony collapses seen in commercial hives. Researchers said given a few more months, even the surviving hives exposed to the pesticide would have died off as well.
Pesticide-free control hives suffered just one collapse from a different threat, the nosema fungus. In that case, bees did not leave the hive.
“We expected the mortality to come much earlier,” said Harvard researcher Chensheng Lu. “One of my beekeepers did a last inception before the holidays. He called me and said ‘Dr. Lu, you’re probably wrong. Everything is fine.’ But then later, one by one, the treated colonies collapsed.”
The “neonic” class of chemicals has long topped the list of culprits in widespread colony collapses over the last decade. Bee researchers have suggested collapses could be due to a toxic confluence of multiple factors, from unknown effects of agrochemical combinations to the rise of pathogens like varroa mites, viruses or nosema. Some chemical companies have promoted multi-factor theories to shift blame away from neonicotinoids. Scientists have even suggested neonics could be weakening the bees’ immune systems, making them more vulnerable to secondary diseases.
But Lu said those extra factors are not required to explain bee deaths. “So in terms of whether this is a smoking gun, I would say yes.”
Dosage and Sample
Critics, including neonicotinoid chemical producer Bayer, have dismissed the Harvard study, saying scientists applied unreasonably large doses to the doomed hives, and used a sample size that was too small.
But Lu lashed back, saying that the sample size, 18 hives, was tuned to the vanishingly small amount of variation in the study’s data. As for the dosage, he said no one has yet defined what level would count as “reasonable exposure,” and the level used in the study was well below levels that would kill off bees instantly. The dose used during the study was .74 micrograms per bee per day over 13 weeks. The hives collapsed weeks after exposure, and mimicked conditions seen in commercial hive collapses.
If you look at Indiana, Illinois or Ohio, I would say the concentration there would be 100 times higher than what we exposed to the bees, because those states grow soy and corn.
Lu said the Harvard study raises more questions about the impact of neonicotinoid chemicals on other organisms. Unrelated studies have blamed the chemicals for impacts on birds, aquatic invertebrates, and other wildlife. “This also has implications for humans,” he said. He said he would like to expand research into Midwest states where neonic use is much higher than in Massachusetts.
“If you look at Indiana, Illinois or Ohio, I would say the concentration there would be 100 times higher than what we exposed to the bees, because those states grow soy and corn.”