Since 2006, honeybees in the United States have been dying at alarmingly high rates. Scientists are still debating the possible causes — but that uncertainty hasn't stopped many news outlets from leaping to conclusions about the role of pesticides.
Bee experts, for their part, tend to cite a variety of culprits: The parasitic Varroa destructor mite has been attacking honeybee hives in the United States. Poor nutrition caused by a lack of crop diversity may make the bees less resilient. And, on top of that, some newer pesticides do seem to make honeybees more susceptible to infections.
In other words, the prevailing view is that pesticides are only one factor here. "If we took pesticides out of the equation tomorrow, I think there's no doubt we would have reduced colony losses," says Dennis vanEngelsdorp, an entomologist at the University of Maryland who has been closely tracking the decline of honeybees. "But even without pesticides, we'd still be seeing significant losses — losses that are unsustainable."
In the past few weeks, however, a number of news outlets have suggested that a newer class of pesticides known as neonicotinoids are, in fact, the main reason why US honeybees are dying. These outlets have been trumpeting a recent study by Harvard researcher Chensheng Lu — a study that, it turns out, has been heavily criticized by other entomologists and beekeepers.
It would be great if the honeybee mystery was solved once and for all. But the latest much-hyped study on pesticides and bees doesn't seem to do that. So here's a rundown of the controversy — and what we know about pesticides and honeybee deaths.
A controversial Harvard study on pesticides and bees
The recent media frenzy started with a paper by Harvard researcher Chensheng Lu that was published in an obscure Italian journal called The Bulletin of Insectology. (Lu has had trouble getting his work on honeybees past peer review in many US journals.)
"We found that neonicotinoids were highly likely to be responsible for colony collapse disorder, " is how Lu summarized his paper to me.
Let's unpack that sentence: Neonicotinoids, or "neonics," are a relatively new class of pesticides that are derived from nicotine and affect the nervous systems of insects. They are usually applied to seeds and taken up by the plant itself. Neonics have become popular because they're considered more effective than older insecticides and less toxic to humans — they're widely used on crops like corn, soybeans, and canola. Studies have also found that honeybees are likely exposed to low levels of neonics through pollen, nectar, or dust.
"Colony collapse disorder," meanwhile, refers to a specific set of symptoms found in many dying bee colonies since 2006. One main symptom is a hive with a live queen but few adult honey bees present — and no corpses. Colony collapse is not the only problem facing bees. In fact, says vanEngelsdorp, it hasn't been observed in the last three winters, even though the bees are still dying at high rates. But it's been an issue in the past.
Now to the paper itself: what Lu did was study 18 honey bee colonies around central Massachusetts in the winter of 2012-2013. At each location, he fed one-third of the colonies a high-fructose corn syrup treated with imidacloprid, a widely-used neonicotinoid pesticide. Another third were fed syrup with clothianidin, another popular neonic. And the final third were "control" hives.
The result: in 6 of the 12 colonies that were treated with neonicotinoids, bees died at elevated rates and fled the hives — exhibiting some (but not all) of the symptoms of "colony collapse disorder." Case closed, right?
Well, not quite. Bee experts quickly criticized the study, complaining that Lu had exposed his bees to an unrealistically high dose of pesticides — roughly 135 parts per billion in the corn syrup. As California beekeeper Randy Oliver points out, that's far above the levels that even pesticide manufacturer Bayer concedes is lethal for bees (about 50 parts per billion), and it's way above what bees are likely to encounter in the wild from things like crop nectar (usually less than 5 parts per billion).
These complaints aren't new — a similar study that Lu did in 2012 was heavily criticized for the same reasons. Lu told me that he lowered the dosage for this newest study. Yet outside experts said that the amounts of pesticide he was using were still unrealistically high, and that Lu hadn't actually simulated colony collapse disorder.
"This study is a total distraction," vanEngelsdorp said. "It's not surprising that those bees died — those doses weren't field realistic. The only surprise was that the bees didn't all die right away."
In that case, this study isn't too useful. After all, everyone agrees that neonicotinoids are lethal to bees in extremely large doses. But bees aren't getting drenched in these pesticides in the real world. What we want to know is whether the smaller everyday doses that bees might encounter through pollen or nectar or dust could have adverse effects. That's a much more important question — but it's also much harder to study.
What we do know about pesticides and bee deaths
Now, one flawed study doesn't mean pesticides are harmless. Far from it. Pesticides can certainly hurt bees. A variety of studies have shown, for example, that exposure to certain fungicides and pesticides (including neonicotinoids) can make bees more susceptible to infections.
Why Europe banned pesticides anyway
The United States, by contrast, has declined to ban neonicotinoids for the time being. Instead, the Environmental Protection Agency will conduct a review on the topic that's expected to take five years. Regulators aren't going to restrict neonicotinoids until they have clear evidence that the pesticides are a major factor in the decline of honeybees.
- The Boston Globe wrote a long story on Chensheng Lu's (controversial) quest to prove that neonicotinoids are the reason for the decline of honeybees. Some sharp criticisms of Lu's research here, here, and here.
- Entomologist Gwen Pearson wrote this accessible introduction to colony collapse disorder in 2012, trying to clear up some of the confusion around it.