Honeybee stings ache for a good reason: This species knows how to brawl. But as it turns out, these black-and-yellow pollinators are quite vulnerable themselves—especially to neonicotinoids, a pesticide commonly used to ward off crop-munching pests. Two new studies, published this week in Science, address this question by studying large populations of bees in multiple locations for months on end. The results add substantial weight to the claim that neonicotinoids damage bee populations.

“I hope that my study kind of makes the debate go away,” says Amro Zayed, an entomologist who studies social insects at York University in Toronto and is co-author of one of the new reports. Even though honeybees are not the intended targets of neonicotinoids, any indication that the resilient insect is suffering from the chemical means less-adaptable species might be in trouble, too. The pesticide is intended to eradicate insects that chew up or suck on grain crops—which is why these substances coat almost all corn and 50 percent of soy seeds in the U.S. “It’s difficult, if not impossible, to find corn not treated with neonicotinoids,” says Shiela Colla, an ecologist also at York who is unaffiliated with the study research.

Most prior research on the bee–pesticide relationship has only involved feeding the chemicals to small populations in lab settings or observing a few populations in nature for a couple of weeks. Such stand-alone studies do not gather enough evidence on the true nature of honeybee behavior, Zayed says. Colla agrees, which is why she praises the York study’s sample size and length.

In their experiments Zayed and his colleagues divided 55 bee colonies among five apiaries close to cornfields and six so far away that inhabitants foraged nowhere near the crops. From early May through September 2014 the researchers only interfered to collect pollen and nectar samples from the hives every few weeks.

Chemical analysis of the samples revealed neonicotinoids were the most threatening compound among all pesticides found in the nectar samples. One neonicotinoid in particular, clothianidin, was the most abundant. As for how it ended up in the samples, the researchers calculated that less than 2 percent of all neonicotinoids found in the hives were from corn and soybeans. The vast majority was from what Zayed calls “bee-friendly” flowers—like goldenrod, willow and clover—that often grow on the perimeters of crop fields. In fact, bees were picking up clothianidin-laced pollen from neighboring plants well before and well after seeds were planted, meaning the chemical pervades and contaminates surrounding areas for much longer than the planting season.

The second study, led by several institutions in Germany, Hungary and the U.K., also raised bees in agricultural environments tainted with pesticides. “It’s actually what farmers are doing normally, and that’s why it’s important,” Richard Shore of the Center for Ecology & Hydrology in England said at a press briefing. They left 33 colonies in test areas that exposed the colonies either to neonicotinoid-treated rapeseed plants or crops untreated with pesticides. The teams assessed bee population survival twice—once when the crops were flowering and again after winter. Clothianidin also struck these bees hard. Over the winter 25 percent of the Hungarian bees and almost all the British bees died. Neonicotinoids also put a dent in those bee populations during the crops’ flowering period. The German bees, on the other hand, not only emerged from the winter unscathed but their population numbers increased in spite of neonicotinoids during rapeseed plants’ flowering season. Although the researchers are not sure why the German bees did so well, they noted this population also consumed less than a third as much rapeseed pollen as their those in the other countries.

The European study stopped at field experiments but Zayed and his team took things a step further. The group fed select bees pollen laced with clothianidin at levels similar to those found in their natural environment and stuck tracking devices on the contaminated bugs’ backs. “We didn’t have to make guesses” about the chemicals involved, Zayed says. “We essentially observed what real colonies in corn were getting exposed to and mirrored that in our experiments.”

This lack of guesswork paid off with precise results. Insects that ate contaminated pollen had a 23 percent shorter life span than their untreated counterparts. They also took up to 45 minutes longer on their forage journeys, suggesting the bees struggled to remember where home was. Treated bees were also worse at identifying and removing diseased individuals from the hive—a task required for maintaining overall hive health. The net effect, Zayed explains, is a slow decline in populations over weeks.

Much of what Zayed and his team observed has been documented by other researchers. Christian Krupke is one of them. An entomologist at Purdue University, Krupke published studies on the appearance of neonicotinoids in crop-adjacent plants last year.* As evidence mounts, Krupke says, the next question is, What can we do? “The current answer,” he says, “is we can’t do much.”

The European Union banned the use of neonicotinoids on certain plants in 2013; the U.S. Environmental Protection Agency has just launched an investigation into the compounds’ harmfulness, the results of which will not appear until 2018. Maryland was the first U.S. state to announce a ban on the chemicals, and it will take effect next year—although farmers are exempted. Farmers may not need the pesticide as badly as they might think, however. John Tooker, an entomologist at The Pennsylvania State University who has published research with his graduate student on the pervasiveness of neonicotinoids in U.S. agriculture, estimates only 10 percent of fields need the chemicals. “We’ve scouted untreated fields for the pests” they aim to defend against, he says, “and we have difficulty finding them.”

Yet Zayed admits honeybees are not the best species for studying the effects of pesticides on insects in general. “The honeybee colony is a marvel of natural selection,” he says, one that often ramps up hive activity in response to an environmental stressor. Krupke agrees, saying this innate adaptability might explain why the European study showed German bee populations surged after exposure to the toxic chemicals. The same research also looked at how other bee species dealt with the insecticide and saw mixed population responses. In the U.S. many of these less-adaptable bee species thrive off the peripheral plants where neonicotinoids ended up. This suggests more delicate insect species may be suffering from exposure to the chemicals as well.“That was probably the worst news in this paper,” Colla says.

Despite Zayed’s hopes, the debate over neonicotinoids will likely continue. A total consensus in science is almost impossible, Krupke explains, and is not necessarily the point of the Canadian and European studies. But still, the assumption pesticides do not harm any animals besides their intended target pests, Krupke says, “is increasingly difficult to make with research like this.”

*Editor's Note (6/30/17): This sentence was edited after posting to correct an error.