A commonly used insecticide might be far more dangerous for some bees than experts have previously thought, cutting their lives short by about 25 percent and damaging the health of their hives.

Research based in Ontario and Quebec, where a type of insecticide called neonicotinoids are popular, found that bees were exposed to the chemicals for three to four months, during their peak pollinating season, and that exposure had a negative effect on their lifespans and on their hygiene. According to a new study in the journal Science, neonicotinoids “increased worker mortality and were associated with declines in social immunity and increased queenlessness over time.” When a common fungicide was also brought into the mix, the problem was compounded, making the insecticide doubly toxic for the bees.

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Having a queen in a hive is important, “because colonies that are unable to rear replacement queens eventually perish,” the study explains.

Neonicotinoid pesticides are also frequently used in other locations throughout Canada and the rest of North America, as well as in Europe. Seeds for crops like corn are treated with the insecticide before they are planted.

Although this is not the first time their effect on bees has been studied, previous research on pesticide poisoning has been controversial because skeptics could not agree on how much exposure the average bee had to the insecticides.

According to the study, the Canadian researchers used tracking devices to follow the movements of honeybees from several colonies, both those close to crops treated with neonicotinoids and those far away from them, to determine their exposure, then replicated that exposure in a lab setting to measure the effect on the critters.

In the field, “pollen from nontarget plants represents the primary route of exposure,” the study notes. This means that even though the insecticides are used on crops like corn, the chemicals leach into the system : When it rains, they go into the soil and are then taken up into other plants through their roots. That includes flowers that bees will visit to gather nectar, and they bring back that contaminated nectar to their hives.

“Worker bees that were exposed to the pesticides lived for about a week shorter,” researcher Amro Zayed explained in a York University video. “And that’s a big effect. …  Almost a 25 percent reduction in lifespan.”

The results showing the Canadian honeybees dying were released at the same time as those of a separate study that looked at the effect of neonicotinoids on bees in Europe, also published in Science. The bees came from hives in the United Kingdom, Germany and Hungary. In the U.K. and Hungary, the bees appeared negatively affected by the insecticide, but German bees fared better. That means that other environmental conditions play a role in how much the neonicotinoids harm bees.

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“The country-specific responses of honey bees and bumble bees strongly suggest that the effects of neonicotinoids are a product of interacting factors,” that study says. “Exposure to low levels of neonicotinoids may cause reductions in hive fitness that are influenced by a number of interacting environmental factors. Such interacting environmental factors can amplify the impact of honey bee worker losses (e.g., through sublethal toxicity effects) and reduce longer-term colony viability.”

Those interacting environmental factors could include the variety and number of flowers available to the bees and the presence of parasites and other diseases. According to a report in NPR, Jeffrey Donald, a spokesman for Bayer, a company that produces many of these insecticides, said the differing results for Germany show “when hives are healthy and relatively disease free and when bees have access to diverse forage, neonics do not pose a danger to colony health.”