One of those alternatives is a pesticide called Sivanto, made by Bayer CropScience. Sivanto is Bayer’s brand name for a pesticide called flupyradifurone, and the company has stressed that the pesticide is safe on bees, to the point of commissioning studies affirming the fact. Bayer’s American director of pollinator safety even gave this quote: “No new insecticide has been as thoroughly tested with respect to bee safety prior to registration.”

Now, researchers from the University of California, San Diego have released the results of a years-long study in which they tested whether Sivanto really is bee-friendly. What they found is that the “thorough testing” may have excluded some pretty common use cases, and that in those, Sivanto is associated with greater mortality and some abnormal behavior than compared with a control group.

The researchers tested Sivanto as it’s used in a “cocktail” with propiconazole, a common fungicide that’s used on wheat, corn, stone fruits, almonds, and many other crops. Sivanto is used on some of those same crops, or can combine with fungicides like propiconazole if used on crops near those treated with the fungicide. Bayer’s instructions for Sivanto do in fact say not to mix with “azole fungicides,” which would include propiconazole—but bees move around, which means that foraging bees could encounter both. Bayer’s instructions may state not to mix these two products, but in the field, things aren’t always so clear-cut.

In that configuration, the researchers found that Sivanto caused bees higher death rates and behaviors including poor coordination and apathy. Few studies have been done on the way fungicides affect bees, since they’re not really designed to kill insects, but the little research that is out there suggests propiconazole is harmless to bees on its own.

The researchers also note that the testing procedures for judging whether a pesticide is dangerous to bees could be at fault. Official guidelines do require testing for danger to bees, but the pesticide is only tested on bees within a hive. Because bees are so specialized, the bees that are actually out foraging—the ones most likely to come into contact with pesticides—are older bees than those inside the hive, and possibly more vulnerable. In fact, the Sivanto damage to foraging bees were found to be four times greater than the damage to in-hive bees.

Sivanto does not list on its packaging or instructions that it should not be used with propiconazole.