Plants that are toxic to bees are not a big issue for most beekeepers. It is safe to assume that most locations have enough diversity of forage that any toxic pollen or nectar would be diluted enough to not be deadly to your bees. However, any given year you may experience some hive “issues” that could be attributed to toxic pollen or nectar. The issues could range from mild (bees are affected but will survive) to severe (it kills the hive).
I am fortunate enough to live in the Tehachapi mountains of California that provide a great spring nectar flow that consists mostly of wildflowers and a solid fall flow that consists mostly of buckwheat. And then there’s the highly toxic California Buckeye tree that blooms from mid/late May to late June. The California Buckeye is found throughout all of the California Sierra Nevada mountain range at an elevation of around 3000 to 5000 feet.
I don’t know if anyone knows whether it’s the Buckeye pollen or nectar (or both) that is toxic to bees, but it sure is toxic (the worst part is that the bees seem to love it.) Buckeye poisoning is easy to spot because it is toxic only to the larvae. If you inspect a hive during and after the buckeye bloom, you’ll see a lot of young larvae but no capped brood. In other words, the larvae die before they are capped. The inability to raise new bees eventually leads to hive collapse. So your hives won’t really “die” until four to eight weeks after the buckeye bloom.
In the past, especially during our historic California drought, we’ve had pretty good success in getting hives to survive through the buckeye bloom, but this year was different. We finally snapped out of the drought and had a very wet winter and spring, which contributed to a massive buckeye bloom. Before we could react to it, some of our hives had already gotten a big dose of it. But as always, for the sake of knowledge, we turned a bad situation into an experiment (might as well right?). We moved out a third of our hives about 2 weeks into the buckeye bloom. We moved out another third of our hives about 3 weeks into the bloom and left the remaining third to tough it out through the bloom. We can only move one truckload of bees (about 18 hives) per weekend, so that’s the way it had to work out.
The bees that we moved out first did show a “dip” but recovered well (when inspected a few weeks later). I suspect that the buckeye only affected about one brood cycle before they used up all of the toxic stuff. The small interruption in brood production should have helped with any varroa mite issues (although I don’t have concrete measurements). The bees that we moved out in the second wave had about a 40% survival rate. The bees that stayed at my house had a 10% survival rate. Oddly enough, the two hives that survived at my house were both very weak splits that we had made for swarm prevention purposes (by removing the queen and a few bees from the strong hives that we didn’t want to swarm). My assumption is that the small splits survived because they didn’t have the foraging capacity to bring in enough of the Buckeye pollen/nectar. But it’s safe to say that any strong hive at my house would have died this year (0% survival rate).
This is why it is important to know if you have any toxic plants in your area. Perhaps you notice hive “issues” at a particular time of the year every year and you haven’t been able to figure out what is causing them. It could very well be that they are foraging on toxic plants. But knowing what it is and when it blooms is the first step to figuring out a plan to overcome it. It may be prudent to move your hives out of the area, or perhaps you can get away with using pollen traps to limit the incoming pollen (or removing all pollen frames) or making splits or a combination of all of the above.
If you live in (or close to) the California Sierra Nevada mountain range or keep your bees there, make sure that you check the area around your apiary to see how many Buckeye trees are present. As long as there are plenty of other flowers for your bees to forage on, they can likely buffer a few trees. But if you live in an area where entire mountainsides turn white from buckeye blooms (like I do), then you really should consider moving your hives out of the area during the bloom. Even if you don’t have any issues with toxic plants, it’s never a bad idea to familiarize yourself with the toxic plants that may exist in your immediate area and your local region. You never know when this information may come in handy.