I just read an interesting article in the San Francisco Chronicle. Apparently, there are several California native plants (e.g. corn lilies, death camas, locoweed) that are toxic to non-native honeybees. Native (American) bees are immune to the toxin. Buckeye trees are among the worst for honeybees. It seems that some plants produce either pollen or nectar that can injure or even kill them.
Poor bees...they seem to be takin' it on the chin lately. But not in my garden...I've actually seen loads there lately. All over the lavender, squash, cucumbers, and tomatillos.