Difficult to diagnose. Titi cause larvae to be purple. Most other plants implicated as causing bee death or a honey that might be toxic to humans are not heavily collected or the collecting bees are killed in the field.
Rhododendron and Mountain Laurel (Kalmia latifolia) from the heath family (Ericaceae) are poisonous to bees and humans. It contains an andromedotoxin.
California buckeye (Aesculus californica) – has caused losses of honey bee colonies throughout its range at lower elevations of the Sierra Nevada Mountains in California. Apparently only the pollen, not the nectar, is toxic
Summer Titi (Cyrilla racemiflora) – is toxic to honey bees and can cause the condition called “purple brood” - it is mostly in the SE US.
Yellow jessamine (Gelsemium sempervirens) poisons humans sucking the nectar from flowers or from eating honey made from this plant. Honey bees foraging on the flowers may appear intoxicated, became paralyzed and die. Yellow jessamine has been reported to cause periodic poisoning of bees from the Carolinas to Mississippi. Carolina jessamine flower contains strychnine-like alkaloids that will cause extreme illness and possibly death when consumed by humans and animals.
Tilia (American basswood or lime tree) is sometimes toxic to honey and bumble bees and dead and dying bees might be found below a tree in bloom.
Jimson weed (Datura stramonium) also sometimes reported poisonious.
There are others reported from time ot time as toxic.
weak colonies, pesticide poisoning
Skinner J. 2019. Are there plants that produce nectar that is poisonous to either honey bees or humans? Bee Heath. Accessed 2023. https://bee-health.extension.org/are-there-plants-that-produce-nectar-that-is-poisonous-to-either-honey-bees-or-humans/
Buzz About Bees. 2021 update. Plants Toxic for Bees. Buzz About Bees. Accessed 2023. https://www.buzzaboutbees.net/Plants-Toxic-for-Bees.html
Pokorny K. 2019. Will creeping buttercups harm wild bee colony?: Ask an Expert. Oregon Live. Accessed 2023. https://www.oregonlive.com/hg/2019/05/will-creeping-buttercups-harm-wild-bee-colony-ask-an-expert.html
Dyer MH. 2022 update. Can Honey Be Poisonous: What Makes Honey Toxic. Gardening Know How. Accessed 2023. https://www.gardeningknowhow.com/garden-how-to/info/can-honey-be-poisonous.htm