Difficult to diagnose. Excessive number of dead adult bees; larvae off-color.
There are several plants that on occasion may cause poisoning to honey bees. The toxic chemicals are allelochemicals, substances developed by the plants themselves to help the plant resist herbivory by insects or grazing livestock. The dosage is important; a chemical toxin from a poisonous plant at a low dose may be a food or a valuable medicine, while at higher doses, the same chemical can be deadly. A substance that is toxic to one organism may not be toxic to another; for example, mountain laurel honey is poisonous to humans but not to honey bees.
Some species of buttercup (Ranunculus spp.) and summer titi (Cyrilla racemiflora; also called leatherwood) have toxic nectar, the latter causing “purple brood.” Swamp buttercup (Ranunculus septentrionalis) has nectar and pollen toxic to honey bees, though it is unclear if other Ranunculus species are toxic. Some other plants that show toxicity to bees include tansy ragwort, linden (toxic sugar), and milkweed, which additionally has pollenia (pollen-filled sacks) that stick to bee bodies, causing stress to foragers.
Rhododendron, from the heath family (Ericaceae), is poisonous to bees and humans. It contains an andromedotoxin. Mountain laurel (Kalmia latifolia) also contains an andromedotoxin that can poison humans. California buckeye (Aesculus californica) annually causes losses of honey bee colonies throughout its range on the western slopes of the Sierra Nevada Mountains in California. Yellow jessamine (Gelsemium sempervirens) can poison humans who might eat honey made from this plant. Bees foraging on the flowers of yellow jessamine appear intoxicated, become paralyzed, and die. Yellow jessamine has been reported to cause periodic poisoning of bees from Florida to Texas.
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) is also sometimes reported as poisonous.
Bee-Health. 2019. Are there plants that produce nectar that is poisonous to either honey bees or humans? Bee Health. Accessed 2023. https://bee-health.extension.org/are-there-plants-that-produce-nectar-that-is-poisonous-to-either-honey-bees-or-humans/
Ask Extension. 2019. Are buttercups toxic to bees? Ask Extension Knowledgebase #562677. Accessed 2023. https://ask2.extension.org/kb/faq.php?id=562677
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
