Plants Toxic for Bees

Updated: 3rd May 2021

It may surprise people to learn that some plants are toxic for bees, or at least have unfortunate effects.  Below is a list of plants believed to be harmful to bees, its effects, and also the name of the published reference, along with the theories of why some plants may have evolved the toxicity. 

I have written about this subject previously with regard to certain species of lime tree (Tillia) which may poison or at the least, may have narcotic effects on bee species.

Below, you will find a list of plants believed to have detrimental effects on bees, and below that, some of the theories surrounding the issue - why, for example, would a plant create a nectar that could harm a potential polllinator?

List Of Plants Toxic For Bees


Note this website receives visitors from all over the world - you may or may not have any of these plants in your own country).

Plant Species
& Family
Effects
On Bees
Publ.
Reference
Aesculus californica (Hippocastanaceae)
Astragalus spp. (Fabaceae)

Cuscuta spp. (Convolvulaceae)
Cyrilla racemiflora (Cyrilliceae)
Gelsemium sempervirens (Loganiaceae)
Kalmia latifolia (Ericaceae)
Solanum nigram (Solacanaceae)
Veratrum californicum (Liliaceae)
Zygadenus cenesosus (Liliaceae)

Toxic to bees
Eckert 1946,
Mussen 1979

Corynocarpus laevigata (Corynocarpaceae)

Toxic to honey bees

Palmer-Jones and Line 1962

Angelica triqueta (Apiciaceae)

Toxic to bees

Bell 1971

Astragalus lentiginosus (Fabaceae)

Toxic to bees

Vansell and Watkins 1934

Camellia thea (Theaceae)

Lethal to honey bee larvae

Sharma et al. 1986

Ochrama lagopus (Bombacaceae)

Toxic to bees and other insects

Paula et al. 1997

Sophora microphylla (Fabaceae)

Toxic to honey bees

Clinch et al. 1972

Tilia spp. (Tiliaceae)
(Read more about Tilia and toxicity to bees)

Toxic to bees and other insects

Crane 1977

Verartrum californicum (Liliaceae)

Toxic to bees

Vansell and Watkins 1933

Asclepias spp. (Apocynaceae)

Toxic to bees

Pryce-Jones 1942

Astragalus miser v. serotibus (Fabaceae)

Toxic to honey bees

Majak et al. 1980

Rhododendrum spp. and hybrids (Ericaceae)

Toxic to bees

Carey et al. 1959


Whilst some plants contain nectar believed to act as a deterrant to bees, other plants produce nectar that results in honey that is unpleasant or creates toxic honey for humans. Indeed, toxic honey was even used in a kind of chemical warfare in ancient times.


Why are some plants toxic for bees?

I have found only a little information to explain why toxic nectar occurs which I outline here.  Indeed, key questions include:

  • Do certain plants seek to deter particular pollinators in order to attract ‘the right ones’? 
  • Why are some plants poisonous for certain insects but not others?

Here are some theories I came across, followed by a list of plants thought to be poisonous for bee species.  The source of this information is an excellent paper I came across - The ecological significance of toxic nectar - Lynn S. Adler (OIKOS 91: 409 – 420. Copenhagen 2000) - see right).

1. A Faithful Partner: – The 'Pollinator Fidelity Hypothesis'


Do some plants require specialization of pollinators?  Such a theory has been proposed by scientists Rhoades and Bergdahl (1981). 

They proposed that toxic nectar may be the way in which flowers increase pollinator fidelity, by repelling rather than attracting the ‘generalist pollinators’, or at least insects which are less effective as pollinators of the plant species.

In other words, toxic nectar may help some plants requiring specialization of pollinators.  Plants are thought to do this in other ways too, for example, by nectar being inaccessible to some pollinators or insects species but not to others.

2. Stop That Thief!:  The 'Nectar Robbery Hypothesis'


Bumble bee nectar robbing pink acquilegia flower.  Could toxic nectar prevent nectar robbery?Could toxic nectar prevent nectar robbery?


Another theory is that toxic nectar may deter nectar robbery - nectar robbery occurring when an insect such as the ant, removes the nectar without actually pollinating the flower.   

Stephenson (1981, 1982) found that ants and skippers who were offered the nectar of Catalpa speciosa, or a sugar solution of the same concentration, preferred the latter, and that the individuals drinking the nectar exhibited signs of disorientation or narcosis. 

Interestingly, ‘legitimate bee pollinators’ of the plant were not affected by the nectar, and showed no sign of preferring the sucrose solution to the nectar.


3. Sozzled Bumble Bees?  Tipsy Wasps?: The 'Drunken Pollinator Hypothesis'

In the orchids Epipactis purpurator and E. helleborine, toxic nectar is produced by ethanol caused by micro-organisms contaminating the nectar, carried from the air or transferred by insects. 

According to Ehlers and Olesen (1997), wasps drinking the nectar become sluggish and intoxicated, and this causes the wasps to groom less  (well.... I suppose drunken humans probably don’t do a very good job of combing their hair either!). 

The prevention of grooming means that more pollen is transferred between the orchid plants themselves, thus serving the pollination aims of the plant.

Drunken effects are noted in other cases.  Bumble bees drinking the nectar of Asclepias flowers also show signs of being…. well….sozzled (Kevan et al 1988). 

The plants may not necessarily get their way though - some raise the question whether getting the bees drunk always benefits the flowers if the narcotic effects result in death, thus preventing pollination (Bell 1971, Clinch et al 1972).







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