Plants Toxic for Bees

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 species of lime tree (Tillia) which may poison or at the least, may have narcotic effects on bee species.

I have found little information to explain why toxic nectar occurs – but I have found some, 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).


‘I Just Want You’ – 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.


'Stop That Thief!' – The 'Nectar Robbery Hypothesis'


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.

'Sozzled Bumblebees?'   '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.  Bumblebees 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).



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

Some plants contain nectar believing to act as a deterrant to bees, whilst some plants produce nectar that results in honey that is unpleasant or poisonous for humans. More about that later!









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Read more about plants toxic for bees:  The ecological significance of toxic nectar - Lynn S. Adler (OIKOS 91: 409 – 420. Copenhagen 2000)

Gardeners Beware!


Did you know?  Bees love lime trees - but some imports are believed to be toxic for bees!

Read more

Excerpt from 'A Sting In The Tale' by Professor Dave Goulson (also author of Bumblebee Behaviour And Ecology):

Page 128:

"Buff-tailed and white tailed bumblebees love the flowers of lime trees, although there is something in the nectar which seems to make them dopey and even sometimes kill them".


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