Plants Toxic for Bees

It may surprise people to learn that some plants are toxic for bees, or at least have unfortunate effects. 

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

Below, you will find a list of plants believed to have detrimental effects on bees, and the published reference source.  I also outline some of the theories regarding the reasons that some plants are toxic to bees.

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).

The source of this information is an excellent paper I came across - The Ecological Significance Of Toxic Nectar by Lynn S. Adler (OIKOS 91: 409 – 420. Copenhagen 2000¹).

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 deterrent 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?

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 three theories I came across, that attempt to explain the existence of plants toxic to bees:

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 benefit those plants requiring specialization of pollinators. 

Plants are thought to do this in other ways too, for example, by nectar being inaccessible to certain pollinators or insects species, whilst being accessible to others.

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

Another theory is that toxic nectar may deter nectar robbery (nectar larceny).  

Nectar robbing 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 might not 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).

References

1. Adler, L.S. (2000), The ecological significance of toxic nectar. Oikos, 91: 409-420. https://doi.org/10.1034/j.1600-0706.2000.910301.x

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