Which Lime Trees Are Toxic For Bees?

Whilst some lime trees are toxic for bees, others provide a very useful nectar source. 

Reputedly some lime trees (tilia) are poisonous for some bee species and less toxic for others, or have at least a narcotic effect.  Such an effect will obvioulsy make the bees vulnerable to predators.

If you find dead bees beneath a lime tree, suffice to say, it’s likely you have a poisonous variety, or at least one which has those narcotic effects. 

An exception to this could be a case I personally heard about. 

Dead bumble bees had been found beneath a lime tree with parts of the abdomen missing.  It transpired that Great tits had taken advantage of bumble bees feeding on a lime tree, and had eaten the parts of the bumble bee they apparently find most appetizing!  

Some lime trees  (also known as Linden trees) are favoured by beekeepers, and Linden honey is especially popular in Romania, for example, but Linden does not produce propolis.  Bees may also harvest the honeydew produced by aphids  on the leaves. 

Which Lime Trees Are Safe For Bees?

Generally, it is thought that non-toxic limes are:   

• Tilia cordata or small-leaved Lime (sometimes referred to as the Small-leaved Linden or Little-leaf Linden).  This species is native to Britain, Europe, Skandinavia, and the Caucasus.  
• Tilia platyphyllos or  large-leaved lime  (but also known as the large-leaved linden).  It is native to Britain and other parts of Europe.   This species is pictured below (courtesy of Wikipedia).
   

Which Lime Trees Are Toxic For Bees?

The following lime trees are regarded as poisonous for bees (or having an unfortunate/narcotic effect, or hamper the bee in some way), and so should be kept out of the bee garden:

• Tilia 'Petiolaris' or weeping silver lime (native to the Balkans)

• Tilia euchlora or Caucasian lime – thought to be a narcotic (i.e. induces a state of sleep of drowsiness – this would increase the vulnerability of bees).
• Tilia tomentosa (silver lime in the UK and silver linden in the US) – especially toxic for bumble bees, but apparently not so toxic for honey bees.  I would avoid growing this species in the bee garden – bumble bees will still visit this tree, and if we plant it for honey bees, bumble bees are still likely to visit and be poisoned
• Tilia dasystila are considered toxic to bees, as are Tilia orbicularis.
• Tilia oliveri (Chinese Lime) is being sold as a pretty garden tree, nevertheless,  it is toxic for bees.

Lime trees are said to have a short flowering season.  My advice would be, that if you are considering buying lime trees, consider Tilia platyphyllos or tilia cordata, since it is generally considered that both of these species of lime trees are safe for foraging bees. However, space may be an issue!

If this is the case, perhaps consider different species of trees instead of lime – take a look at these trees, shrubs and hedgerows for bees. 

Why Do Some Linden Trees Effect Some Bees In This Way?

This is as yet unclear. 

One theory is that some bumble bees simply starve themselves to death by drinking so much lime tree nectar that they run out of energy and die before returning back to the nest.

Having read a number of research papers, this theory lacks credibility in my view. 

It seems highly unlikely to me because if the bees keep feeding, surely, they are obtaining energy from the nectar they have just been feeding on, and so how can they have 'run out' of energy?  Why don't they 'run out of energy' after feeding at other highly melliferous plants, such as borage and comfrey?

It also fails to explain a number of other results I have read from research. 

My own (untested) hypothesis is based on the theory of the founder of toxicology, Paracelsus, which is:

"The dose makes the poison"

Or

"All things are poison and nothing is without poison; only the dose makes a thing not a poison."

In other words, it is healthy to drink clean water, but if you drink far too much of it in one go, even water can kill you.

Thus I wonder if the same is true for some bees and lime trees - i.e. sometimes the bumble bees enjoy the nectar so much, they consume too much of it, such that it has a narcotic or even poisonous effect. 

Why this should be the case sometimes for bumble bees and not honey bees is unclear, but here are my possible explanations:

• Could it be connected with the strength of the 'homing instinct'.  We already know that a bumble bee may fly up to 1.7km for food, whereas honey bees are known to fly 1 – 6 km but also up to 13.5 km.  Does this suggest that in the case of honey bees, the natural instinct to return to the colony is stronger, enabling a successful return from longer distances? 
  
  
• Then again, honey bees are super-organisms, meaning the colony is an interdependent, super-efficient structure.  Perhaps honey bees simply gather the food that is required for the colony, without becoming intoxicated on the nectar they are gathering for the colony.  As marvellous as bumble bees are, their colonies are not 'super-organisms'.

• On the other hand, perhaps it could be due to subtle differences in biochemistry.  In the same way that some humans experience allergic reactions to different foods, honey bees and bumble bees may have differing tolerance levels to certain nectars.
  
  
• Perhaps bumble bees are not the perfect pollinators for particular species of lime tree, which may have a different 'perfect pollinator'.  This phenomenon is not, after all, unique to lime trees - see my link below about other plants that seem to be detrimental for bees.
  
  

I don't know what the answer is, but since nobody has any proven, definite explanation as yet, it's interesting to guess! 

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