I recently had the pleasure of taking part in a series of talks at the RHSI Garden Show at Russborough House and was delighted to be counted amongst some well-known icons from the garden and plant world including Oliver Schurmann from Mount Venus Nursery, Eanna Ni Lamhna - famed zoologist, Micheal Kelly from GIY and Garden Designer Peter Donegan. The overarching theme of the talks was plant care and biodiversity so I concentrated on invasive plants and what species to avoid adding to your collection. Researching the topic was a real eye-opener, in particular, discovering what plants are most toxic to bees namely the invasive Rhododendron ponticum which is a pest found growing across the Killarney National Park after it was introduced during the Victorian period for hunting, the silver-leaved lime and lupins.
It seems like a contradiction in nature that nectar could be toxic to some species of bee but this is the case as was determined by a team of scientists from Trinity College Dublin. Led by Dr Erin Jo Tiedeken and Professor Jane Stout, they found nectar in Rhododendron ponticum, in particular was toxic to honeybees so much so they would perish shortly after foraging on the flowers of this ericaceous rogue. The study found that honeybees were not the only species that fell victim to the toxic nectar, it also had a detrimental effect on solitary bees who after consuming the nectar slowly became paralysed and met a slow untimely death. Interestingly, not all bee species were affected. Bumblebees happily foraged with no ill effects, a finding that will surely be investigated further.
What makes the nectar so deadly? It’s a toxin called Grayanotoxin which is found present not only in the nectar but has also been identified in the foliage and flowers of some ericaceous plants. Other members of this plant family, namely Pieris japonica and Kalmia latifolia also contain these toxins so are best avoided in a pollinator-friendly garden. One theory as to why these plants are poisonous to some pollinators proposes perhaps the plants are attempting to deter inefficient pollinators from using up their nectar but what a cruel deterrent to pay with their lives.
Naturally enough, honey from regions where these species are commonly found is also toxic to humans when taken in high doses and can cause respiratory problems as well as dizziness and nausea but people continue to consume the honey as the plants are perceived to have natural health benefits. In fact, there is a history of honey containing these toxins being ingested by ancient Greek soldiers only to find they became extremely ill and depending on the quantities they consumed they exhibited behaviour like they were drunk to full-blown unconsciousness which could last for days at a time. The symptoms became known as and to this day are still referred to as ‘Mad Honey Disease’. Luckily enough heather honey does not contain these same toxins so we can continue to indulge our sweet tooth.
Another study which was led by scientists at Kew Botanic Gardens looked at the unusual behaviour bees demonstrated after foraging on lime trees in particular the silver-leaved lime Tilia tomentosa. Anecdotally bees had been reported to appear to lie in a drunken state underneath the trees, a common sight which was witnessed each summer when they came into flower. Upon further investigation, the study found the nectar was not toxic but contained high levels of caffeine which improved the memory of the bees as they returned to the same tree but as nectar sources were depleted the bees no longer had a reward for foraging on the trees and fell to the ground with exhaustion. Another case where a plant species lured an inefficient pollinator to its flowers only to result in the ill health of the bee.
Toxins in plants are not isolated to trees and shrubs. Another study found lupins also had toxins present in their pollen. Once bees foraged on these flowers the pollen became attached to their fur which was introduced to the hive as a poison. This had a detrimental affect on the population in the hive, in particular the quantity of males borne which are vital for a productive hive.
The number of plants which produce toxic nectar or pollen is quite small and the National Biodiversity Data Centre provides us with lots of pollinator-friendly plants which we can garden with and are readily available on the All Ireland Pollinator Plan which can be downloaded from their website www.biodiversityireland.ie
Fascinating stuff altogether!
Gardening Workshop – Knockrose House and Garden
The Wonderful World of Edible Ornamentals
Saturday August 17th 11am to 3pm
Join me for this fascinating workshop to learn all about ornamental and wildflowers commonly found in our gardens. The day includes a hands-on workshop, light lunch and a guided tour of the wonderful garden at Knockrose House. Places are limited so book your place today!
As Ever Happy Gardening Folks