Natural toxins in nectar and pollen can poison insects and affect their memory, behaviour and reproductive success, researchers have found. Toxins in lupin pollen cause bumble bees to produce fewer offspring while chemicals found in rhododendron nectar are toxic to honeybees but not bumble bees, toxic effects that could be contributing to the worrying decline in pollinator species. The new research will be presented at this week’s joint BES/SFE conference in Lille.
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Professor Phillip Stevenson from the Royal Botanic Gardens Kew and Natural Resources Institute at the University of Greenwich with colleagues from Trinity College Dublin studied the impact on bees of naturally-occurring toxins in lupin and rhododendron flowers. Plants produce these chemicals in their leaves and stems as a defence against herbivorous insects such as aphids, but they can also accumulate in pollen and nectar and affect the behaviour of insect pollinators visiting the flowers.
In this study, bumble bees given pollen treated with lupanine, a chemical found in lupin plants, at natural concentrations produced fewer, smaller males. The consequences could be severe where lupins are cultivated and present the major food source for bees at particular times of the year. Other chemicals called diterpenoids, which are found in the nectar of Rhododendron ponticum, were found to be toxic to honeybees and a wild mining bee species (Andrena carantonica), but bumble bees (Bombus terrestris) were unharmed by the compounds.
Plant chemicals are not all bad though. Another recent study by Stevenson and collaborators at Newcastle University showed that caffeine occurs in nectar of Citrus and Coffea species where it has the effect of improving honeybee memory for the flower odours that they associate with their food reward. This makes bees more likely to return to flowers from the same species to get their caffeine fix and so increase the chance of pollination success.
This new study shows how plant chemicals can work in beneficial ways. “Plant chemicals in nectar and pollen can mediate specialisation in pollinators, can drive plant pollinator interactions and can simply be toxic to pollinators where they have been selected for another purpose in the plant, such as defence against herbivores,” Professor Stevenson explains.
The findings also highlight important considerations for landscape management of wild plants. Rhododendron ponticum is an invasive species and this study demonstrated that rhododendron toxins are poisonous to honeybees and mining bees. “Where rhododendron dominates landscapes we know that this may be to the detriment of other plant species but now it seems it could also impact invertebrates as well,” he says.
Although bumble bees are unaffected by the toxins in rhododendron, and the plant may be providing an important food source for them, more needs to be known about how plant toxins are contributing to declining bee populations.
“Plant toxins need to be considered alongside other stressors on pollinators for their potential to impact ecosystems, so the next phase of our research will be to look at the interactive effects of plant chemicals with other stressors such as disease, so that we can determine whether plant chemicals exacerbate or ameliorate the effects of bee diseases,” he says.