Wild bees and managed honeybees are important pollinators in agricultural and natural ecosystems around the world. But as they move through the landscape, bees are exposed to chemicals used in agriculture and released from domestic and industrial sources. Researchers are striving to understand the effects (if any) of these exposures through a combination of laboratory experiments, field studies and in silico modelling.
For laboratory testing, assessments involving bees generally start with acute studies in the honeybee, Apis mellifera. But can acute effects observed in this model species translate to different species and for longer exposure periods? These are key issues for robust hazard assessment.
In our experimental project, we have developed and trialled toxicity tests that allow for long-term exposure (up to 240 hours) for three bee species: A. mellifera, the bumblebee Bombus terrestris and a solitary bee Osmia bicornis. We used a number of insecticides (clothianidin, dimethoate, tau-fluvalinate), other pesticides (propiconazole, 2,4-D) and environmental contaminants (cadmium and arsenic), as well as selected mixtures of these chemicals.
The experiments with honeybees suggested that toxicity decreased in the order clothianidin > dimethoate > cadmium > arsenic > tau-fluvalinate > propiconazole >= 2,4-D.
Using a physiological-based model we were able to track the progression of toxicity over time, not only to describe how toxicity changes over the 10-day test duration, but also to project how it would be further modified over a period equivalent to the average summer and winter life-spans of a worker bee.
This analysis indicated that extending exposure to the lifetime (720 hours) of an Apis worker bee may reveal higher toxicity levels than those recorded in standard 48-hour tests.
We found that the sensitivity to the chemicals in honeybees was broadly similar in other species. The only chemical that showed any marked species variation in sensitivity was tau-fluvalinate, which was not toxic for honeybees at the tested concentrations, whereas effects were seen in the other two species.
Chemicals combined in mixtures can have additive effects through the same or through different modes of action. In these cases, risk assessors can apply dose addition or response addition, respectively. Additionally, chemicals may also interact leading to either an increase (synergism) or a decrease (antagonism) in toxicity. Where interactions are known or expected, risk assessors would need to take into account the magnitude of the interaction.
For our project, mixture tests showed that most commonly tested combinations were additive and non-interactive, although studies with clothianidin and propiconazole pointed to a slightly increased toxicity for the neonicotinoid in the presence of the fungicide. For dimethoate and clothianidin in B. terrestris and, to an extent O. bicornis, weak antagonistic interactions were found. These finding suggest that, at least for initial assessment, current mixture models provide relevant indications of likely joint effects.
David Spurgeon is Principal Scientist in Ecotoxicology at the UK Centre for Ecology and Hydrology and co-author of a report for EFSA on “Chronic oral lethal and sub-lethal toxicities of different binary mixtures of pesticides and contaminants in bees”.