Dr Dave Goulson, from Sussex University explains "Garden centres frequently market nectar- and pollen-rich ornamental plants as “pollinator-friendly”, however these plants are often treated with pesticides during their production. We tested 29 different ‘bee-friendly’ plants for 8 commonly used insecticides and 16 fungicides. Only two plants (a Narcissus and a Salvia variety) did not contain any pesticide, but 23 plants (almost 80%) contained more than one pesticide." Equally shocking is that Dr Goulson's team found neonicotinoids at concentrations known to be dangerous to bees.
The news that newly-purchased garden plants could pose dangers to bees and other beneficial insects is a shock to gardeners. These plants are a rich source of flowers in urban environments, and thus are highly attractive to insects such as butterflies, hoverflies and bumble bees. Equally worrying is the lack of information at the point of sale. Gardeners do not know that plants have been treated, and whether they may actually pose a threat to, rather than benefit, the health of pollinating insects.
"The organic grower mixes flowers with vegetables in their plot in order to encourage beneficial and beautiful insect life," says James Campbell, Garden Organic's Chief Executive. " We feel that buyers have been deluded, and bees may suffer. Garden Organic will be writing to the major retailers asking them to pledge a phase out of these unnecessary chemicals - just as B&Q has done recently. We also urge members to contact their local garden centres to ask them to come clean about their 'Bee Friendly' plants. We cannot risk our pollinators' health."
Recent attention on the negative effects of pesticide pollution has been focussed on agricultural - not domestic - growing. Research has shown that bees in particular, grow more slowly, produce 85% fewer queens, find it hard to collect pollen, and encounter problems of navigation. See Neonicotinoids. However, in 2014, Greenpeace published a report which found pesticides in 97% of 35 popular ornamental garden plants sourced from garden centres in 10 European (but not UK) countries. Goulson's study is the first to provide data on purchases of ornamental plants in the UK.
The report concludes "The concentrations of individual chemicals found overlap with, and sometimes considerably exceed, those known to do measureable harm to bees. Residues of pesticides in plants bought by members of the public will decline over time, and unless large numbers of contaminated plants are bought and planted together, it is likely that the total residues to which pollinators are exposed will be diluted by their also feeding on other, uncontaminated plants nearby. However, many ornamental plants are bought in spring, which may provide a pulse of exposure of bees to pesticides at a critical time in the early development of bumblebee colonies and when honey bees colonies are normally undergoing rapid growth.
With the current state of knowledge, we are not able to evaluate whether the net effect of planting ‘pollinator-friendly’ flowers contaminated with pesticides is likely to be positive or negative. But it is clear that levels of pesticides found in some plants may well be sufficient to do harm, and the purchaser currently has no way of knowing what residues are in the different plants on sale. "
See here for a full copy of the report Ornamental plants on sale to the public are a significant source of pesticide residues with implications for the health of pollinating insects by A Lentola, A Tapparo, Department of Chemical Sciences, University of Padova, Italy; and A David, A. Abdul-Sada, D Goulson and E M Hill, School of Life Sciences, Sussex University, UK.
For those with sharp eyes, the neonics detected include thiamethoxam, clothianidin and imidacloprid, which were banned in 2013 for farmers to use in the EU. However, they are allowed on non-flowering crops, and on ornamental flowers, but not in the year in which they are to flower. Unfortunately they are very persistent, so remain in the plant and soil for several years after treatment.