Contrasting impacts of pesticides on butterflies and bumblebees in private gardens in France

Private gardens are an important food source and refuge for animals in urban areas because they represent a large part of the green space. It has been shown that garden management regime (water use, floral composition) may impact the species they shelter. However, due to access restrictions, lack of regulations and the difficulty of data collection on private property, the impact of management practices and in particular pesticide use has seldom been assessed in private gardens. Using data collected in the framework of a nationwide participatory monitoring scheme in France, we assess here, for the first time, the effect of private garden management on two important groups of flower-visiting insects, i.e. butterflies and bumblebees, at a large scale. We show that the correlation between butterfly and bumblebee abundance and use of insecticides and herbicides is negative, whereas the use of Bordeaux mixture (fungicide approved for organic use), fungicides and anti-slugs is positively correlated with butterfly and bumblebee abundance. We hypothesize that herbicides have an indirect negative impact on insects by limiting the amount of available resources, and that the Bordeaux mixture, fungicides and slug repellants have an indirect positive impact on these insects by fostering healthier plants, probably offering higher level of resources to pollinators. Moreover, we show that the impact of pesticides varies according to the landscape, the negative effect of insecticides being more important in highly urbanized areas. Overall, our results show that gardener practices can have a positive impact on flower-visiting insects, even in a highly anthropized, urban landscape.