Assessing the impact of gardening practices and pesticide use on butterflies and bumblebees in private gardens

Background/Question/Methods

Private gardens represent an important food source and refuge for animal species in urban areas. It has been shown that garden management regime (water use, floral composition) may have an impact on the species they shelter. However, due to access restriction, lack of regulations and difficulty in data collection, impact of management practices and in particular phytochemical use has never been assessed in private gardens. We assessed this effect at a large scale on two important groups of flower-visiting insects, i.e. butterflies and bumblebees, using data collected in the framework of the Observatoire de la Biodiversité des Jardins, a French nationwide participatory monitoring scheme. Following a light protocol, participants identify and count butterflies and bumblebees in their garden. They also provide information regarding garden environment and gardening practices. Since 2006, more than 9000 gardens have been monitored all over France, and two million abundance data have been gathered.

Results/Conclusions

Butterfly and bumblebee abundances were negatively impacted by the level of urbanization, and positively by garden area and flower diversity. We show that the correlation between butterfly and bumblebee abundance and use of insecticides and herbicides is negative, whereas the use of Bordeaux mixture, fungicides and anti-slugs is positively correlated with their abundance. We hypothesize that herbicides have an indirect negative impact on insects by limiting the amount of available resources, and that 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. Overall, our results show that gardener practices can have a positive impact on flower-visiting insects, even in a highly anthropized, urban landscape. Our results also highlight the interest of monitoring schemes performed by volunteers, where the potential drawbacks are outnumbered by their advantages, including access to biodiversity trends from private areas such as domestic gardens.