This week's paper is lead-authored by Professor Scott McArt at Cornell University, and is published in Proceedings of the Royal Society B. McArt and his colleagues have examined the factors associated with declines in populations of four declining bumblebee species in the US: Bombus occidentalis, Bombus affinis, Bombus pensylvanicus and Bombus terricola. McArt and his colleagues collected over 10,000 bumblebees from 284 sites in the USA. Bees belonged to these species known to be in decline as well as bumblebee species whose populations are currently stable (Bombus bimaculatus, Bombus impatiens, Bombus vosnesenskii). At each of the 284 sites they quantified 24 landscape variables including: latitude and longitude, habitat types, total human population densities, and pesticide usage including insecticides, herbicides, and fungicides. They also screened the collected bees for two parasites: Crithidia bombi and Nosema bombi. These two parasites are suspected to be an important component of bumblebee population declines. McArt and his colleagues then used multi-variate model selection to assess the impact of individual habitat and pesticide variables on infection prevalence and population declines in bumblebees. They found that the best predictor of both N. bombi infection and bumblebee population declines was use of the fungicide Chlorothalonil. Chlorothalonil is frequently used to combat fungi, molds and mildew, on trees, turf, and fruits. The maps below (from McArt et al. 2017) show the prevalence of Chlorothalonil usage overlaid with parasite infection and population declines in bumblebees.
Chlorothalonil has already been shown to be associated with honeybee parasite infection, and has negative impacts on colony growth in the common eastern bumblebee, Bombus impatiens. The mechanism by which chlorothalonil affects bees is uknown, but one possibility is that it disrupts the bee's normal gut microbiota, making them more susceptible to parasite infection. Other interesting findings from this study include a negative relationship between parasite infection and urbanization. McArt and his colleagues suggest that the diversity of flower resources in urban gardens may actually enhance bumblebee nutrition, buffering them against pathogen infection.