A study with data from 284 sites across 40 US States has revealed that common fungicides were the strongest factor in the decline of four bumblebee species.

http://rspb.royalsocietypublishing.org/content/284/1867/20172181

https://www.theguardian.com/environment/2017/dec/29/alarming-link-between-fungicides-and-bee-declines-revealed

Bees feed on fermented pollen – i.e. bees need yeasts to assist with digestion of pollen – so fungicide would be expected to harm bees. This research links the presence of the fungicide chlorothalonil with the presence of a bee gut pathogen – Nosema bombi.

Chlorothalonil is a widely used fungicide in UK – especially in cereal crops.