Sub-lethal effects of thiamethoxam, a neonicotinoid pesticide, and propiconazole, a DMI fungicide, on colony initiation in bumblebee (Bombus terrestris) micro-colonies

This laboratory experiment investigated whether a neonicotinoid (thiamethoxam) or a fungicide (propiconazole) had sublethal effects on nest building or brood production in queenless Bombus terrestris micro-colonies. Micro-colonies consisting of three worker bees were established and treated with thiamethoxam, propiconazole, a solvent control of acetone at the levels in the pesticide treatments, or left untreated. 

There were no significant effects on mortality of any of the treatments. Some uncoordinated movement and excessive grooming was recorded in the thiamethoxam treatments, but not in the propiconazole or control groups. There were significant effects on honey water consumption, with the treatment groups consuming less honey water than the controls. Time to initiation of nest building was only affected in the 10 µg/kg thiamethoxam group, where only 20% of the micro-colonies built a nest within the 28-day experiment. Wax cell building was significantly reduced compared to the controls in both of the thiamethoxam treatments and the 23 mg/kg propiconazole treatment. The 10 µg/kg thiamethoxam treatment had significantly fewer eggs laid, and no larvae produced throughout the experiment.

The authors note that significant effects on honey water (artificial nectar) consumption in all treatments and wax cell construction in the thiamethoxam treatments could lead to a significant reduction in food consumption that could impact colony initiation and reproduction in the field. The reduced reproductive capacity seen in the 10 µg/kg thiamethoxam treatments (only two micro-colonies constructed nests and no larvae were produced) could have severe implications for colonies in the field, although this may only be likely if there was no access to uncontaminated forage within the range of the colonies. Small reductions in colony size and the ability of queens to establish new colonies in the spring could have adverse effects on queen production and hence colony survival. 

Propiconazole however showed few effects on the bees, and even when effects were seen on nectar consumption there were no significant effects on nest building, brood production, activity or coordination. The authors do note that this study did not address potential synergism of propiconazole and thiamethoxam, which has been indicated in other studies of neonicotinoids and DMI fungicides in honey bees. They suggest that pesticide-free alternative forage could reduce exposure in the field.