Unintended pitfalls: Irrigation boxes sink bumblebee populations

Bumble bee ranges are shifting, and populations are in decline, with several species already extinct or likely to be soon. Diverse factors are driving these declines, including shifts in land use, pathogen prevalence, anthropogenic chemicals and climate change. Most studies of these and related factors focus on effects on workers and colonies during the growing season; queens emerging in spring are arguably the most important life stage affecting population growth, but we know surprisingly little about the factors that affect their survival. We noticed several spring queens trapped in buried boxes used for irrigation control; subsequent inspection of over 100 irrigation boxes on the university campus revealed that this was a widespread phenomenon, with dozens of these boxes having at least 1 recently dead queen and several containing more than 50 queens at various stages of decay. We hypothesized that the dark hole (for opening the box) on a green background presents a strong visual cue for nest-searching spring queens which enter the the box but cannot escape due to the slanted walls far from the small exit hole in the lid. Our experiments reveal that spring (but not fall) queens are attracted at high rates to irrigation boxes, that they cannot escape once they enter, and that they likely die within 48-72 hours due to depletion of energy reserves. Given replacement rates and estimates of box densities in urban landscapes, we posit that these boxes could be causing massive mortality of spring bumble bee queens.