Examining the individual and additive effects of cold storage and CO 2 narcosis on queen survival and reproduction in bumble bees.

Diapause is a pre-programmed arrest of development allowing insects to survive in unfavorable environments. In adult insects, diapause termination is often followed by a reallocation of macronutrients and a transition to reproduction, and in some insects, this transition can be achieved using narcosis with CO2 . However, whether CO2 narcosis and diapause act in concert to affect reproduction remains unknown. Here, we investigated the separated and combined effects of diapause and CO2 on female reproduction in queens of the common eastern bumble bee Bombus impatiens. Queens were treated with CO2 following a cold storage period (zero days, two weeks, two and four months) and were compared with untreated queens at the same timepoints for survival, colony initiation, egg-laying latency, and offspring production. We found that both CO2 and a period of at least two months in cold storage induced a transition to egg laying in gynes, and as expected, survival decreased with cold storage length. When CO2 and cold storage were combined, CO2 narcosis positively affected egg laying in the earlier timepoints but its impact diminished following a longer cold storage. These data suggest that the impacts of CO2 narcosis and cold storage are partially additive, and application of CO2 is effective only after a short cold storage. It further demonstrates that CO2 has complex effects on insect reproduction that are independent from diapause.