Photoperiod effect on Megachile rotundata (Hymenoptera: Megachilidae) female regarding diapause status of progeny: the importance of data scrutiny

Authors: Pitts Singer, Theresa

Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/30/2019
Publication Date: 1/21/2020
Citation: Pitts Singer, T. 2020. Photoperiod effect on Megachile rotundata (Hymenoptera: Megachilidae) female regarding diapause status of progeny: the importance of data scrutiny. Environmental Entomology. 49(2):516–527. https://doi.org/10.1093/ee/nvaa004.
DOI: https://doi.org/10.1093/ee/nvaa004

Interpretive Summary: Alfalfa leafcutting bees are managed to pollinate alfalfa for seed production in western North America. Some of the offspring they produce in summer enter diapause, i.e., remain larvae for eventual hibernation. These offspring later spend the winter in hibernation and become adults the following year. Other offspring do not hibernate and instead hatch in the summer, mate so that females can build nests and lay eggs. It is not known how bees make the determination to hibernate or not; the process is thought to involve an array of complex factors. This four-year study was performed to determine effects of the exposure of mother bees to short, long, very long, and natural photoperiods (daylengths) on their production of non-hibernating and hibernating offspring. Just-emerged adult bees from both Utah and Canada sources were exposed to different photoperiods for three days in incubators and then released into field cages placed over blooming alfalfa. Control bees were those exposed to only outdoor conditions. Reproduction was monitored for each female bee, and whether offspring remained larvae for hibernating or became adults right away was recorded. Not all collected data sets were reliable because many offspring died as eggs or young larvae, and therefore, this report makes conclusions using only two of the four study years. Generally, offspring of Utah mother bees exposed to long and very long photoperiods were more likely to skip hibernation compared to offspring of mothers exposed to short and control photoperiods. Exposing Canada mothers to short and long (but not very long) photoperiods increased the likelihood that their offspring would skip hibernation. This study highlights the importance of performing multiyear field studies to reveal environmental challenges and inconsistent bee performance that can make some results unusable. Future similar studies are needed to more fully evaluate how photoperiod length can affect whether bee offspring will become adults right away or hibernate over winter before becoming adults in the next year.

Technical Abstract: Megachile rotundata F. (Hymenoptera: Megachilidae) populations are managed to pollinate alfalfa, Medicago sativa L. (Fabaceae), for seed production in western North America. Some progeny produced in summer by overwintered M. rotundata females enter diapause as prepupae to overwinter and become adults the following year. Other offspring avert diapause to undergo adult emergence in summer. The regulatory mechanism(s) of diapause induction or aversion is unknown; the process apparently involves multiple and integrated factors. This four-year study sought to determine effects of the exposure of mother bees to short, long, very long, and natural photoperiods on their production of non-diapausing and diapausing progeny. Just-emerged adult bees from both Utah and Canada sources were exposed to different photoperiods for three days in incubators and then released into field enclosures placed over blooming alfalfa. Control bees were those exposed to only outdoor conditions. Reproduction was monitored for each female bee, and offspring diapause outcome was recorded. Progeny outcome data were informative for only two of the four study years. Generally, progeny of Utah mothers exposed to long and very long photoperiods were more likely to be non-diapausers compared to progeny of mothers exposed to short and control photoperiods. Short and long (but not very long) photoperiod maternal exposures increased the likelihood of diapause aversion in Canada progeny. Performing multiyear field studies on geographically distinct populations is imperative for revealing environmental challenges and inconsistent bee performance that can impair analyses and interpretation. Future similar studies are needed to more fully evaluate photoperiod effects on diapause.