Food absence is a cue for metamorphosis in the solitary bee Megachile rotundata through a conserved physiological mechanism

Authors: Grula, Courtney; KITTILSON, JEFFERY - North Dakota State University; HOLTHOUSEN, JASON - US Department Of Agriculture (USDA); Rinehart, Joseph; BOWSHER, JULIA - North Dakota State University

Submitted to: Proceedings of the National Academy of Sciences (PNAS)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/15/2025
Publication Date: 12/29/2025
Citation: Grula, C.C., Kittilson, J., Holthousen, J., Rinehart, J.P., Bowsher, J.H. 2025. Food absence is a cue for metamorphosis in the solitary bee Megachile rotundata through a conserved physiological mechanism. Proceedings of the National Academy of Sciences (PNAS). https://doi.org/10.1073/pnas.2511035122.
DOI: https://doi.org/10.1073/pnas.2511035122

Interpretive Summary: Insect metamorphosis is the developmental transition from the egg to adult stage. The mechanisms that control metamorphosis have not been extensively studied across all insect taxa, and there is evidence that these mechanisms vary among insects. Many insects start metamorphosis when they reach a certain size, but the goal this study was to determine the cue for entry into metamorphosis in the solitary bee, Megachile rotundata. We found that a period of time without food available cued M. rotundata larvae to enter metamorphosis. This metamorphic cue differs from the cue found in model organisms, but it is common among insects that are resource limited in the larval stage, which is the case for M. rotundata. This was accomplished through manipulating the amount of food larvae were provided. We provided larval bees with excess amounts of food, or we removed all available food. Once the cue for metamorphosis was determined to be a period without food, we were able to use these feeding treatments to measure the physiological changes during metamorphosis. This included measuring hormone levels, and gene activity related to metamorphosis. We found that starved larvae entered metamorphosis earlier than bees provided excess food, and this was confirmed by the physiological and genetic mechanisms. Since these bees rely on a fixed amount of food provide by their mothers and cannot gather more, running out of food may be a natural sign to transition into adulthood. However, even though their trigger for metamorphosis is different from many other insects, the underlying biological processes are very similar.

Technical Abstract: The timing of metamorphosis in insects determines adult body size and influences aspects of fitness including flight performance and fecundity. The regulation of metamorphosis has been well studied in model insects but determining the cue that initiates the physiological shift toward metamorphosis has been challenging. In most insects, the critical weight is a cue to initiate metamorphosis. Our goal was to determine the cue for metamorphosis in the solitary bee Megachile rotundata and to characterize the physiological mechanisms that are initiated by this cue. Larval bees were either fed ad libitum food or were starved and had all food removed. We measured the time to initiate metamorphosis, Juvenile Hormone (JH) titers, and the abundance of transcripts involved in the endocrine response to metamorphosis. We experimentally manipulated JH titers by topically applying precocene, which interrupts the production of JH. We found that starvation triggered the initiation of metamorphosis and that M. rotundata does not have a critical weight. Starvation was associated with low JH titers, and the application of precocene mimicked the response to starvation. Starvation decreased transcripts associated with JH production, and increased transcripts involved in ecdysone synthesis and insulin signaling. Food absence is an ecologically relevant cue for a M. rotundata larva, which is provisioned a finite amount of food by the mother and is unable to forage for additional food. Despite having food absence cue metamorphosis, the physiological mechanisms associated with metamorphosis are conserved with those found in other insects.