Growth rate and the timing of when growth stops determine body size, an important trait for survival and reproduction. The cues that stop growth are not fully understood, even in well-studied model species. Solitary bees provision their offspring, restricting the food available for larval growth. Experiments indicate that the complete consumption of these provisions induces metamorphosis. Studying the cues to metamorphosis in solitary bees provides a unique opportunity to physiologically compare individuals of the same age and size that either have or have not transitioned to metamorphosis. The proposed research will investigate (1) whether the hormonal cues underlying starvation-triggered metamorphosis in solitary bees are the same as those found in insects that metamorphose upon reaching a particular size, (2) how variation in the timing of metamorphosis predicts body size variation in natural populations and different bee species, and (3) how the determinants of adult size in bees shape differences between queens and workers in honeybees. Bee body size predicts pollination performance, and a physiological understanding of body size control has direct implications for pollinator health and performance in both natural and managed bee species, whether solitary or social. Undergraduates, graduate students and a postdoctoral fellow will participate in this research, and K-12 learning modules on pollinator life cycles will be developed.

Body size is an important organismal trait that correlates with most aspects of performance and fitness. In determinant-growing organisms, body size becomes fixed at maturation; therefore the mechanisms regulating maturation also influence size variation among individuals. Insects grow as larvae until attainment of a critical weight, at which point the mechanisms regulating metamorphosis are irreversibly initiated. This established model of body size control may not be generalizable to species with larval ecologies that differ from model insect species. Preliminary data demonstrate that completely consuming the larval provision initiates metamorphosis in the solidary bee Osmia lignaria. This result challenges the existing conceptual model for insect body size by suggesting that the critical weight is not a universal trait in insects. The proposed research will address three central aims: 1) characterizing the physiological regulation of metamorphosis in the solitary bee Osmia lignaria, 2) characterizing factors determining body size variation among populations and species of solitary bees that share similar larval ecologies; and 3) testing the degree to which diet quantity contributes to caste differences in eusocial bees. These mechanisms have the potential to explain much of the body size variation observed among Hymenopterans. Finally, the proposed research will develop a mechanistically explicit understanding of body size determination for bees, which are key pollinators for natural and managed ecosystems. A deeper, mechanistic understanding of body size variation may yield insights into improving pollinator health at the scale of individuals or even populations. Results from these studies will be disseminated in peer reviewed journals and through presentations at scientific meetings.

National Science Foundation (NSF)
Division of Integrative Organismal Systems (IOS)
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Kathryn Dickson
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North Dakota State University Fargo
United States
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