Genetic and usurpation data support high incidence of bumble bee nest invasion by socially parasitic bumble bee, Bombus insularis

Authors: Koch, Jonathan; McCabe, Lindsie; Love, Byron; Cox-Foster, Diana

Submitted to: Journal of Insect Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/6/2021
Publication Date: 9/3/2021
Citation: Koch, J., McCabe, L.M., Love, B.G., Cox-Foster, D.L. 2021. Genetic and usurpation data support high incidence of bumble bee nest invasion by socially parasitic bumble bee, Bombus insularis. Journal of Insect Science. 21(5):1-7. Article 3. https://doi.org/10.1093/jisesa/ieab063.
DOI: https://doi.org/10.1093/jisesa/ieab063

Interpretive Summary: Cuckoo bumble bees are a unique lineage of bees that depend exclusively on a host bumble bee species to provide nesting material and labor to rear offspring. Usurpation by a cuckoo bumble bee female typically results in the death of resident gyne and many adult workers. In this study we document usurpation incidence and population genetic data of the cuckoo bumble bee, B. insularis, on field deployed B. huntii colonies. Within 12 days, 14 of the 16 deployed B. huntii colonies were documented with a B. insularis female. In one colony, up to five B. insularis females were detected, with all but one dead upon the time of inspection. We failed to detect a significant effect of the number of B. huntii workers present at deployment, colony weight, or site location on the probability of B. insularis detection within a colony. Furthermore, we failed to detect a significant difference in the number of living workers present across invaded B. huntii colonies. Based on genetic data, we determined that B. insularis have the capacity to disperse across the landscape in search of host colonies at distances of at least 3.52 km and potentially up to 7.04 km. Our study underscores the significant impact Psithyrus bumble bees have on the fitness of the host bumble bee colony. As B. insularis significantly impacts the fitness of bumble bee colonies, we briefly discuss how the utilization of Psithyrus excluders may be useful for commercial bumble bee colonies that are used to pollinate open field crops.

Technical Abstract: Cuckoo bumble bees (Psithyrus) are a unique lineage of bees that depend exclusively on a host bumble bee species to provide nesting material and labor to rear offspring. Usurpation by a Psithyrus female typically results in the death of resident gyne and many adult workers. In this study we document usurpation incidence and population genetic data of B. insularis females on field deployed B. huntii colonies. Within 12 days, 14 of the 16 deployed B. huntii colonies were documented with a B. insularis female. In one colony, up to five B. insularis females were detected, with all but one dead upon the time of inspection. We failed to detect a significant effect of the number of B. huntii workers present at deployment, colony weight, or site location on the probability of B. insularis detection within a colony. Furthermore, we failed to detect a significant difference in the number of living workers present across invaded B. huntii colonies. While our results suggest that bumble bee colonies with = 20 workers are highly susceptible to B. insularis usurpation, applying a Psithyrus excluder to prevent the inquiline from invading a colony was 100% effective. Finally, based on microsatellite genotype data, we determined that B. insularis have the capacity to disperse across the landscape in search of host colonies at distances of at least 3.52 km and potentially up to 7.04 km. Our study underscores the significant impact Psithyrus bumble bees have on the fitness of the host bumble bee colony. As B. insularis significantly impacts the fitness of bumble bee colonies, we briefly discuss how the utilization of Psithyrus excluders may be useful for commercial bumble bee colonies that are used to pollinate open field crops.