Social apoptosis in Varroa mite resistant western honey bees (Apis mellifera)

Authors: Ihle, Kate; De Guzman, Lilia; Danka, Robert

Submitted to: Journal of Insect Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/4/2021
Publication Date: 4/7/2022
Citation: Ihle, K.E., De Guzman, L.I., Danka, R.G. 2022. Social apoptosis in Varroa mite resistant western honey bees (Apis mellifera). Journal of Insect Science. 22(1):13. https://doi.org/10.1093/jisesa/ieab087.
DOI: https://doi.org/10.1093/jisesa/ieab087

Interpretive Summary: Honey bees use a variety defenses to protect colonies against Varroa destructor. Varroa mites were introduced to the Western honey bee from their sister species, the Eastern honey bee, Apis cerana. Unlike the Western honey bees common in the United States, Eastern honey bees are able to thrive while keeping Varroa mite levels low. Part of this resistance to mite infestation comes from a newly discovered trait in the developing brood. Varroa mites reproduce inside the closed brood cells of honey bees, and parasitize the pupae to extract nutrients and other substances needed for growth. Female brood from the Eastern honey bee, slows development and dies when parasitized by Varroa in a process termed “social apoptosis.” This prevents the mites from reproducing in the very common female brood, limiting their reproduction to the rare male brood cells and helping to keep mite populations low. We tested for the presence of the social apoptosis trait in two Varroa resistant stocks of honey bees developed by the USDA Honey Bee Lab in Baton Rouge, LA, the Pol-line and Russian honey bees (RHB), as well as commercially produced Italian honey bees (IHB) as mite susceptible control. We deliberately inoculated newly sealed brood cells with a foundress Varroa mite and measured development, mortality and mite reproduction. We report the first evidence of social apoptosis in a Western honey bee stock, the RHB, which had increased mortality when experimentally infested with a mite. Further testing is needed to determine if this counter-intuitive trait can be beneficial to the beekeeping industry.

Technical Abstract: Honey bees are eusocial animals that exhibit both individual and social immune responses, which influence colony health. This is especially well-studied regarding the mite Varroa destructor, a parasite of honey bee brood and disease vector. This mite was introduced relatively recently to Apis mellifera, and is a major driver of the catastrophic die-off of honey bee colonies in the last decade. In contrast, the original host species, Apis cerana, can maintain mite populations at low levels in healthy colonies. This resilience is due in part due to a newly identified social immune response expressed by developing worker brood. Varroa infested female A. cerana brood often slows development and eventually dies in a process called “social apoptosis.” Here, an individual susceptibility to a common parasite results in colony level resistance. In this study, we tested for the presence of the social apoptosis trait in two Varroa resistant stocks of Apis mellifera with different selection histories: Pol-line and Russian honey bees. We assessed the survival and development of worker brood that were either uninfested, experimentally inoculated with a Varroa mite, or wounded with a sterile microneedle to simulate Varroa damage. We found that Russian honey bee pupae experimentally infested with Varroa mites had decreased survival and lower mite offspring numbers. This is the first evidence that genetic variation for social apoptosis exists in Western honey bee populations and could be a potential target for Varroa resistance.