When delicate brood makes for strong colonies

Authors: Ihle, Kate

Submitted to: Bee Culture
Publication Type: Popular Publication
Publication Acceptance Date: 1/18/2023
Publication Date: N/A
Citation: N/A

Interpretive Summary: Honey bees have a variety of traits that contribute to disease and pest resistance. Some of these traits benefit the individual and some, called social immunity traits, benefit the colony. Often times social immunity traits come at the expense of the individual exhibiting them. We identified a social immunity trait that developing pupae express called "social apoptosis." Here, pupae who's cells are infested by a Varroa mite have higher rates of death than those that aren't infested. This stops the mite from reproducing, but comes at a high price for the individual. However, this helps the colony by keeping mite populations low. We identified this trait in Russian Honey Bees, a Varroa resistant stock developed and released by the USDA Honey Bee Breeding, Genetics, and Physiology Lab in Baton Rouge, Louisiana.

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. Varroa 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 is able to survive mite infestations with little effect on colony health and survival. This resilience is due in part to a newly identified social immune response expressed by developing worker brood. Varroa infested female A. cerana brood experience delayed development and eventually die in a process called “social apoptosis.” Here, an individual’s susceptibility to Varroa results in colony level resistance. We tested for the presence of the social apoptosis trait in two Varroa resistant stocks of Apis mellifera (Pol-line and Russian) with different selection histories and compared them to a known Varroa-susceptible stock (Italian). We assessed the survival and development of worker brood reared in either highly or lightly infested host colonies, then receiving one of three treatments: uninfested, experimentally inoculated with a Varroa mite, or wounded to simulate Varroa damage. We found that response to treatment was only differentiated in brood reared in lightly infested host colonies, where experimentally infested Russian honey bees had decreased survival relative to the mite-susceptible Italian stock. This is the first evidence that social apoptosis can exist in Western honey bee populations.