The effect of Israeli acute paralysis infection on honey bee brood care behavior

To protect themselves from communicable diseases, social insects utilize social immunity—behavioral, phsyiological, and organizational means to combat disease transmission and severity. Within a honey bee colony, larvae are visited thousands of times by nurse bees, representing a prime environment for pathogen transmission. We investigated a potential social immune response to Israeli acute paralysis virus (IAPV) infection in brood care, testing the hypotheses that bees will respond with behaviors that result in reduced brood care, or that infection results in elevated brood care as a virus-driven mechanism to increase transmission. We tested for group-level effects by comparing three different social environments in which 0%, 50%, or 100% of bees were experimentally infected with IAPV. We investigated individual-level effects by comparing exposed bees to unexposed bees within the mixed-exposure treatment group. We found no evidence for a social immune response at the group level; however, individually, exposed bees interacted with the larva more frequently than their unexposed nestmates. While this could increase virus transmission from adults to larvae, it could also represent a hygienic response to increase grooming when an infection is detected. Together, our findings underline the complexity of disease dynamics in complex social animal systems.

Methods
Honey bees were either experimentally infected with Israeli acute paralysis virus (IAPV) or treated with a control solution. Each bee was paint-marked for individual tracking of behaviors. The bees were grouped in sets of 10 to create three different types of social environment—0% of bees exposed to IAPV, 100% of bees exposed, and a mixed-treatment group with 50% of bees exposed to IAPV. The bees were introduced to a larval queen, at which point they were observed for five minutes. The number of events and duration of behaviors such as "external-antennation" around the opening of the queen cell and entering inside the queen cell ("insertion") were recorded for each individual bee, as were the number of unique visitors ("responders") to the queen cell, and summed for the entire social group. The group-level responses were standardized by dividing by the number of living bees in the behavioral assay arena. Virus titers in the bees were measured with qPCR, and the dry head masses of nurse bees were also recorded.