Honey bee virus causes context-dependent changes in host social behavior

Abstract

Anthropogenic changes create evolutionarily novel environments that present opportunities for emerging diseases, potentially changing the balance between host and pathogen. Honey bees provide essential pollination services, but intensification and globalization of honey bee management has coincided with increased pathogen pressure, primarily due to a parasitic mite/virus complex. Here, we investigated how honey bee individual and group phenotypes are altered by a virus of concern, Israeli acute paralysis virus (IAPV). Using automated and manual behavioral monitoring of IAPV-inoculated individuals, we find evidence for pathogen manipulation of worker behavior by IAPV, and reveal that this effect depends on social context; that is, within versus between colony interactions. Experimental inoculation reduced social contacts between honey bee colony members, suggesting an adaptive host social immune response to diminish transmission. Parallel analyses with double-stranded RNA (dsRNA)-immunostimulated bees revealed these behaviors are part of a generalized social immune defensive response. Conversely, inoculated bees presented to groups of bees from other colonies experienced reduced aggression compared with dsRNA-immunostimulated bees, facilitating entry into susceptible colonies. This reduction was associated with a shift in cuticular hydrocarbons, the chemical signatures used by bees to discriminate colony members from intruders. These responses were specific to IAPV infection, suggestive of pathogen manipulation of the host. Emerging bee pathogens may thus shape host phenotypes to increase transmission, a strategy especially well-suited to the unnaturally high colony densities of modern apiculture. These findings demonstrate how anthropogenic changes could affect arms races between human-managed hosts and their pathogens to potentially affect global food security.

Keywords: honey bee; host–pathogen evolution; pathogen manipulation; virus.

Fig. 1.

Viral infection decreases honey bee social interactions but not overall movement in automatically monitored colonies. Violin plots with inset box-plots (horizontal line within rectangle indicates median; upper and lower hinges denote first and third quartiles, respectively, upper (Lower) whiskers extend from the hinge to the largest (smallest) value no further than 1.5 * interquartile range from the hinge). (A) Number of trophallaxis interactions per hour. IAPV-inoculated bees had significantly fewer trophallaxis interactions than sucrose-only control bees (GLMM for gamma distributed response variables, fixed effect: treatment, random effect: trial; likelihood ratio test against null model, χ² = 12.69, 1 d.f., P < 0.0004). (B) Distance moved (in millimeters) per hour. IAPV-inoculated bees covered significantly greater distances than sucrose-only bees (GLMM for gamma distributed response variables, fixed effect: treatment; random effect: trial; likelihood ratio test against null model, χ² = 9.50, 1 d.f., P < 0.003). Both measures were adjusted for lifespan on an individual bee basis. (C) Typical image obtained from the automatic system for identifying honey bees and detecting trophallaxis, showing barcoded bees inside the observation hive. The hive entrance is in the lower-right corner. (Inset) Close-up of two bees that were automatically detected performing trophallaxis. For these two bees, the results of applying our computer vision algorithms for head and trophallaxis detection are visualized in magenta and green, respectively.

Fig. 2.

Intracolonial reduction in social interactions is caused by a general immune response. Violin plots with inset box-plots (as in Fig. 1) showing counts of select behaviors, and results from negative binomial GLMM followed by estimated marginal means (EMM) with a Tukey post hoc test. (A) Honey bees treated with IAPV or dsRNA were about half as likely to engage in trophallaxis as untreated focal bees [Control-dsRNA: P = 0.007, Z = −3.028 exp(β) = 2.00; Control-IAPV: P = 0.003, Z = 3.275, exp(β) = 2.186; IAPV-dsRNA: P = 0.947, Z = 0.316]. (B) Measurement of movement did not differ significantly between treatment groups. (C) Antennation of treatment bees by partners did not significantly differ between treatment groups.

Fig. 3.

Intercolonial reduction in aggression is IAPV-specific. Violin plots with inset box-plots (as in Fig. 1) showing counts of select behaviors and results of negative binomial GLMM followed by EMM with a Tukey post hoc test: (A) IAPV-inoculated bees experienced significantly reduced aggression from putative guards compared to immunostimulated bees, with controls intermediate [dsRNA-IAPV: P = 0.0143, exp(β) = 5.601, Z = 2.796; Control-IAPV: P = 0.2345; dsRNA-Control: P = 0.4397]. (B) IAPV-inoculated bees elicited significantly more trophallaxis behaviors between treatment bees and putative guards compared to immunostimulated bees [dsRNA-IAPV: P < 0.0001, exp(β) = 0.38, Z = −5.012; Control-IAPV: P = 0.0592, exp(β) = 0.66, Z = −2.276; dsRNA-Control: P = 0.0148, exp(β) = 1.747, Z = 2.784]. (C) A significantly greater proportion of IAPV-treated intruder bees were accepted at foreign colony entrances than control or dsRNA-immunostimulated bees (Pearson’s Chi-squared test, IAPV-Control, χ2 = 5.9665, d.f. = 1, P = 0.0218; IAPV-dsRNA, χ2 = 19.13, d.f. = 1, Benjamini–Hochberg-corrected P < 0.0001; Control-dsRNA, χ2 = 1.099, d.f. = 1, P = 0.294; N_control = 42, N_dsRNA = 31, and N_IAPV = 30; sample sizes indicated inside bars).

Fig. 4.

Viral infection changes honey bee cuticular hydrocarbon profile. Violin plots with inset box-plots (as in Fig. 1) of cuticular hydrocarbon relative abundance (A, B, E, and F) and nanogram (C, D, G, and H) values found to be significantly different between groups (statistics are reported in SI Appendix, Table S1). Comparisons were made with a standard GLMM followed by an EMM analysis with a Tukey post hoc test. Letters denote significant differences (P < 0.05). (I) Separation of CHC profiles (relative abundance) visualized by using linear discriminant analysis shows distinct profile patterns of IAPV-inoculated, dsRNA-treated, and control bees. Variance explained by each LD is as follows: LD1: 0.522, LD2: 0.478.