Inside Honeybee Hives: Impact of Natural Propolis on the Ectoparasitic Mite Varroa destructor and Viruses

Abstract

Social immunity is a key factor for honeybee health, including behavioral defense strategies such as the collective use of antimicrobial plant resins (propolis). While laboratory data repeatedly show significant propolis effects, field data are scarce, especially at the colony level. Here, we investigated whether propolis, as naturally deposited in the nests, can protect honeybees against ectoparasitic mites Varroa destructor and associated viruses, which are currently considered the most serious biological threat to European honeybee subspecies, Apis mellifera, globally. Propolis intake of 10 field colonies was manipulated by either reducing or adding freshly collected propolis. Mite infestations, titers of deformed wing virus (DWV) and sacbrood virus (SBV), resin intake, as well as colony strength were recorded monthly from July to September 2013. We additionally examined the effect of raw propolis volatiles on mite survival in laboratory assays. Our results showed no significant effects of adding or removing propolis on mite survival and infestation levels. However, in relation to V. destructor, DWV titers increased significantly less in colonies with added propolis than in propolis-removed colonies, whereas SBV titers were similar. Colonies with added propolis were also significantly stronger than propolis-removed colonies. These findings indicate that propolis may interfere with the dynamics of V. destructor-transmitted viruses, thereby further emphasizing the importance of propolis for honeybee health.

Keywords: Apis mellifera; deformed wing virus; plant-insect interactions; resin; sacbrood virus; social immunity.

Figure 1

(a–d) Effects of the high (dark grey) and low (light grey) propolis treatment on the development of (a) colony strength; (b) Varroa destructor infestation rates (natural mite fall/colony strength); (c) sacbrood virus titers (SBV); and (d) deformed wing virus titers (DWV) over the course of the experiment from July to September 2013. Viral titers are expressed as log₂ transformed, efficiency corrected ΔCq values (Cq = quantification cycles). Each boxplot represents median values of both treatment groups (N = 5) per month with default ranges for boxes (75th and 25th percentile), whiskers (±1.5) and outliers (dots).

Figure 2

(a,b) Effect of the high (black) and low propolis (grey) treatment on the correlation between viral loads and the V. destructor mite infestations of (a) deformed wing virus (DWV); and (b) sacbrood virus (SBV). Each dot represents data from one colony for one month. Virus titers and V. destructor infestation were measured for each colony once per month from July to September 2013. Lines represent linear regressions between DWV virus titers (log₂ efficiency corrected fold-change relative to housekeeping gene, Cq = quantification cycles) and V. destructor infestation rates (natural mite fall/colony strength) for each treatment group according to significant interaction between treatment and V. destructor infestation (see Table 2).

Figure 3

(a,b) Effect of viral infection with (a) deformed wing virus (DWV); and (b) sacbrood virus (SBV) on the amount of resin (g) collected by bees. Each dot represents data from one colony for one month. Lines represent linear regression between resin collection and virus titers (log efficiency corrected fold-change relative to housekeeping gene) for each treatment group (black = ”high propolis treatment”, gray = “low propolis treatment”) with * indicating a significant correlation with p < 0.05 and n.s. a non-significant correlation.

Figure 4

Kaplan-Meyer survival curves showing survival rates of Varroa destructor mites when exposed to propolis obtained from colonies treated with thymol (Propolis B = dashed line N = 58), not treated with thymol (Propolis A = gray line, N = 58) or not exposed to propolis (Control = black line, N = 58) under laboratory conditions. Dotted lines mark 95% confidence intervals.