Honey bee colony parameters and mite infestation rates.

<p>The progression of the mean values of (A) the total amount of bees and brood in the colony, (B) the proportion of brood production in the colony and (C) the mite infestation rates for the mite-resistant (MR) colonies (purple lines) and mite-susceptible (MS) colonies (green lines) during the late summer to autumn of 2009 on Gotland, Sweden. Standard error bars are shown.</p

Results of the maximum likelihood repeated measures model analysis.

<p>Results of the maximum likelihood repeated measures model analysis.</p

Persistence of subclinical deformed wing virus infections in honeybees following <i>Varroa</i> mite removal and a bee population turnover

<div><p>Deformed wing virus (DWV) is a lethal virus of honeybees (<i>Apis mellifera</i>) implicated in elevated colony mortality rates worldwide and facilitated through vector transmission by the ectoparasitic mite <i>Varroa destructor</i>. Clinical, symptomatic DWV infections are almost exclusively associated with high virus titres during pupal development, usually acquired through feeding by <i>Varroa</i> mites when reproducing on bee pupae. Control of the mite population, generally through acaricide treatment, is essential for breaking the DWV epidemic and minimizing colony losses. In this study, we evaluated the effectiveness of remedial mite control on clearing DWV from a colony. DWV titres in adult bees and pupae were monitored at 2 week intervals through summer and autumn in acaricide-treated and untreated colonies. The DWV titres in Apistan treated colonies was reduced 1000-fold relative to untreated colonies, which coincided with both the removal of mites and also a turnover of the bee population in the colony. This adult bee population turnover is probably more critical than previously realized for effective clearing of DWV infections. After this initial reduction, subclinical DWV titres persisted and even increased again gradually during autumn, demonstrating that alternative non-<i>Varroa</i> transmission routes can maintain the DWV titres at significant subclinical levels even after mite removal. The implications of these results for practical recommendations to mitigate deleterious subclinical DWV infections and improving honeybee health management are discussed.</p></div

Average DWV titres in pupae.

<p>The average DWV titres with standard error bars in pupae in Apistan treated colonies (light bars) and untreated control colonies (dark bars) are presented on the right y-axis. The symptomatic bee with deformed wings and the asymptomatic bee represent the difference in titre amounts between clinical and sub-clinical DWV on the right y-axis. The average mite infestation rates with standard error bars in treated colonies (light red lines) and untreated colonies (dark red lines) are presented on the left y-axis. The sampling points are presented progressively in weeks (and months) in relation to when the Apistan treatment was administered.</p

Average DWV titres in adult bees.

<p>The average DWV titres with standard error bars in adult bees in Apistan treated colonies (light bars) and untreated control colonies (dark bars) are presented on the right y-axis. The symptomatic bee with deformed wings and the asymptomatic bee represent the difference in titre amounts between clinical and sub-clinical DWV on the right y-axis. The average mite infestation rates with standard error bars in treated colonies (light red lines) and untreated colonies (dark red lines) are presented on the left y-axis. The sampling points are presented progressively in weeks (and months) in relation to when the Apistan treatment was administered.</p

Contingency table analyses for virus co-prevalence in both bees and mite samples.

<p>For the <i>Varroa</i>-infested region, separate comparisons were made for the virus prevalences and co-infection in bee samples and in mite samples (n = 41). The contingency tables were derived through comparing the observed incidence of co-infection with the expected values derived from the individual prevalences in bees and mites. For significant non-random associations (bold; p<0.05) is also indicated whether the association is positive (+), i.e. a higher incidence of co-infection than expected, or negative (−), i.e. a lower incidence of co-infection than expected.</p

Quantitative analysis of the phoretic <i>Varroa</i> infestation.

<p>(<b>A</b>) Varroa prevalence. The proportion of colonies where mites could be retrieved (black) versus not retrieved (white) is presented in terms of the sampling site location and number of years <i>Varroa</i> had been detected in the area. A significant increase in <i>Varroa</i> prevalence along the sampling transect is symbolised by the red curve (GLMM, Z = 4.14, p<0.001, 27≤n≤39). (<b>B</b>) <i>Varroa</i> infestation levels according to the number of years of confirmed exposure to <i>Varroa</i>. Number of phoretic mites per 100 bees (27≤n≤39). Stars indicate significant differences between years of infestation (Pairwise post-hoc comparisons, p<0.01).</p

Principal component analyses of pathogen titres in honeybee and <i>Varroa</i> samples.

<p>(<b>A</b>) Barplot of the eigenvectors of the PCA performed on the variables measured in bees. Variables included in the Principal Component Analysis (PCA) are the titres of 5 viruses (DWV, BQCV, CBPV, KBV, SBV) and the <i>Varroa</i> infestation rate (Var). Numbers above each bar indicate the cumulative percentage of variability explained by the successive eigenvectors. The two principal eigenvectors, represented by black bars, correspond to the axes used to plot the colonies in <a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1004323#ppat-1004323-g004" target="_blank">Figure 4.B</a>. (<b>B</b>) Scatterplot of colonies analysed by PCA for the titres of 5 viruses plus the <i>Varroa</i> infestation rates in bees (n = 191). The colony values for the two principal components are plotted, with each colony represented by a filled circle. The colonies are clustered by colour and bound by an ellipse according to the number of years since the first detection of <i>Varroa</i>, indicated by the number located at the centre of gravity of each ellipse. The ellipse covers 67% of the samples belonging to the cluster. The colour code is the same as for <a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1004323#ppat-1004323-g001" target="_blank">Figure 1</a>. (<b>C</b>) Barplot of the eigenvectors of the PCA performed on variables measured in bees and in <i>Varroa</i>. Variables included in the PCA are the titres of 4 virus species in bees (DWV, BQCV, KBV, SBV), titres of 4 virus species in <i>Varroa</i> (DWV.V, BQCV.V, KBV.V, SBV.V) and the <i>Varroa</i> infestation rates (Var). The numbers above each bar indicate the cumulative percentage of variability explained by the successive eigenvectors. The two principal eigenvectors, represented by the black bars, correspond to the axes used to plot the colonies in <a href="http://www.plospathogens.org/article/info:doi/10.1371/journal.ppat.1004323#ppat-1004323-g004" target="_blank">Figure 4.D</a>. (<b>D</b>) Scatterplot of colonies analysed by PCA for virus titres in bees and mites plus the <i>Varroa</i> infestation rates (n = 83). The colony values for the two principal components are plotted, with each colony represented by a filled circle. The colonies are clustered by colour and bound by an ellipse, according to the number of years since the first detection of <i>Varroa</i>. Each ellipse covers 67% of the samples belonging to the cluster.</p

Map illustrating the spread of <i>Varroa</i> across New Zealand and the location of sampling sites.

<p>Colours indicate the date <i>Varroa</i> was first confirmed in each area. Shaded tones from dark red to light yellow show the progression of the front of <i>Varroa</i> infestation. Control regions where the mite had not yet been detected are presented in white. Black dots indicate the location of the apiaries sampled in each region. The sampling transect crosses the front of infestation.</p

Honeybee virus prevalence across the <i>Varroa</i> front of infestation.

<p>Pathogen prevalence across the front of infestation, in bee samples and <i>Varroa</i> mite samples. The percentage of colonies assigned positive for each of the seven viruses monitored is compared between <i>Varroa</i>-free areas for bee samples (white bars, n = 39), <i>Varroa</i>-infested areas for bee samples (black bars, n = 75), and <i>Varroa</i> mite samples (grey bars, n = 34). Stars indicate significant differences between proportions (Chi-square, p<0.05). Viruses are presented in decreasing order of prevalence. The average pathogen prevalence in bee samples across all regions sampled is indicated on the x-axis below the pathogen acronym.</p