Replicative Deformed Wing Virus Found in the Head of Adults from Symptomatic Commercial Bumblebee (Bombus terrestris) Colonies

Abstract

The deformed wing virus (DWV) is one of the most common honey bee pathogens. The virus may also be detected in other insect species, including Bombus terrestris adults from wild and managed colonies. In this study, individuals of all stages, castes, and sexes were sampled from three commercial colonies exhibiting the presence of deformed workers and analysed for the presence of DWV. Adults (deformed individuals, gynes, workers, males) had their head exscinded from the rest of the body and the two parts were analysed separately by RT-PCR. Juvenile stages (pupae, larvae, and eggs) were analysed undissected. All individuals tested positive for replicative DWV, but deformed adults showed a higher number of copies compared to asymptomatic individuals. Moreover, they showed viral infection in their heads. Sequence analysis indicated that the obtained DWV amplicons belonged to a strain isolated in the United Kingdom. Further studies are needed to characterize the specific DWV target organs in the bumblebees. The result of this study indicates the evidence of DWV infection in B. terrestris specimens that could cause wing deformities, suggesting a relationship between the deformities and the virus localization in the head. Further studies are needed to define if a specific organ could be a target in symptomatic bumblebees.

Keywords: DWV; bumblebee; commercial Bombus terrestris; honey bee pathogens; replicative virus; spillover; strand-specific RT-PCR.

Figure 1

Bombus terrestris workers with crippled (A) and normal (B) wings. Individual A shows yellow and white patches of anomalous pigmentation.

Figure 2

Evidence of several degrees of wing deformities in symptomatic DWV infected Bombus terrestris. Wings of adult bumblebees from the inspected colonies: bilateral (A,C) and unilateral (D) deformity in workers; bilateral deformity in newly hatched male (B); asymptomatic worker (E). Deformed individuals show yellow and white patches of anomalous pigmentation.

Figure 3

DWV titre of deformed and asymptomatic adults and juvenile individuals. Averages +/− standard error are shown. The same letter indicates a nonsignificant difference.

Figure 4

DWV copies detected in both head and rest of the body of deformed and asymptomatic B. terrestris adults. Averages (columns) and standard errors (vertical bars) are shown. The same letters highlight nonsignificant differences (p ≤ 0.05).

Figure 5

Molecular phylogenetic analysis for RNA-dependent RNA polymerase of deformed wing virus (DWV) by maximum likelihood method. The evolutionary history was inferred using the maximum likelihood method based on the Tamura–Nei model. The branch lengths of the tree measured the number of substitutions per site. The analysis involved 28 nucleotide sequences. There were 255 positions in the final dataset. Accession number, host, state, and year of available GenBank DWV sequences are shown. DWV sequence accession numbers are reported and associated with year and site of origin. The DWV sequence obtained from the tested B. terrestris samples is highlighted by a red box.