RESULTS
Change in virus composition before and after exposure to the
environment
Overall, the most prevalent viruses in the bumble bee colonies were
DWV-A, DWV-B, BQCV, LSV, ARV-1, which are all honey bee associated
viruses . In addition, several bumble bee associated viruses were
abundant: BcDV, CBV and MV 1 . Total viral load across all viruses (log
scale) increased on average by 1.34±2.21 (mean±SD) from the beginning to
the end of the field exposure (day 0 = 3.67±1.47; day 45 = 5.00±1.61;
lm: estimate±SE = 1.335±0.368, p<0.001), while total virus
richness on average increased by a factor of 1.46±1.75 from day 0 to day
45 (day 0 = 3.14±1.03; day 45 = 4.60±1.31; lm: 1.457±0.282,
p<0.001). Among the 9 screened viruses, only 1 to 5 were
present at day 0, whereas at day 45 between 2 and 7 viruses were present
among the colonies (Fig. 2).
The generally large increase in viral richness from day 0 to day 45 was
driven by the appearance of new and mostly bumble bee specific viruses,
such as CBV (lm: 2.852±0.467, p<0.001) or the MV 1 (lm:
2.291±0.404, p<0.001) as well as the DBV (lm: 3.006±0.309,
p<0.001). At day 0 colonies had either viral loads dominated
by the BcDV (significant increase; lm: 0.804±0.376, p = 0.036) or a
combination of two honey bee viruses: DWV-B (significant decrease; lm:
-0.990±0.363, p = 0.008) and BQCV (significant increase; lm:
0.639±0.198, p = 0.002; Fig. 2, SI Fig. S2).
Influences of initial colony viral loads on colony
development
Among all measured colony development parameters, only the moth
infestation index was related to the initial viral loads of viruses,
with increasing loads of DWV-B colonies were related to higher moth
infestation rates at the end of the field exposure
(estimate±SE=0.210±0.081, p=0.012), while increasing BcDV loads led to
fewer moth infestations (-0.218±0.079, p=0.008; SI Table S3).
Change in viral patterns in relation to landscape
structure
Total Viral Load Change – The mean distance among forest patches (1250
m radius) had a marginally significant and positive relationship with
the total viral load change in bumble bee colonies, while mean distance
among agricultural patches showed a large positive and significant
relationship (Table 2A, Fig. 3a), meaning that bumble bee colonies
located in more isolated (or less connected) agricultural patches had a
higher increase in viral loads during the field exposure (SI Fig. S3a).
Virus Richness Change – Habitat diversity (300 m), a measure of
landscape heterogeneity, was significantly negatively related to virus
richness change, showing that bumble bee colonies placed in areas with
fewer different habitat types (lower heterogeneity) were infected with a
higher number of different viruses after field exposure (Table 2B, Fig.
3b). On the other hand, forest area (100 m) was significantly positively
related to virus richness change; with increasing forest cover within
the close surroundings of a colony, the number of viruses infecting the
colony increased (Table 2B; SI Fig. S3b, c).
Virus Turnover – The area covered by forests (100 m) had the strongest
effect on viral turnover. Colonies in areas with more forest showed a
higher change in their virus composition (Table 2C). However, with
increasing distances (decreasing connectivity) among forest patches (400
m), virus turnover also increased significantly. Also, fragmentation of
vegetated vineyards (400 m) significantly influenced the turnover, such
that the viral composition changed less for colonies in areas with
higher fragmentation (Table 2C; SI Fig. S4a-c).
Virus Appearance – Habitat diversity and forest area both were
significantly related to virus appearance, with habitat diversity (300
m) again showing a negative effect, such that colonies in areas with a
lower number of different habitat types were more frequently infected
with viruses that were not yet present at the beginning of the field
exposure. Increasing forest area within the wider surrounding of a
colony (500 m) led to the appearance of new virus infections (Table 2D;
SI Fig. S4d, e).
Overall, among the different spatial scales tested (ranging from 100 m
to 1500 m), the predictors included in the final models had their
strongest influence on the different response variables within smaller
scales (100 m to maximum 500 m), indicating that the pattern of viral
infections in bumble bee colonies influenced by the landscape is
influenced by local rather than meso- or landscape scale heterogeneity.
Regarding the responses of colony development parameters to the
landscape and field structure, we found very similar responses as for
the viral patterns with generally positive relationships with
agricultural or residential areas and habitat diversity but negative
links to forest cover and connectivity. For detailed results see SI
section 3.