During 2017 and 2018, a total of 43920 trap nights were performed, capturing 1325 bank voles (976 and 349 captures in 2017 and 2018, respectively). Altogether 5372 tick larvae were collected from the trapped voles at all altitudes of the gradient. Larvae of I. ricinus were 7.4 times more numerous compared to I. trianguliceps (88.1% and 11.9% of ticks, respectively). Larval burdens ranged from 1-100 ticks for I. ricinus, and from 1–29 for I. trianguliceps, and more than half of the bank voles (57.4%) were infested with at least one larva of either tick species. For I. ricinus and I. trianguliceps, respectively, 46.8% and 18.0%, of voles carried at least one larva. Of all the voles, 7.3% were infested with at least one larvae of both tick species. For a total number of ticks collected from voles at every altitude see Fig. 2. Mean burdens can be found in Table 1.
I. ricinus
The best model shows that there was a clear negative overall effect of altitude on the prevalence of I. ricinus presence (z=-3.954, p < 0.001), and a significant interaction between altitude and season (c2 = 12.3, df = 2, p = 0.002) illustrate that this effect was stronger in autumn than in spring and summer (Table 2, Fig. 3). In addition, a significant interaction between altitude and site (c2 = 11.1, df = 1, p < 0.001) shows that the negative altitude effect was stronger in Lærdal than in Lifjell (Table 2). This is mainly caused by the fact that, while the prevalence is low at 1000 m at both sites, the prevalence at the lower altitudes was higher in Lærdal (close to 1) compared to Lifjell (around 0.75 or less) (Fig. 3). The model further shows that the overall prevalence of I. ricinus was highest among voles captured in spring, lower, but not significantly so, among voles captured in summer (z=-1.14, p = 0.25) and significantly lower among voles captured in autumn (z=-2.65, p = 0.08) (Table 2, Fig. 3). The prevalence of I. ricinus was overall lower in 2018 (z=-2.06, p = 0.04), but the season effect, with lower prevalence in autumn, was much stronger in 2017 than in 2018, as indicated by a significant interaction between year and season ((c2 = 26.7, df = 2, p < 0.001) (Table 2, Fig. 3). Finally, the significant interaction between site and season shows that the season effect was stronger in Lærdal than in Lifjell (c2 = 12.3, df = 2, p = 0.002) with especially the autumn prevalence being lower in Lærdal (z=-2.73, p = 0.006) (Table 2, Fig. 3).
While host weight did not affect the prevalence of I. ricinus (c2 = 0.106, df = 1, p = 0.75), it was significantly more often found on male than on female host, resulting in a significant sex effect in the best model (c2 = 15.0, df = 1, p < 0.001).
I. trianguliceps
The best model, explaining the prevalence of I. trianguliceps on bank voles, ended up being much simpler than that for I. ricinus (Table 2). Also for this species there was a clear negative overall effect of altitude on the prevalence (c2 = 11.7, df = 1, p < 0.001) (Table 2). However, the overall prevalence was much lower than what was found for I. ricinus (Fig. 3). The overall prevalence of I. trianguliceps was significantly lower in Lærdal than in Lifjell (c2 = 34.7, df = 1, p < 0.001) and significantly higher in 2018, compared to 2017 (c2 = 27.0, df = 1, p < 0.001) (Table 2, Fig. 3). The model further shows a clear effect of season (c2 = 35.7, df = 2, p < 0.001) with the overall prevalence of I. trianguliceps being higher among voles captured in autumn, and with an equal prevalence in spring and summer (Table 2, Fig. 3)
Neither host weight (c2 = 2.66, df = 1, p = 0.10) nor host sex (c2 = 1.11, df = 1, p = 0.29) did affect the prevalence of I. trianguliceps presence on bank voles.
Table 1
Mean larval burden sizes ± SD of I. ricinus and I. trianguliceps found on bank voles at every altitude during 2017 and 2018 in Lifjell and Lærdal. NA indicates either that no voles were captured, or that not enough data was available to compute a standard deviation.
Lærdal
|
2018
|
Autumn
|
2.1 ± 1.6
|
1.6 ± 2.8
|
4.3 ± 3.0
|
1.4 ± 3.5
|
0.5 ± 0.7
|
0.4 ± 1.0
|
0.3 ± 0.6
|
1.2 ± 3.3
|
1.0 ± 1.8
|
1.6 ± 2.3
|
0.2 ± 0.4
|
0.7 ± 1.1
|
0.0 ± 0.0
|
1.1 ± 3.0
|
0.2 ± 0.4
|
1.0 ± 2.4
|
NA
|
NA
|
0.0 ± 0.0
|
0.0 ± 0.0
|
Summer
|
6.3 ± 4.6
|
0.2 ± 0.4
|
19.4 ± 19.1
|
0.1 ± 0.3
|
2.8 ± 4.0
|
0.2 ± 0.4
|
1.0 ± 1.4
|
0.6 ± 1.2
|
2.6 ± 3.2
|
1.4 ± 4.3
|
0.4 ± 0.9
|
0.5 ± 0.9
|
0.4 ± 0.9
|
0.2 ± 0.4
|
0.4 ± 0.9
|
0.0 ± 0.0
|
0.0 ± 0.0
|
0.2 ± 0.7
|
0.0 ± NA
|
0.0 ± NA
|
Spring
|
43.3 ± 32.6
|
0.0 ± 0.0
|
31.6 ± 25.0
|
0.5 ± 0.9
|
10.6 ± 6.9
|
0.2 ± 0.4
|
9.5 ± 15.3
|
0.3 ± 0.5
|
12.7 ± 10.5
|
0.1 ± 0.3
|
2.1 ± 2.6
|
0.3 ± 0.8
|
0.3 ± 0.6
|
0.0 ± 0.0
|
0.2 ± 0.4
|
0.0 ± 0.0
|
NA
|
NA
|
NA
|
NA
|
2017
|
Autumn
|
10.5 ± 9.8
|
0.6 ± 15
|
8.9 ± 4.9
|
0.4 ± 08
|
0.7 ± 1.1
|
0.7 ± 14
|
0.2 ± 0.6
|
0.3 ± 05
|
0.0 ± 0.2
|
0.5 ± 1.9
|
0.0 ± 0.0
|
0.1 ± 0.3
|
0.0 ± 0.0
|
0.1 ± 0.4
|
0.0 ± 0.0
|
0.5 ± 1.0
|
0.0 ± 0.0
|
0.1 ± 0.6
|
0.0 ± 0.0
|
0.2 ± 0.4
|
Summer
|
19.7 ± 20.3
|
0.1 ± 0.3
|
13.2 ± 9.1
|
0.3 ± 0.7
|
3.1 ± 3.3
|
0.0 ± 0.0
|
1.6 ± 1.6
|
0.0 ± 0.2
|
4.1 ± 2.6
|
0.3 ± 1.4
|
1.6 ± 1.5
|
0.0 ± 0.0
|
1.4 ± 1.9
|
0.1 ± 0.5
|
0.2 ± 0.4
|
0.1 ± 0.4
|
0.9 ± 0.9
|
0.0 ± 0.0
|
0.0 ± 0.0
|
0.0 ± 0.0
|
Spring
|
84.0 ± 25.2
|
0.3 ± 0.6
|
23.8 ± 9.8
|
0.0 ± 0.0
|
19.5 ± 13.7
|
0.0 ± 0.0
|
8.1 ± 7.3
|
0.0 ± 0.0
|
14.5 ± 8.0
|
0.1 ± 0.3
|
13.8 ± 12.9
|
0.1 ± 0.4
|
2.0 ± 3.3
|
0.3 ± 0.8
|
NA
|
NA
|
NA
|
NA
|
NA
|
NA
|
Lifjell
|
2018
|
Autumn
|
4.8 ± 5.3
|
4.6 ± 6.5
|
NA
|
NA
|
NA
|
NA
|
0.0 ± NA
|
4.0 ± NA
|
NA
|
NA
|
NA
|
NA
|
NA
|
NA
|
0.0 ± 0.0
|
2.0 ± 1.4
|
NA
|
NA
|
0.0 ±
|
0.0 ± NA
|
Summer
|
0.0 ± 0.0
|
1.0 ± 1.4
|
NA
|
NA
|
NA
|
NA
|
0.0 ± 0.0
|
0.0 ± 0.0
|
NA
|
NA
|
0.0 ± 0.0
|
0.5 ± 0.7
|
0.0 ± NA
|
0.0 ± NA
|
NA
|
NA
|
NA
|
NA
|
0.0 ± NA
|
0.0 ± NA
|
Spring
|
19.0 ± NA
|
0.0 ± NA
|
NA
|
NA
|
NA
|
NA
|
0.5 ± 1.0
|
0.5 ± 0.6
|
0.5 ± 1.2
|
1.2 ± 1.3
|
0.0 ± NA
|
0.0 ± NA
|
NA
|
NA
|
0.0 ± 0.0
|
0.0 ± 0.0
|
NA
|
NA
|
NA
|
NA
|
2017
|
Autumn
|
0.2 ± 0.5
|
1.6 ± 3.4
|
2.9 ± 1.6
|
0.9 ± 1.1
|
0.6 ± 1.4
|
1.6 ± 2.8
|
0.0 ± 0.0
|
2.3 ± 4.9
|
0.0 ± 0.2
|
1.5 ± 3.9
|
0.1 ± 0.3
|
0.5 ± 1.1
|
0.0 ± 0.0
|
0.2 ± 0.5
|
0.0 ± 0.0
|
0.2 ± 0.7
|
0.0 ± 0.0
|
0.3 ± 0.8
|
0.0 ± 0.0
|
0.4 ± 0.5
|
Summer
|
0.8 ± 1.0
|
0.2 ± 0.7
|
8.1 ± 7.4
|
0.0 ± 0.0
|
1.5 ± 2.4
|
0.0 ± 0.2
|
0.9 ± 1.9
|
0.6 ± 1.1
|
0.1 ± 0.4
|
0.5 ± 0.9
|
1.1 ± 1.2
|
0.1 ± 0.9
|
0.0 ± 0.0
|
1.3 ± 5.9
|
NA
|
NA
|
0.1 ± 0.4
|
0.1 ± 0.4
|
0.2 ± 0.4
|
0.0 ± 0.0
|
Spring
|
0.3 ± 0.7
|
1.3 ± 2.2
|
15.0 ± 10.9
|
0.4 ± 1.0
|
1.1 ± 0.7
|
0.0 ± 0.0
|
0.9 ± 1.3
|
0.3 ± 0.6
|
0.5 ± 0.8
|
0.3 ± 0.6
|
0.2 ± 0.4
|
0.4 ± 0.7
|
2.8 ± 7.9
|
0.0 ± 0.0
|
NA
|
NA
|
NA
|
NA
|
NA
|
NA
|
|
|
|
I. ric.
|
I. tri.
|
I. ric.
|
I. tri.
|
I. ric.
|
I. tri.
|
I. ric.
|
I. tri.
|
I. ric.
|
I. tri.
|
I. ric.
|
I. tri.
|
I. ric.
|
I. tri.
|
I. ric.
|
I. tri.
|
I. ric.
|
I. tri.
|
I. ric.
|
I. tri.
|
|
|
|
100 m
|
200 m
|
300 m
|
400 m
|
500 m
|
600 m
|
700 m
|
800 m
|
900 m
|
1000 m
|
Table 2
Parameter estimates for the infestation prevalence on bank voles indicating the probability to be infested with at least one tick, using a binomial distribution model. Baseline for study area is Lifjell, year 2017, spring season and male voles.
|
Estimate
|
Std. Error
|
z - value
|
p -value
|
I. ricinus larvae
|
|
|
|
|
Intercept
|
1.7429
|
0.5243
|
3.32
|
0.001
|
Study area Lærdal
|
4.2441
|
0.5824
|
7.29
|
< 0.001
|
Altitude
|
-0.0044
|
0.0011
|
-3.95
|
< 0.001
|
Sex Female
|
-0.6724
|
0.1719
|
-3.91
|
< 0.001
|
Year 2018
|
-0.7789
|
0.3780
|
-2.06
|
0.039
|
Season Summer
|
-0.6515
|
0.5701
|
-1.14
|
0.253
|
Season Autumn
|
-1.6039
|
0.6048
|
-2.65
|
0.008
|
Altitude : Season Summer
|
0.0012
|
0.0012
|
1.03
|
0.305
|
Altitude : Season Autumn
|
-0.0023
|
0.0014
|
-1.59
|
0.112
|
SiteLærdal : Altitude
|
-0.0029
|
0.0009
|
-3.39
|
0.001
|
Study area Lærdal : Season Summer
|
-0.7681
|
0.4656
|
-1.65
|
0.099
|
Study area Lærdal : Season Autumn
|
-1.5402
|
0.5646
|
-2.73
|
0.006
|
Year 2018 : Season Summer
|
-0.6665
|
0.4740
|
-1.41
|
0.160
|
Year 2018 : Season Autumn
|
1.4512
|
0.4858
|
2.99
|
0.003
|
I. trianguliceps larvae
|
|
|
|
|
Intercept
|
-1.1554
|
0.2461
|
-4.70
|
< 0.001
|
Study area Lærdal
|
-0.9831
|
0.1690
|
-5.82
|
< 0.001
|
Altitude
|
-0.0011
|
0.0003
|
-3.37
|
0.001
|
Year 2018
|
0.9297
|
0.1785
|
5.21
|
< 0.001
|
Season Summer
|
0.0412
|
0.2257
|
0.18
|
0.855
|
Season Autumn
|
0.9133
|
0.2114
|
4.32
|
< 0.001
|