Small mammal abundance and tick infestation prevalence
Over the 15 capture sessions (totalling 8,820 trapping nights), 1,371 captures were recorded in total, corresponding to 906 different rodents captured per session over the 15 sessions (hereinafter called rodent-sessions) and 564 different individuals over the same 15 sessions: 276 Myodes glareolus , 287 Apodemus flavicollis , 1 Microtus agrestis . Overall, 342 rodents were captured during two or more trapping sessions. Data on each individual’s weight and sex were completed for more than 90% of individuals for each trapping session, except in September and October 2012 when 33 individuals died in traps due to the predation of a stone marten.
The populations of bank vole (M. glareolus ) and yellow-necked mice (A. flavicollis ) peaked in 2012 (453 individuals trapped), dropped dramatically in 2013 (32 individuals) and was intermediate in 2014 (85 individuals) (Figure 4). Every year, the density of small mammals peaked during season 2 (June and July). Both species were similarly abundant in 2012 whereas in 2013 and 2014 —during the years of the lowest small mammal abundances—, A. flavicollis was captured slightly more often than M. glareolus (Figure 4). In 2012, bank voles were more abundant than yellow-necked mice in April and in September-October. Overall, 74 individuals were captured as juveniles, of which nine (12%), 41 (55%) and 19 (26%) were respectively captured in April, June and July.
The proportion of rodents infested at least once by ticks significantly varied between years and seasons (Table 1). For all years, the prevalence of tick infestation on rodents was the highest in season 2 compared to the two other seasons (Chi², 2012, p-value < 0.001, 2013, p-value = 0.001, 2014, p-value < 0.001). The prevalence of tick infestation was lower in 2012 than 2013 and 2014 except in season 1. In season 2, this prevalence varied from 44% (95% CI: 38.8 – 49.3) in 2012 to 94.2% (95% CI: 85.8 – 98.4) in 2013 and 100% (95% CI: 71.5 – 100) in 2014.
The best GLM for tick infestation prevalence on rodents included the small mammal species, season, year, the interaction between season and year and the interaction between season and small mammal species (Figure 5). Small mammals were significantly more likely to be infested by ticks in season 2 than in season 1 and 3. Yellow-necked mice were about twice as more likely to be infested than bank voles in season 2 of 2012. In other seasons and years, there was no significant difference in the prevalence of tick infestation between species.
A total of 349 larvae of Ixodes sp ., 17 nymphs of I. ricinus and 4 females of I. trianguliceps were collected on 152 rodent-sessions in 2012 and 2013. During June and July, the mean number of larvae and nymphs per infested rodent-session (the intensity of tick infestation) was higher in 2013 (5.9 ticks +/- 4.8) than in 2012 (2.2 ticks +/- 2.2) (Mann-Whitney U test, p-value = 0.01, Table 2) and was not significantly different between species (Mann-Whitney Utest, 2012: p-value = 0.64, 2013: p-value = 0.45, Table 2). The proportion of rodent-sessions infested by nymphs and co-infested by larvae and nymphs was higher in 2013 (6/12 and 5/12 respectively) than in 2012 (3/67 and 2/67 respectively, Table 2, Fisher’s exact test, p-value<0.001). As a consequence, only 2.7% (4/127) of larvae fed simultaneously with nymphs in 2012 vs. 57.8% (37/64) in 2013.