Species composition and numbers of fleas (Siphonaptera) in a local population of the field vole Microtus agrestis (L.)

In 1968 and 1969, 2 078 specimens of fleas were collected from 501 live-trapped field voles, Microtus agrestis (L.) on an estate in southern Finland. The fleas belonged to 12 species, the dominant species being Megabothris walkeri (Rothsch.) (44.2 %) and Ctenophthalmus agyrtes fennicus Peus (37.l %). Seasonal changes in the percentage infestation and mean infestation per host were observed: one maximum occurred in spring and another in autumn. Examination of the species composition of the fleas at different seasons revealed some species with a restricted seasonal occurrence and others occurring throughout the year. An excess of females was observed in Ctenoph thalmus agyrtes fennicus and C. uncinatus uncinatus (Wagner). In Megabothris walkeri the total sex ratio was roughly 1:1. Reproductive males were the host group most heavily infested. Reproductive females were more heavily infested than subadult and juvenile hosts. Among the non reproductive animals there was no difference in numbers of fleas between the sexes. Data on seasonal fluctuations in the infestation of these different host categories are presented. The infestation level and species composition of the fleas in two sub populations of the field voles were studied in two different habitats. In the drier habitat there was a slightly heavier infestation than in the moist habitat. A few differences in species composition were observed, too.