OBSERVATIONS ON POPULATIONS OF ADULT BEAVER-BEETLES, PLATYPSYLLUS CASTORIS

Abstract

While trapping beaver in the Mississippi River bottom-lands below Hastings, Minnesota in March, 1960, many of the twoyear- oIds were found to be heavily infested with the coleopterous ectoparasite, Platypsyllus castoris Ritsema. Since the beaver taken in previous years were old, mature beaver and did not have a heavy infestation, it was suspected that the beetles were segregating out on the two-year-oIds. It was suggested that this could be a mechanism to insure infestation of the new colonies that would be established by the young beaver. 1£ this segregation really existed, it was hypothesized that sampling of the population during the winter (when the beaver are icebound), and then during the spring (at the beginning of dispersal), would show differen~es in beetle densities per beaver that would be correlated with the age of the beaver and with the time of sampling. On this basis, two samples were taken; one during the December-January period (1960-1961) and the other during the late March and early April legal trapping season (1961), It was hoped that both samples would be similar in size and representation of colonies, but the spring work was partially interrupted by the activities of local fur trappers. The previous published investigations dealing with P. castoris have been primarily of two types: taxonomic (Ritsema, 1869; Riley, 1892) and parasite surveys concomitant with investigations of beaver biology (Lawrence, Hays & Graham, 1961; Erickson, 1944; Parks & Barnes, 1955). Essentially no consideration has been given to parasite populations per beaver and there appears to be a complete lack of discussion of the behavior of the beetle. There appear to be some differences in the life history of the beetle between the Michigan Upper Peninsula (Lawrence, Hays & Graham, 1961·) and the area below Hastings. Despite the fact that no evidence was gathered in support of differential infestation of beaver age-classes, it seems advisable to report the observations made on the beaver-parasite relationship and beetle behavior.