A Ten-Year Study Of Louse Populations On Deer Mice1

From 1952 to 1961 over 2800 specimens of Peromyscus maniculatus gracilis (Le Conte) were collected in northern Minnesota. Of these 2452 were suitable for the study of louse populations. The host population density varied considerably from year to year with the density being over 5× as large during the peak year as during the low years. Only the louse Hoplopleura hesperomydis was found. The infestation rates varied from 17.5 to 58.0% with an overall rate of 33.8% on the ♂, and from 12.3 to 48.4% with an overall rate of 26.9% on the ♀ hosts. There was no demonstrable relationship between the rate of infestation and the density of the host population. There was a positive relationship between the infestation rates from ♂ and ♀ hosts over the several years. There was no significant change in infestation rate with age of the ♀ host but they did increase with the age of the ♂ host. The infestation rates were similar on the young ♂ and ♀ hosts but were significantly higher on the old ♂♂ than on the old ♀♀. The average infestation on infested ♀ hosts was 19.8 but varied from 1 to 1261. The median population size was 5. On the ♂ hosts the average was 16.2 with extremes of from 1 to 758. The median population size was 6. There was no correlation between infestation size and infestation rate. The average population sizes on ♂ and ♀ hosts showed positive correlation for the years studied. The louse population on the ♀ hosts was composed of 31.5% adults of which 41.8% were ♂♂. The immature portion of the population was composed of 73.9% 1st, 19.1% 2nd, and 6.9% 3rd instars. On the ♂ hosts the population was composed of 46.6% adults of which 40.8 are ♂♂. The immature portion of the population was composed of 69.6% 1st, 19.7% 2nd and 10.7% 3rd instars. The immature-to-adult ♀ ratio was 2.74 on the ♀ and 1.94 on the ♂ hosts. The immature-adult ♀ ratio varied positively with the louse population size. The factors which cause variation in infestation rate and population structure of the lice are apparently different from those which affect the host population.