The population dynamics and host utilization of ... - Phthiraptera

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298 1%. W. Rust litters and between litter mates. This explains the adult sex differences and small louse population (inoculum) on juvenile gophers. Miller (1946) found an average of 5 young/litter and 2 litters/year for gophers inhabiting irrigated fields in the Davis area. Thus, ideally 84% of the female's louse population could be lost twice a year. Actual measurements of the proportion of the population lost ranged from 56% to 66% for litter sizes of 2 and 4 (N=4). Lice are generally found on the head and foreparts of the small mammals and may spread over the dorsal and lateral aspects of the host as the population increases (Vysotoskaya, 1950; Dubinin, 1953; Murray, 1961). In general, louse distribution is controlled by the grooming characteristics of the host (Murray, 1961). Observations of gopher grooming indicate that lice can be nipped from the ventral body and possibly the hind quarters. The short neck and large pectoral girdle prevent more extensive preening with incisors. Preening with the hind and fore claws in the form of scratching probably does not remove many lice because of their tenacity. This action could destroy stationary nits, but gophers were observed to rub the head region with their fore claws rather than scratch with claw tips. Preening is an important control agent in the distribution of nits (Murray, 1961), but hair size and configuration (Murray, 1957b-d; Hopkins and Chamberlain, 1969) and host regional body temperatures (Murray, 1957e, 1963b, 1965) also affect their distribution. Another factor on the gopher is the structure of the hairs on which eggs are glued. Most hair follicles on the gopher contain 9-18 hairs (Morejohn and Howard, 1956), but the character of hairs per follicle differs in the various body regions. Hair follicles on the dorsal and lateral aspects of the head bear 4-6 large guard hairs and 7-12 fur hairs, whereas follicles on the dorsal body lack the stiff guard hairs. Follicular separation is nearly equal over the body, with the hairs follicles being separated by 0.25-0.33 ram. The response of female lice to a particular hair configuration for oviposition may result in the limited distribution as seen on over 80% of the gophers sampled. On juveniles, all hairs are of the fur type and thus the possible wider distribution of nits, but as the gopher loses its juvenile pelage the hair per follicle change to the fur hair-guard hair arrangement. The wider distribution of nits on gophers with large infestations (800-~) possibly indicates a spreading to less favorable sites. Molting restricts the availability of oviposition sites and may result in a loss of eggs already laid. Evidence for the latter is sparse, although Vysotoskaya (1950) considered molting a major factor for population decline. Three gophers with active molt lines in and around the ears and eyes had no unhatched nits in the area. When examined 30 days later the
Population Dynamics of the Gopher Louse 299 molt line had moved posteriorly and the empty eggs were gone. Morejohn and Howard (1956) studied the molt patterns of captive and field gophers. They found two distinct seasonal pelages, winter and summer, and a constant molt period with a short non-molt period from December to February. It is therefore possible to have a reduction in the hair follicles available twice a year. The dense replacement hairs and thickened dermis during active molting renders an area inaccessible to lice and oviposition. Lice were not observed in this thick, new hair. Using an average juvenile gopher inoculum of 86 lice and no upper restriction on population growth, the average mean density of lice on subadults can be reached in about 200 days and for adult males in 300 days. The mean louse density of adult female gophers would also be reached in 200 days, and this represents an approximation of sexual maturity (Miller, 1946). On male gophers, the louse density would reach nearly 1000 in 400 days, which is the mean longevity of male gophers (I-Ioward and Childs, 1959). The small sample of gophers with 1000 or more lice suggests that factors must be operating to maintain the density at a lower level than predicted. To examine the population growth curve, an arbitrary measure of time was necessary because of the difficulty in accurate aging of gophers. The measurement selected was the increase in surface area of the gopher (cm 2) and was obtained by combining body length and circumference of the thorax cavity, legs and tail excluded from the measurement. An accuracy check was made by measuring fresh, fiat skins. It has been shown that gophers continue to grow as adults (Miller, 1952 ; Howard and Chflds, 1959). Therefore, a clumping of older animals in a younger age group is not expected. The average densities on categoric age groups showed nearly equal numbers of lice on subadult and adult female gophers. Thus, the louse population on a female gopher should reach an asymptote at some body size or point in time. The relationship between female gopher area and lice per area was curvilinear (Fig. 6) and the curve was defined by a second degree polynomial, Y=--8.9990+O.15884X--O.OOO56X 2. On female gophers, the louse population leveled at 2.2 lice per cm ~ over 140 cm 2 of body area, or 308 lice. The mean densites of lice on male gophers differed according to age groups, and the relationships were analyzed for both linear and eurvilinear regression. The test for curvilinear regression (Snedecor, 1956) was highly significant (F--~7.1604, P>0.001), and a linear relationship was rejected. The curve (Fig. 6) was defined by a second degree polynominal, Y------5.4723-~0.09352X-- 0.0026X 2. The louse population on male gophers leveled at around 2.9 lice per cm ~ over an area of 175 cm ~, or 507 lice. The respective asymptotes are equal to the overall mean densities for adult animals.
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