Oecologia (Berl.) 15, 287--304 (1974) by Springer-Verlag 1974 The Population Dynamics and Host Utilization of Geomydoecus oregonus, a Parasite of Thomomys bottae R. W. Rust Department of Entomology and Applied Ecology, University of Delaware, Newark, Delaware 19711 Received January 28, 1974 Summary. This study treats the population dynamics and host utilization of Geomydoecus oregonus Price and Emerson, a mallophagan parasite of the pocket gopher, Thomomys bottae (Eydous and Gervais). Over 135000 lice were collected from 393 gophers over a period of 20 months. Average infestation on all gophers was 357 lice. Bimonthly mean densities showed an increase in June-July of both years, and these data were statistically different from the rest. Population age structure remained relatively constant in time with 9.3% females, 7.2% males, 47.4% nymphs and 36.1% eggs. Sex-age class separation of the gophers showed juveniles of both sexes to average 86 lice; subadult males averaged 210 lice; adult males averaged 544 lice; subadult and adult females averaged 255 and 296 lice, respectively. Lice were not randomly distributed on the gopher, but were most numerous on the head and anterior dorsal body. Lice eggs were restricted to hairs around the ears and eyes of the host. Over 80% of the animals sampled had eggs restricted to that region. Embryonic development and eelosion of G. oregonus proceeded over a wide range of environmental parameters. Over 80% of the ova tested survived and hatched in conditions between 33 ~ and 37~ and 22 % and 84 % relative humidity. The greatest survival was 98% at 35~ 75% R. H. and in a 3% COs atmosphere. The generation time of G. oregonus was 40• days. Duration of embryogenesis and nymphal stadia approximated 10• days for each. Adult lice lived 30-t- days on gophers. Age frequency mortalities were calculated as 0.02 for eggs, 0.18, 0.24, and 0.06 for nymphal instars and 0.50 for adult lice. This indicates a Type II survivorship curve. There was a direct linear relationship between the number of female lice on a gopher and the number of lice eggs. The average number of eggs per female was four. Using the pivotal frequency for reproduetives, it was possible to calculate R o for the louse at 1.272. Thus, r was equal to 0.24 per generation or 0.006 per day, and the population doubles in 2.8 generations. Speculations regarding population regulation are also included. Introduction The relationshi p between an ectoparasite and its host is governed by a complex set of interacting biotic and abiotie parameters. For the ecto- 1913 Oecologia(Bet1.), Vol. 10 288 R.W. Rust parasite, the biotic parameters are in general related to or originate from the host. The degree of adaptation of the parasite to the biotic parameters corresponds to the host specificity of the parasite. The degree of host specificity also indicates the adaptations of the parasite to the abiotie parameters influencing the host. Mallophaga, biting lice, are host specific (Hopkins, 1949; Werneck, 1950), and the restriction of all life stages to the body of the host refines the specificity and adaptations of a louse to its host and host's environment (see Clay, 1949; Ash, 1960; Baum, 1968; Foster, 1969, for avian lice; Ewing, 1924; Hopkins, 1949; Waterhouse, 1953; Craufurd-Benson, 1941; Scott, 1952; Murray, 1957b-d, 1965; Hopkins and Chamberlain, 1969, for mammalian lice). For lice, the combination of interacting mortality factors and a species innate capacity for increase will determine the population level on a particular host. Evans and Smith (1952), in their classic demographic analysis of Pedi. culus humanus Linnaeus, were the first to study the reproductive poten- tials of an ectoparasite. Murray and Gordon (1969) modeled the popu- lation dynamics of Bovicola ovis (Linnaeus), a biting louse of sheep. They showed mathematically the development of the population from a low summer density to a winter high density. The genus Geomydoecus, parasites of the fossorial rodent family Geomyidae, possesses morphological adaptations not found in other trichodectid lice (Ewing, 1936). Degenerate adaptations as the absence of eyes and lack of abdominal spiracles and tergites might be a response to the altered conditions of the host's burrow environment (Kennerly, 1964; McNab, 1966; Darden, 1970), and specializations of the antennae and well-developed hair-groove could relate to life on the gopher. Re- cently, Price and Emerson (1971) studied Geomydoecus and found 45 taxa on 29 of the 36 species of gophers. Most gophers have only one louse species; exceptions were hosts with extensive geographical distributions. The relative accessibility, small size, and peculiar habitat of the host provide a system for analyzing a parasite's population dynamics. Thus, Geomydoecus oregonus Price and Emerson, a parasite of Thomomys bottae (Eydoux and Gervais), was selected for analysis and interpretation. Methods and Materials Field A minimum of 20 pocket gophers per month as live-trapped from two irrigated alfaHa fields located on the University of California, Davis Farm, and 2 consecutive months of collection data were combined for analysis. The field sizes, agricultural practices, measurement of environmental parameters, gopher collecting and labora- tory handling techniques and parasite recovery methods are treated in Rust (1973a, b). Three age classes were established for both sexes of gophers: juvenile, subadult and adult. Separation was based on reproductive condition, skull features, and body Population Dynamics of the Gopher Louse 289 measurements (Miller, 1946, 1962; Hansen, 1960; Thaeler, 1968). Juveniles had the following sutures unfused: parietal-squamosal, maxilla-alisphenoid, and alisphenoid- squamosal; the dorsal surface of the maxilla (in the zygomatic arch) and palatine showed distinct porosity. Juveniles had gray pelage and were still in association with their mother, as determined from trapping. Subadults had partial fusion of the cranial sutures, reduced porosity and brownish pelage. Subadult females had a closed pubic symphysis and limited follicular development; subadult male testes were only 3-6 mm long. Adults had skull sutures and nonporous bones. Adult females had the pubic symphysis open, placental scars, embryos, or were lactating. Adult males had testes from 7-15 mm long. Body length and maximum skull length and width showed overlap due to a continuous breeding population (Miller, 1946) but were also used in the separation. Time for the age categories would be approximately one and one-half to two months for juveniles, 2-5 months for subadults and the mean length of life for adult gophers has been estimated by Howard and Childs (1959) as 7.6 months for males and 12.3 months for females. Laboratory Eelosion and first instar survival in vitro were examined under various environ- mental conditions. 8 different temperatures (27-41~ were maintained in Percival environmental chambers. The chambers were set for total darkness with the photophase only when eggs or nymphs were checked. Eight different relative humidities (10-96%)were maintained in liter desiccator jars containing various saturated salt solutions (Winston and Bates, 1960). Eclosion was tested in normal air and a 3% COz atmosphere. The high CO 2 atmosphere was created by removing 3 % of the gas volume from the jars and replacing it with CO 2. The CO 2 concentration was measured with a Unieo Gas Analyzer. Four replicates of 25 eggs each were tested at 64 different temperature-relative humidity combinations. Translucent eggs were plucked individually from gophers for the temperature-relative humidity tests. Eclosion and nymphal survival in 3% CO 2 atmosphere were recorded at 33 ~ and 35~ both at 62%, 75% and 84% R. It. Nymphal survival was also checked in normal CO 2 atmosphere. The duration of embryonic development was obtained by allowing female lice to oviposit in small petri dishes. This gave eggs of a known age (-4-12 h), and these eggs were placed in separate containers in various temperature-relative humidity conditions. Louse survival on the host was checked in a normal CO 2 atmosphere at various humidities. Humidities tested were 35 % • 5, 50 % =J:5, 75 % • 5 and 100 % -- 2 and were maintained in environmental chambers at 21~1771. Louse demography was studied (in rive) in an artificial burrow system simulat- ing natural conditions (Darden, 1970). Four artificial burrows were housed in adjoining chambers in a soil box (60.9 X 91.4 X 121.9 cm). Burrows were constructed of 7 • 7 mm hardware cloth welded together. A system consisted of an above ground holding cage (14.2 • 20.3 X 30.4 ore), an initial diagonal tunnel (30 cm), a horizontal tunnel (24 cm) and a nest chamber (19x24 era). The tunnels were 7.5 cm in dia- meter. Before an animal was placed in a burrow system, carrot pieces and fresh alfalfa were placed in the next chamber. The system was packed with soil and buried in the soil box, with 25 to 30 em of soil over the nest chamber. The surface was moistened with approximately 2 1 of water and allowed to stand for 24 h before a gopher was introduced. Additional fresh alfalfa was provided daily. Two Plexiglas tubes (6 mm O. D.) were attached to the roof of the nest chamber and horizontal tunnel. Both gas samples and temperatures were measured via the tubes. The entire system was housed in a greenhouse with a regime of controlled temperature and 290 R.W. Rus~ natural light. Every seventh day of an experiment an additional 21 of water were added to the surface. Two different treatments were designed for gophers placed in the artificial system. Newly eclosed first instar nymphs were introduced on a clean gopher, and the animal was released in a system for a set period--6, 10, 15, 20, 25, and 30 days.