2006 SOUTHEASTERN NATURALIST 5(1):157–164 Cuterebra fontinella Parasitism on Peromyscus leucopus and Ochrotomys nuttalli CHAD A. JENNISON1, LUIS R. RODAS1, AND GARY W. BARRETT1,* Abstract - Peromyscus leucopus (white-footed mice) and Ochrotomys nuttalli (golden mice) were live-trapped in eight experimental plots of lowland and upland deciduous forest during 2001 and 2004. An outbreak of Cuterebra fontinella (botfly) parasitism occurred on both species of small mammals during the 2001 and 2004 trapping seasons, with peaks in mid-July each year. A second peak of parasitism was observed in late October 2004, which differed greatly from 2001 where only one peak occurred. We suggest that a greater three-dimensional home-range size and pattern of behavioral activity exhibited by P. leucopus led to a greater incidence of parasitism (41.7%) compared to the more arboreal O. nuttalli (6.3%–12.5%). The second outbreak of parasitism appeared to have been the result of a late-summer deluge of tropical weather caused by an exceptionally active hurricane season affecting the southeastern United States. Introduction Three species of Cuterebra are known to infect Peromyscus leucopus Rafinesque (white-footed mice) and Ochrotomys nuttalli Harlan (golden mice), namely C. fontinella Clark, C. angustifrons Dalmat, and C. grisea Coquillett (Catts 1982). Our study focused on C. fontinella. Botfly (Cuterebridae) parasitism of small mammals has been studied since the 1940s (Dalmat 1943, Hunter and Webster 1973, Miller and Getz 1969, Timm and Lee 1981, Wecker 1962, Wolf and Batzli 2001, Xia and Millar 1990). Many questions remain unanswered, however, such as how small mammal movement behavior influences rates of parasitism, how rates of parasitism are influenced by weather conditions, and how patterns of parasitism compare across temporal and spatial scales. Although many studies have investigated the effects of Cuterebra on P. leucopus (Barko 2003, Dunaway et al. 1967, Miller and Getz 1969, Munger and Karasov 1991, Timm and Cook 1979, Wecker 1962), few of these studies were conducted in the southeastern United States, and no study was related to the effect of tropical weather on rates of parasitism in this area. Even fewer studies have investigated botfly parasitism on Ochrotomys nuttalli (but see Clark and Darden 2002, Dunaway et al. 1967). Both P. leucopus and O. nuttalli were parasitized by C. fontinella during our investigation. Dunaway et al. (1967) is the only study that compared rates of parasitism between these two small mammal species. They found that 24.7% of P. leucopus were parasitized during a seven-year study, while < 1.5% O. nuttalli individuals were parasitized. 1Institute of Ecology, University of Georgia, Athens, GA 30602-2202. *Corresponding author - gbarrett@uga.edu. 158 Southeastern Naturalist Vol. 5, No. 1 Our study addressed the need to quantify and compare rates of parasitism between P. leucopus and O. nuttalli, to investigate how aspects of small mammal behavior might influence rates of parasitism, to document the effects of weather conditions related to hurricanes on frequency of parasitism, and to expand knowledge about botfly parasitism in the southeastern United States. Study Area The HorseShoe Bend (HSB) Ecological Research Site located in Clarke County near Athens, GA (33°57'N, 83°23'W) served as the site of this study. HSB is located in a 14.2-hectare riverine peninsula formed by a meander of the North Oconee River. Upland and bottomland deciduous forests characterize this peninsula. Though the bottomland is prone to flooding, recent droughts in the southeastern United States prevented flooding during the 2001 and 2004 periods of study. Both habitats are dominated by Smilax spp. (greenbriar), Lonicera mackii (Rupr.) Herder (honeysuckle), Quercus nigra L. (water oak), and Ligustrum sinense Lour. (Chinese privet). Q. alba L. (white oak) and Fagus grandifolia Ehrh. (American beech) are also abundant in the upland, whereas Betula nigra L. (river birch) and Liriodendron tulipifera L. (tulip poplar) are common in the bottomland (Klee et al. 2004). Materials and Methods Census procedures Four 0.21-ha experimental grids were established in each of the bottomland and upland habitats. Each grid consisted of 12 trapping stations, with live traps located at approximately 10-m (± 2 m) intervals along two parallel transects. Each station consisted of two Sherman live traps (7.6 x 7.6 x 25.4 cm; H.B. Sherman Traps, Inc. Tallahassee, FL): one trap situated on a platform 1.5 m high on the trunk of a tree and the second trap located on the ground within 2 m of the base of the same tree. Live trapping was conducted twice weekly from 29 March to 16 November 2001, and 17 March to 3 November 2004. Traps were baited with black oil sunflower seed, set before dark, and checked the following morning. Date and location of captured P. leucopus and O. nuttalli were recorded, and the mice were marked with ear tags for identification, sexed, weighed to the nearest g, examined for reproductive condition (open or closed vaginal orifice, abdominal or scrotal testes, pregnant, and/or lactating), and examined for general health (particularly the presence of botflies). Captured animals were released at the site of capture immediately following examination. All animals were handled in accordance with the guidelines provided by the American Society of Mammalogists (ASM Animal Care and Use Committee 1998). Although other species of small mammals were captured, we restricted our analysis to P. leucopus and O. nuttalli. 2006 C.A. Jennison, L.R. Rodas, and G.W. Barrett 159 Mean population densities of P. leucopus and O. nuttalli, as well as frequency of botfly parasitism per grid, were estimated by minimumnumber- known-alive methods (MNKA; Krebs 1996). The frequency of parasitism for each small mammal species was calculated weekly, including the percentage of each species parasitized. Weather data were provided by a weather station at The Horticulture Research Farm, University of Georgia, Watkinsville, GA, located approximately 11.7 kilometers (7.0 miles) from HSB. Statistical methods A repeated measures ANOVA (SAS, alpha = 0.05) was used to test for differences between proportion of P. leucopus and O. nuttalli parasitized within the two individual years (Zar 1996). Additionally, we conducted a t-test to determine whether observed differences within each species between the two years were significant. Results Population densities of P. leucopus and O. nuttalli were based on 13,056 trap-nights in 2001 and 6432 trap-nights in 2004. In 2001, the maximum mean density per grid for P. leucopus was 23.1 individuals (22–28 April); the maximum mean density for O. nuttalli was 12.3 individuals per grid (20–26 May; Fig. 1A). Each population steadily declined to 1.5 (11–17 November) and 0.3 (21 October–6 November) mean individuals per grid, respectively. Similar trends were observed during 2004 when P. leucopus and O. nuttalli mean population densities per grid peaked at 34.3 (28 March– 3 April) and 6.1 (14–20 March), respectively. Each population declined to 1.9 (31 October–6 November) and 0.3 (31 October–6 November) individuals per grid, respectively. Botfly parasitism of P. leucopus in 2001 first occurred during 3–9 June, and the last occurrence was observed during the week of 12–18 August (Fig. 1B). The highest proportion of parasitism occurred during 15–21 July (37.1%). Parasitism of O. nuttalli began 17–23 June and lasted until 12–18 August, when the highest proportion of parasitism occurred (23.3%). In 2004, however, the first parasitism of P. leucopus occurred 6–12 June; the proportion increased to a maximum of 20.7% (27 June–3 July) and declined to zero during 22–28 August. Unexpectedly, parasitism was again observed beginning 5–11 September. This late-season parasitism peaked at 41.7% during 17–23 October and lasted through the end of the trapping period (Fig. 1B). Parasitism of O. nuttalli during 2004 was first observed during 13–19 June, reached a peak of 3.6% (18–24 July), and declined to zero during 15–21 August. Proportions of late-season parasitism of O. nuttalli were 6.3% during 25 August–11 September, 6.3% during 3–9 October, and 12.5% during 17–30 October. Peromyscus leucopus were parasitized more often than O. nuttalli during 2001 (F = 0.24, df = 33, p < 0.01). In 2004, P. leucopus were also parasitized 160 Southeastern Naturalist Vol. 5, No. 1 2006 C.A. Jennison, L.R. Rodas, and G.W. Barrett 161 more frequently than O. nuttalli (F = 3.59, df = 33, p < 0.01). Additionally, P. leucopus were parasitized more frequently in 2004 than 2001 (p < 0.05). Conversely, O. nuttalli did not show a significant difference in proportion parasitized between the two years (p = 0.77). These late-season pulses of parasitism appear to be associated with the increased minimum temperature (Fig. 1C) and increased weekly rainfall (Fig. 1D) observed during 2004 compared to 2001. For example, in 2004, the southeastern United States was impacted by numerous tropical storms and hurricanes. These storms brought warm, humid air from tropical ocean waters, resulting in increased precipitation and higher air temperature. Average weekly rainfall during 9 August to 6 November 2004 was 3.8 cm, with minimum temperatures well above the minimum temperature of 15 °C needed for botfly egg survival (Catts 1982) until 19 September 2004. The average weekly rainfall for the same period during 2001 was 0.6 cm, and the average minimum was about 4 °C cooler than during 2004. Discussion The mean population density of P. leucopus was significantly larger than O. nuttalli at our experimental research site. P. leucopus was also more frequently parasitized than O. nuttalli. Because O. nuttalli has a smaller, more arboreal home-range compared to P. leucopus (Goodpaster and Hoffmeister 1954, Lackey et al. 1985, Pruett et al. 2002) and because golden mice prefer dense canopies containing ample climbing structures (Christopher and Barrett 2006, Morzillo et al. 2003), we attributed differences in rates of parasitism to patterns of movement and use of habitat space. Bioenergetics studies also confirmed that P. leucopus exhibits higher metabolism rates, greater foraging behavior, and greater utilization of the three-dimensional habitat than O. nuttalli (Christopher and Barrett, in press; Knuth and Barrett 1984; O’Malley et al. 2003). Botfly eggs are especially abundant at entrances of small mammal burrows (Catts 1982, Dalmat 1943, Timm and Cook 1979). When a mouse passes by an egg, its body heat triggers the larva to hatch, attach to the fur, and travel into the body via a mucosal orifice (Catts 1982). Because parasitism is the result of passive encounters with botfly eggs, the greater activity of P. leucopus helps to explain higher rates of parasitism. Catts (1982) suggests that aberrant infections occur in non-target species by virtue of sharing habitat (egg-infested territory) with a common host, which explains why both species were parasitized. Supporting our results are the findings of Dunaway et al. (1967) in which only three infections out of more than 200 captures of O. nuttalli (< 1.5%) were noted, whereas 24.7% of P. leucopus were parasitized. Figure 1 (opposite page). Mean weekly population density per grid (0.21-ha) of Peromyscus leucopus and Ochrotomys nuttalli during 2001 and 2004 (A), comparison of proportion parasitized by Cuterebra fontinella between species (B), minimum weekly temperature (°C) between years (C), and weekly rainfall (cm) during periods of study (D). 162 Southeastern Naturalist Vol. 5, No. 1 Numerous studies have reported infections of P. leucopus in October (Dunaway et al. 1967, Hensley 1976, Hirth 1959, Layne 1958, Timm and Cook 1979, Wecker 1962), but our study represents the first record of high rates and late-seasonal parasitism of O. nuttalli. In an effort to explain this occurrence and the differences we found between 2001 and 2004, we investigated differences in weather—a series of variables that influences the life history of C. fontinella and that was significantly different between years. For example, adults will not fly, and therefore not mate or lay eggs, below temperatures of 20 °C (Hunter and Webster 1973). In addition, egg development is slowed by low temperatures (less than 15 ºC) and reduced humidity (Catts 1982). The active hurricane season of 2004 extended warmer, wetter weather into October and November and appears to have extended the viability and hatching of botfly eggs later into the year. This combination of abundant rainfall and warmer temperature allowed botfly activity that could not have occurred during October 2001. Also, the tropical weather likely created a greater number of eggs because the warmer conditions would have extended adult activity as well. Most studies investigating rates of botfly parasitism have occurred in the northern ranges of P. leucopus (Barko 2003, Dalmat 1943, Hensley 1976 , Miller and Getz 1969, Timm and Cook 1979, Wecker 1962, Wolf and Batzli 2001, Xia and Millar 1990). Our study addresses rates of botfly parasitism and their temporal variation as found in the southern range. The botfly season is generally restricted to summer months, when temperatures and humidity are high. In the northern range, this season generally falls from July to October. The southern range exhibits a season that is one to three months longer than the botfly season in the north (Dunaway et al. 1967). Our investigation corroborates the range and temporal variation findings of other studies conducted in the southeast. To date, most studies investigating botfly infestation have focused on the genus Peromyscus, with only limited data related to small mammals of similar body mass and natural history. We contribute information about the parasitism of an infrequently studied species, O. nuttalli, of similar body mass and natural history as P. leucopus (Christopher and Barrett, in press). Jaffe et al. (2005) provide an excellent long-term (20-year) overview of botfly parasitism on Peromyscus maniculatus Wagner, P. leucopus, and Tamias striatus L. We also stress the need for long-term observations on a diversity of small mammal species, ecosystem types, and spatial scales. Such observations are necessary for better understanding the impact of botfly parasitism on small mammal population dynamics. Acknowledgments Special thanks are extended to T. Barrett, J. Chastant, A. Mahoney, M. O’Malley, C. Payton, A. Pruitt, and M. Shuman for field assistance during 2001, and to M. Beres, A. Howington, T. Luhring, K. Meeks, A. Peachy, C. Schmidt, and S. Shivers for field assistance during 2004. We also thank D. Hall and M. Atkinson 2006 C.A. Jennison, L.R. Rodas, and G.W. Barrett 163 of the Statistical Consulting Center, Department of Statistics, University of Georgia for their assistance with statistical analysis. We especially thank C. Christopher for providing unpublished data from 2001. Editorial advice from the Guest Editor and two anonymous reviewers was invaluable and deeply appreciated. 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