DIFFERENTIAL PARASITISM OF PEROMYSCUS MANICULATUS AND PEROMYSCUS TRUEI BY CUTEREBRA LARVAE

— During a 21-month field study of sympatric Peromyscus maniciilatus (Deer Mouse) and Peromyscus truei (pinyon mouse), 12/152 (7.9 percent) P. maniculatus and 1/45 (2.2 percent) P. truei were parasitized by Cuterebra larvae. Both parasitized P. maniculatus and P. truei and nonparasitized P. maniculatus selected different habitats in the community. Differential parasitism of P. maniculatus and P. truei was possibly caused by host specificity of Cuterebra, whereas differential parasitism of P. maniculatus may have been due to habitat selection. Cuterebrid flies are widely distributed in North and South America where their larvae are found embedded in the subcutaneous tissues of many rodents (chipmunks, deer mice, wood rats, etc.), rabbits, and sometimes cats, dogs, deer, cattle, humans, and other mammals (Sillman and Smith 1959, Catts 1965). They have also been found in birds (Artmann 1975). Frequently, certain species of Peromyscus (deer mice) harbor Cuterebra (botfly) larvae while other species of Peromyscus apparently do not. Why one species is parasitized, but a second closely related species is not parasatized, is not fully understood. During a 21-month field study of sympatric Peromyscus maniculatus (deer mouse) and Peromyscus truei (pinyon mouse), differential parasitism of the two species by Cuterebra larvae was found. Whitaker (1968) suggested that differential parasitism of Peromyscus may be caused by a species not being in the right place under the proper conditions to be parasitized, or Cuterebra will not parasitize certain species. Previous field investigators have found a correlation between the habitat and parasitism of a species, and laboratory experiments have shown a strong degree of host specificity by Cuterebra. I suggest that these two factors caused certain P. maniculatus, but not P. truei, to be parasitized. Materials and Methods Sympatric populations of P. maniculatus and P. truei were monitored from April 1975 through December 1976 by live trapping in a permanent 1.4 hectare study plot at an elevation of 2025 meters. The study plot was located in the Geiger Grade portion of the Virginia Range, which is situated about 18 km SE of Reno, and 10 km NW of Virginia City, Nevada. The Geiger Grade area consists of a pinyon-juniper woodland dominated by single-leaf pinyon {Pinus monophylla) and Utah juniper {Juniperus osteosperma). Between the trees and in the larger open areas a total of 15-20 species of shrubs are found. The most common of these include low sagebrush (Artemisia arbuscula), snakeweed (Gutierrezia sarothrae). Mormon tea (Ephedra viridis), bitterbrush (Purshia tridentata), and big sagebrush (Artemisia tridentata). Numerous boulder piles and talus rock slides, in which P. truei are abundant, occur throughout the area (for a complete description of the area see Llewellyn 1977). From 25 April 1975 to 21 October 1976, the study plot was sampled for three successive nights on a biweekly basis, using 140 Sherman live traps situated at 10 m intervals. Four additional nights of trapping were completed during a rainy period in August 1975; only one three-night sequence was completed in May 1976; and the last sequence in October 1976 consisted of four trap periods. In November and December of 1976, a five-night sequence was completed during the middle of each month. During the 21-month period, there was a 'Department of Biology, University of Nevada, Reno, Nevada 89557.