Sylvatic Plague Studies in Southeast New Mexico. II. Relationships of the Desert Cottontail and its fleas

The Desert Cottontail ( Sylvilagus audubonii ) was trapped over a 5-year period during a study of flea-host relationships ncar Roswell in Southeast New Mexico. Fleas collected from these rabbits were predominantly Euhoplopsyllus glacialis affinis . This flea was collected in highest numbers in May and June in all years except 1, when severe drought conditions prevailed during the winter and spring months. This annual peak in flea numbers coincided with the main breeding period of the rabbit, but no hormonal link between the breeding cycles of these 2 species was indicated; detailed data from 2 years indicated no disproportionate increase in flea numbers on female rabbits during the rabbit breeding period, and gravid female fleas were collected year-round from both male and female rabbits. In a 2-year study conducted by other workers near Santa Fe in northcentral New Mexico, peak numbers of E. g. affinis were collected from S. audubonii in July and August. Climatic conditions at Santa Fe (elevation 1980 m) are similar to those at Roswell (elevation 1200 m), except that average temperatures are lower at Santa Fe. Average temperature conditions that occur in the May–June period at Roswell occur in the July–August period at Santa Fe. The ambient temperature conditions in each locality, which directly influence temperature conditions in the rabbit burrows, apparently regulate the time of year when flea population expansion occurs. In addition, there was a significant correlaton between total rainfall during the May–June period and the flea index for that period each year at Roswell. Thus, burrow humidity conditions, resulting from the interaction of ambient conditions and soil moisture, are apparently important in determining the extent of flea population growth each year.