The Ecology of Ticks Transmitting Rocky Mountain Spotted Fever in a Study Area in Virginia1

Abstract

This study was undertaken in 1963 on a 40-acre tract near Montpelier, Va., to which 20 adjacent acres (largely tall, deciduous and evergreen conifer habitats) were added for the studies in 1964 and 1965. Six tick species were found established here, namely, Dermacentor variabilis (Say), Amblyomma americanum (L.), Haemaphysalis leporispalustris (Packard), Ixodes cookei Packard, I. dentatus Marx, and I. texanus Banks. Sampling plans were designed and implemented to provide data for quantitating the population dynamics of these species. Only the first 2 were sufficiently numerous to permit a quantitative study. However, seasonal abundance and host relationships of the other 4 (which were taken only from hosts) were observed and are reported. Population indices of adult D. variabilis were estimated during the period of activity from late March to the end of August in each year. Males tended to appear earlier than females, especially in 1963, but they also declined more rapidly in that year than did females. Both sexes reached peak abundance in June, when total estimated populations of 56,000, 89,000, and 45,000 active adults were noted in 1963, 1964, and 1965, respectively. The estimates were dependent on activity, and were probably closer to the actual numbers present in the spring, when activity apparently is stronger, than in the summer, when it is diminishing. There was a significant decrease in host-seeking behavior on overcast or rainy days, in contrast to this activity on clear days. The ticks showed little tendency to migrate once they had commenced activity, and the great majority of adults were recaptured in the same localities where they were marked. Larvae were most abundant on small rodents in March of 1963, but not until mid-April in 1964 and 1965. Their numbers declined until they were almost absent in July, but moderate infestations reappeared in August. Nymphs were present in small numbers in March, and peak abundance occurred in May. No nymphs were observed after August in 1963, but a second nymphal infestation appeared during August and September 1964, and during September 1965. The bimodal larval and nymphal activity curves suggest the presence of 2 populations, with the spring larval population consisting of survivors from the previous season and with larvae which emerged from eggs in spring and summer composing the late summer population. The distribution of D. variabilis on the study area showed a positive correlation with the occurrence of vegetative types. The categories recognized, on the basis of dominance, were: evergreen; 3 types of woody, deciduous vegetation (low, medium, and tall); and a combined grasses-and-herbs type. This tick was most abundant in the low, woody-deciduous habitats. The epidemiological significance of this relationship is discussed. The active adult population of A. americanum on the study area increased rapidly during the spring, and reached estimated seasonal peaks of 12,000 in May 1963, 13,000 in June 1964, and 25,000 in June 1965. Adults were not found on drags after July in any year, but small numbers were taken on hosts as late as September. Nymphs apparently were less abundant than adults. No larvae were taken by dragging, though they were numerous on hosts during August and September. H. leporispalustris and I. dentatus were taken occasionally from cottontail rabbits and ground-feeding birds, and a larva of the latter was found on a meadow vole. Specimens of I. cookei were collected from skunks, foxes, opossums, and raccoons. I. texanus was found only on raccoons.