The conditions within the soil level microclimate regulate the initiation of activity, and the ambient water stress condition regulates the termination of activity. Both these factors combine to regulate the decline in seasonal activity by limiting tick survival. Two cohorts of adults determine the distribution of activity within a season. Diapause adults, emerging from overwintering, enter seasonal activity in April. Diapause nymphs, after overwintering, feed in the spring and molt to active adults in July. The sum of seasonal activity represents the breeding potential of the population. The factors which limit seasonal activity regulate the population size by Jimiting the reproductive potential of the tick population. The variation in seasonal adult tick activity found across south-eastern Massachusetts was related to the climatological gradient over this area