TRANSMISSION PATTERN AND INTRASPECIFIC COMPETITION AS DETERMINANTS OF POPULATION-STRUCTURE IN PINWORMS (OXYURIDA, NEMATODA)

The transmission pattern of Zonothrix columbianus (Nematoda: Oxyurida) in its host Tropisternus columbianus (Coleoptera: Hydrophilidae), an aquatic beetle, was studied to determine whether parasites were dispersed with their hosts and to examine the possible role of intraspecific competition in limiting population size. Beetles were sampled at regular intervals from fall 1986 through fall 1989 and examined for worms. Worms, absent in larval stages of the host, were uncommon in newly metamorphosed beetles and therefore probably do not infect adult stages until after they have dispersed; worms are not dispersed with the host. Prevalence reached its lowest points in spring and fall when newly metamorphosed beetles were most common, but it was near 100% for most of the year. Worms were uniformly distributed in the host population. Many hosts had exactly 1 male and 1 female worm; the high prevalence suggests that this infrahost population results from interference competition between males on the one hand and females on the other. Only 3 of 285 beetles contained more than 1 male. Females shared the host with members of the same sex more commonly than males, but females from hosts harboring more than 1 female had significantly fewer eggs than lone females in hosts. Numbers of adult stages of beetles were estimated during spring, summer, and fall of 1989 and were lower in early spring and late fall. Because worms do not disperse with hosts, the panmictic unit could be estimated from the number of infected beetles; this probably was about 50 individuals during the winter bottleneck.