Parasite-mediated selection in host populations is thought to vary in magnitude temporally. We monitored variation in life history traits that are known or suspected to influence fitness in a population of damselflies parasitized by larval water mites. Mite prevalence and intensity varied considerably over 5 y and was often higher in females. Prevalence and intensity were highest in the years when the damselfly emergence periods were early and of short duration, which also corresponded to damselflies emerging at larger sizes. Mites appeared to exert negative effects on apparent survival in some years only, and only for females, suggesting that parasite-mediated selection on damselflies is variable and dependent on other factors such as emergence times, weather, and sex and body size of hosts.
Study species and study site
Lestes disjunctus is the predominant Lestes spp. in an isolated marsh located near the Queen’s University Biology Station in Ontario, Canada (45° 37' n, 76° 13' w). Emigration and immigration of L. disjunctus from Barb’s Marsh is low (Anholt, 1997). Lestes disjunctus overwinter as eggs (Sawchyn & Gillott, 1974) and have a relatively late emergence period for this region, beginning in mid-June. After several weeks, mating begins (ending in mid August; Anholt, 1997). Males are not territorial. Arrenurus pollictus is the only mite that parasitizes L. disjunctus at this site (and it rarely parasitizes other hosts at this site). Mites are unable to move to another host once feeding begins, soon after host eclosion. Engorged mites drop off their hosts when the damselflies are close to water during breeding, usually during the first breeding attempt.
We monitored damselfly emergence from the marsh using emergence traps and used hand nets to sample the mature adult population. Mite prevalence and intensity (sensu Bush et al., 1997) were assessed for damselflies caught in these samples. To estimate numbers of emerging L. disjunctus and measure parasite prevalence and intensity at emergence, 10 emergence traps were placed over vegetation at the margins of the marsh. Traps had wooden frames with 1-m2 base and 0.25-m2 top, with a height of 1.25 m. Traps were provided with wood frames and windowscreen meshing (1 mm2). The mesh and emergent vegetation provided substrate for emerging damselflies to crawl out of the water and eclose.