Insects can resist parasites using the costly process of melanotic encapsulation. This form of physiological resistance has been studied under laboratory conditions, but the abiotic and biotic factors affecting resistance in natural insect populations are not well understood. Mite parasitism of damselflies was studied in a temperate damselfly population over seven seasons to determine if melanotic encapsulation of mite feeding tubes was related to degree of parasitism, host sex, host size, emergence timing, duration of the emergence period, and average daily air temperature.
Although parasite prevalence in newly emerged damselflies was > 77% each year, hosts did not resist mites in the early years of study. Resistance began the year that there was a dramatic increase in the number of mites on newly emerged damselflies. Resistance continued to be correlated with mite prevalence and intensity throughout the seven-year study. However, the percentage of hosts resisting only ranged from 0-13% among years and resistance was not sex-biased and was not correlated with host size. Resistance also was not correlated with air temperature or with timing or duration of damselfly emergence.
Resistance in host damselflies was weakly and variably expressed over the study period. Factors such as temperature, which have been identified in laboratory studies as contributing to resistance by similar hosts, can be irrelevant in natural populations. This lack of temperature effect may be due to the narrow range in temperatures observed at host emergence among years. Degree of mite parasitism predicted both the appearance and continued expression of resistance among parasitized damselflies.
Barb's Marsh is a 1-ha, isolated marsh surrounded by hay fields and mixed woods near the Queen's University Biology Station in eastern Ontario, Canada (45°37'N, 76°13'W). The mite Arrenurus pollictus is specific to Lestes disjunctus at this site. Further, L. disjunctus is not parasitized by other mite species there. Larval mites initially challenge the final aquatic larval instars of lestids (where resistance to them takes the form of grooming similar to that in a coenagrionid larvae ). They are phoretic on these hosts, but when the damselflies eclose, the mites pierce the host cuticle with their chelicerae and form a blind-ended feeding tube. Mites cannot move to other hosts once feeding begins, so enumerating them upon host emergence provides accurate data on degree of parasitism for individuals.
After a pre-reproductive period of about 12 d (unpublished data), female L. disjunctus damselflies lay multiple clutches of eggs at intervals of 1- 5 d . It is during damselfly oviposition and mate guarding that fully engorged mites drop off their hosts (leaving a scar on the damselfly which can become obscured with age). The larval mite then goes through predatory nymphal and adult stages punctuated by quiescent protonymphal and tritonymphal stages.
L. disjunctus is the most common lestid damselfly at this site, and dispersal is very low . Lestes rectangularis and L. congener are present in low numbers and are rarely parasitized by A. pollictus (unpublished data.). Female L. disjunctus oviposit endophytically; eggs overwinter and hatch in mid-May. Emergence begins in mid-June, and the flight season ends in early August. Resistance occurs within 24 h of host emergence in lestids, and dead mites are always associated with a melanised feeding tube [14, 69]. Resistance expression should not be influenced by investment in reproduction, since resistance occurs during the first 24 h to few days of the pre-reproductive period. Surveys of mature damselflies in the weeks after emergence reveal whether or not mites have engorged successfully, or if the damselfly has mounted a melanotic encapsulation response, resulting in dead mites still attached to the host .