Infestation intensity and prevalence of an ectosymbiotic midge under variable environmental and host conditions

Host-symbiont theory suggests that the abundance of an obligate symbiont will correlate positively with the density and size of its host. We examined these expectations using an obligate chironomid ectosymbiont - host fishfly system in 13 streams of southern Maine over 2 years (1996-1998). In addition, we investigated field conditions and macroinvertebrate-community composition, to search for correlates with symbiont prevalence and mean infestation intensity. Prevalence and infestation intensity were high in both years of the study, but neither infestation intensity nor prevalence was correlated with host population density across sites. Within sites, prevalence increased significantly when host density declined, contrary to our expectation. Symbiont-infestation intensity was not related to host size. No symbiont-population attributes correlated with physicochemical conditions in either sample year but prevalence was positively correlated with infestation intensity. Ectosymbiotic midges exhibited a significant host and attachment-site bias. Collectively, these data suggest that this symbiont-host interaction is obligate and stable, but the dynamics in our streams do not support the general expectation of increasing symbiont-population size with either increasing host-population or body size. Intraspecific interactions between symbionts are thought to have a stronger influence on population dynamics than do host populations or abiotic conditions for this ectosymbiotic midge.