Parasites of Larval Black Flies (Diptera: Simuliidae) and Environmental Factors Associated with Their Distributions

Abstract

A total of 26,673 midto late-instar larval black flies were collected from 115 stream sites in South Carolina. Host larvae with patent, externally visible infections of nematodes, fungus, or microsporidia were identified to species either morphologically or cytologically. Six parasite taxa were identified: nematodes of the family Mermithidae; the fungus Coelomycidium simulii; and the microsporidia Janacekia debaisieuxi, Polydispyrenia simulii, Amblyospora bracteatalvarians, and Caudospora simulii. Of 43 species of black flies, 25 harbored at least one of these parasite taxa. Records from museum specimens increased the number of infected host species in South Carolina to 34 species. Prevalence of patent infections for each parasite taxon was low, ranging from 0.1 to 6.5% of all larvae. The distribution of nematodes among stream sites was nonrandom and correlated with season, host abundance, and stream-site conditions. The association of nematode distributions with stream parameters might reflect environmental influences on the nematodes during their free-living period and on host susceptibility to infection. Additional keywords: Mermithidae, Coelomycidium simulii, microsporidia Larval black flies (Diptera: Simuliidae) are a major component of the macroinvertebrate fauna in streams and rivers (Adler & McCreadie 1997). They anchor themselves to a silken pad spun on solid substrates (e.g., rocks, trailing vegetation) and filter food from the water column or graze it from the substrate. Larval and pupal development is temperature dependent and requires from one week to half a year (Crosskey 1990). After emergence from the pupa, females of most species mate, acquire sugar for energy and a vertebrate bloodmeal for egg maturation, and find a suitable stream for oviposition. Larvae are hosts of a variety of parasites, especially mermithid nematodes (Mermithidae), microsporidian protozoans (Microspora), and the chytrid fungus Coelomycidium simulii DEBAISIEUX. Preparasitic mermithid nematodes crawl about on the stream substrate and use a protrusible stylet to penetrate the host body (Molloy 1981). Infected host larvae are recognized by the presence of one or more coiled worms visible through the abdominal cuticle. Mermithids either exit and kill the host while it is still in the larval stage or pass into the adult stage, exiting shortly thereafter (Crosskey 1990). Postparasitic worms molt to adults, mate, and deposit eggs in the streambed (Poinar 1981). a Author for correspondence. E-mail: jmccrd@clemson.edu Microsporidian infections cause pathological changes, primarily to the host fat body (Weiser & Undeen 1981). Patent infections are recognized by the presence of large, irregular cysts that distend the host abdomen. Life cycles of microsporidia that attack black flies are not well understood, but larval hosts become infected both by transovarial transmission and by ingesting the free-living spores (Crosskey 1990). Spores might require some form of environmental conditioning before they become infective, and alternate hosts might be required to complete the life cycle (Jamnback 1973; Lacey & Undeen 1987). Larvae with patent infections of the fungus C. simulii are packed with minute, spherical thalli throughout the body cavity. Thalli give rise to spores that are released into the water column after the death of the host (Tarrant 1984). An intermediate host might be required to complete the life cycle (Lacey & Undeen 1987). In the present study, we survey selected parasites of larval black flies in South Carolina. Given that nematodes have free-living stages, we ask whether these parasites occupy a restricted range of stream conditions within the broader range of conditions under which their hosts are found. In other words, can occurrence (presence/absence) of nematodes in a particular host species, at a particular location, be predicted This content downloaded from 157.55.39.84 on Wed, 15 Jun 2016 06:30:20 UTC All use subject to http://about.jstor.org/terms Parasites of larval black flies by local stream conditions? Our objective is to examine factors that can explain parasite distributions, with the tenet that patterns of distribution can provide insight into the basic ecology of these organisms. Other parasites (microsporidia, C. simulii) were collected in numbers too low to address these questions.