THE BIOLOGY OF SIMULIUM ARCTICUM MALLOCH IN ALBERTA. PART II. SEASONAL PARITY STRUCTURE AND MERMITHID PARASITISM OF POPULATIONS ATTACKING CATTLE AND FLYING OVER THE ATHABASCA RIVER

Abstract

AbstractDissections of flies caught attacking cattle and flying over the Athabasca River (14 June through 11 September 1974) revealed two large peaks of nulliparous females and two peaks of parous females. In both cases, peak numbers of nullipars preceded the peaks in pars. A small third cohort emerged in late August, and some nullipars were collected over the river on all sampling dates.Around cattle, nullipars had two prominent, but brief, peaks, followed by two subsequent prolonged periods when pars were most abundant. Parity of captured females was typical of the expected seasonal pattern of parity for an anautogenous species and corroborated our previous determination of obligate anautogeny of laboratory-reared females.Females examined for parity status were categorized as being teneral, blood-fed, gravid/ovipositing, or parasitized with a mermithid parasite (Isomermis spp.). Dissection of 2414 flies caught over the river revealed little difference in the infection rates of males and females throughout the season. Mermithids occurred primarily in nulliparous flies of the first generation and retarded the development of the reproductive organs in both sexes. Parasitism did not prevent nulliparous females from mating, nor feeding on carbohydrate or blood. A blood meal actually seemed required for maturation of the mermithid parasite. Blood-engorged females with mermithids were collected both around cattle and flying over the Athabasca River. When parasitized females with digested blood meals joined swarms of ovipositing females, mermithids were returned to the river. Nearly all flies harbored just one worm; of the 1435 parous females examined for parity, only 1 had a small mermithid. By killing flies from which they exited, the worms prevented females from taking a second blood meal.Although flies were killed by exit of the mermithid, this parasite is ineffective against the major pest species, Simulium arcticum Malloch, because it does not prevent parasitized females from seeking and taking a blood meal and thereby harassing cattle. However, in years when nearly 50% of the emerged adults are parasitized (like 1974), this mermithid may be a principal factor regulating the population density of the S. arcticum population in subsequent years.