Ecological Studies of Culiseta Melanura (Diptera: Culicidae) in Relation to Eastern and Western Equine Encephalomyelitis Viruses on the Eastern Shore of Maryland

Abstract

Mosquito larvae were caught by larval traps and dipping, and adults with CDC miniature light traps, vacuum sweeping, and other means near Pocomoke City, Maryland. Culiseta melanura (Coquillett) was caught in greatest numbers, persisted through the summer, and was the primary source of eastern and western equine encephalomyelitis (EEE and WEE) virus isolations. Larvae of C. melanura were found in “typical” sites, in crypts, in water held in the saturated root mat that forms the substratum of much of the swamp, and in freshly dug holes in the root mat, even in winter. Very few were found in open pools or in areas with mucky soil near the river. Some adult C. melanura emerged into funnels placed over “typical” breeding sites, and a few from the root mat into emergence traps which enclosed neither exposed water nor crypts. The catch of C. melanura in CDC miniature light traps appeared be influenced by temperature, cloud cover, precipitation, water table, and activity of the mosquitoes. Light traps captured more females than males. Vacuum sweep samples fluctuated less, had more males than females, a larger proportion of blood-engorged and gravid females, and a higher virus isolation rate than did the light trap samples. All but 1 of the C. melanura blood meals identified (191) were avian. Both Culex salinarius Coquillett, and Aedes canadensis (Theobald) contained bovine and avian blood. Dispersal of C. melanura increased in late summer, into the vicinity of nearby farms. In 1966, 1 isolation of EEE virus and 16 of WEE virus were made, all from female C. melanura ; in 1967, there was 1 isolation of EEE virus (from Culex salinarius ) and 8 of WEE virus (all from C. melanura , 1 of which was caught in a nearby farmyard).