TRAPPING BLACK FLIES (DIPTERA-SIMULIIDAE) IN NORTHERN CALIFORNIA .1. SPECIES COMPOSITION AND SEASONAL ABUNDANCE ON HORSES, HOST MODELS, ANDIN INSECT FLIGHT TRAPS

Catches of black flies (Diptera: Simuliidae) on CO2-baited three-dimen sional horse/cow models with adhesive-treated ears and bodies, and in CO2-baited insect flight traps, accurately reflected the species compo sition and relative seasonal densities of four of five species caught attacking free-ranging horses in northern California. On host models, the specific ear-landing site behavior of Simulium argus Williston, me mbers of the Simulium vittatum Zetterstedt complex, and Simulium virga tum Coquillett complex was the same as that observed on pastured horse s. However, although both Simulium griseum Coquillett and Simulium tri vittatum Malloch were caught feeding almost exclusively on the undersi des of horses, both were caught in large numbers on the ears of animal head models, even when a head model was accompanied by a body model. The large numbers of S. argus collected on ears of CO2-baited horse he ad models and CO2-baited insect flight traps, and the small numbers co ncurrently collected from the ears of horses, suggest that horses are not a preferred host, and that they may even be repellent for this spe cies. During concurrent periods of study the observed landing rates of S. argus/S. vittatum on horses' ears exceeded catch rates on the adhe sive-treated ears of horse head models by 2X to 5X. This discrepancy w as related to the ear flicking behavior of horses, which often resulte d in multiple landings by dislodged flies. On models, all species loca ted the ears whether CO2 was released from the front of the head or be low the body. The large peaks in numbers of several black fly species attacking horses and caught by models and traps during September, Octo ber, and November revealed a previously unrecognized problem in northe rn California.