The larval instars and population dynamics of five species of black flies (Diptera: Simuliidae) and their responses to selected environmental factors

Abstract

The larval instars of 4 spp. of black flies were determined. Prosimulium fuscum Syme and Davies and P. mixtum Syme and Davies had 7 instars; but, they passed through only 6 instars when development occurred at warmer temperatures. Simulium vittatum Zetterstedt and Stegopterna mutata (Malloch) had 7 and 6 larval instars, respectively. The instars of Cnephia dacotensis (Dyar and Shannon) could not be positively segregated. Stream temperature was the most important physical factor regulating larval black fly population dynamics, determining hatching time and developmental rates. P. mixtum fuscum larvae required 240 D.degree. C (day-degrees) above 0.degree. C for maturation, while S. mutata and C. dacotensis needed 250-275 and 475 D.degree. C, respectively. Stream discharge influenced the annual dynamics of simuliid populations, as spring floods produced 2nd cohorts of some univoltine species which normally have 1 cohort of overwintering larvae per generation. Changes in discharge affected estimates of larval densities by influencing the number of larvae colonizing artificial substrates.