EFFECT OF CURRENT VELOCITY ON INGESTION RATES OF BLACK FLY LARVAE

In situ feeding measurements were made to quantify the effect of curre nt velocity on ingestion rates of larvae of the major pest species of black flies in southern Quebec (Simulium venustum/verecundum, S. tuber osum, and Prosimulium mixtum/fuscum) and to compare these results with those obtained in seminatural conditions (gutter system). Ingestion r ates of all three species increased with current velocity by a factor of 2.5 (P. mixtum/fuscum) to 10 (S. tuberosum) between 25 and 100 cm.s (-1). Measured field rates were significantly correlated with those pr edicted for similar larval sizes, temperatures, and seston concentrati ons in gutters, although field rates were only 56-87% of those in gutt ers once corrected for differences in current velocity. These results suggest that the preference of these species for current velocities of about 100 cm.s(-1) reflect their ability to feed faster at these velo cities. The effectiveness of larvicides can therefore be strongly affe cted by current velocities, and determinations of effective doses from gutter experiments may slightly underestimate the required quantity o f larvicide.