FIELD-EVALUATION OF THE PHOTOTOXIN, ALPHA-TERTHIENYL, FOR REDUCING LARVAL POPULATIONS OF BLACK FLIES (DIPTERA, SIMULIIDAE) AND ITS IMPACT ON DRIFT OF AQUATIC INVERTEBRATES

The efficacy of alpha-terthienyl (alpha-T) for reducing larval populations of black flies was determined in two streams in southeastern Ontario. By 24 h post-treatment, larval black fly population reductions greater than 90% resulted from exposure to calculated dosages of 0.04 and 0.10 mg.L-1 alpha-T emulsifiable concentrate over distances of 1.0 and 1.5 km, respectively. Significant population reductions were not observed 2.3 km downstream from injection of the 0.10 mg.L-1 dosage, probably because a marshy area upstream from this sampling site slowed the transport and enhanced photodegradation of alpha-T. Alpha-terthienyl initiated catastrophic drift of benthic invertebrates that was not selective for any functional feeding group. Significant increases in post-treatment drift, relative to pre-treatment densities, were observed downstream from treatment for eight of 10 taxa studied and included filter-feeders, grazers, and predators. Upstream from treatment, significant increases between sampling days were observed for only two taxa. The catastrophic and nonselective impact of alpha-T on invertebrate drift preclude its usefulness as an alternative to black fly larvicides used currently, although specialized applications for alpha-T may exist in integrated black fly control programs.