Adult emergence of Simulium verecundum S. and J. and S. decorum Walk. was reduced 58–94% by exposure of late instar larvae to 0.1 ppm Altosid or Altosid SR-10 for 1 h. Pupae were not affected by the presence of 0.1 ppm Altosid SR-10 in the rearing water. Adult emergence of S. verecundum was reduced 95% when mixed larval instars were reared in the presence of 0.001 ppm Dimilin. Last instar larvae died as a result of abnormal metamorphosis. Black fly mortality was 86–100% with 1-h exposures to 0 1–0.2 ppm Dimilin, and 100% with a 15-min exposure to 1 ppm at 22 ± 2 °C, lower temperatures giving similar results.

Release of Altosid over a 48-h period from formulated blocks gave a concentration of 0.002–0.006 ppm in a small stream. Emergence of adults was completely suppressed for 23 days 18 m downstream and 17 days 43 m downstream. Applications of 1 ppm Dimilin for 15 min to two streams resulted in the gradual elimination of black fly larvae from artificial substrates, a high reduction in adult emergence, and the formation of abnormal larvae following the tests. The maximum concentration of Dimilin detected in the water was 0.31 ppm during application; however, submerged vegetation residues as high as 18.9 ppm were found 1 h following the second application, decreasing to less than 0.05 ppm 14 days later.