The effects of the organophosphate insecticide fenitrothion in streams in New Brunswick from operational spraying against spruce budworm larvae, Choristoneura fumiferana (Clem.), have been investigated for four years. A number of streams have been studied and several agencies have documented insecticide concentrations in streams, effects of the insecticide on benthos drift and mortality, changes in benthos standing crop, and changes in fish populations, biomass, and growth.

The concentration of fenitrothion found in streams immediately following forest spraying by aircraft varies greatly and is influenced by spray regime, weather conditions, forest cover, and water depth. At application rates of 210 g/ha or two applications of 140 g/ha, fenitrothion concentrations in streams with one exception were less than 15 ppb. Concentrations in streams usually peaked within the first hour following spray application and diminished to less than 1.0 ppb within 24 to 48 hrs. Insecticide applications at 210 g/ha have resulted in measurable concentrations of fenitrothion in streams as far as 4.0 km from the area of application.

Using nets, sharply increased numbers of drifting Ephemeroptera and Plecoptera were measured in several streams that had peak fenitrothion concentrations up to 7 .6 ppb, following operational sprays over all or part of the drainage basins.

Benthic sampling in one stream sprayed with fenitrothion in 1971, 1972, and 1973, but not in 1974, indicated a substantial decline in benthos between 1971 and 1972, and that recovery had not occurred by 1974. Kill of arthropods in another stream, containing a peak fenitrothion concentration of 6.38 ppb, was measured using drift nets. The stoneflies Leuctra spp. , Amphinemoura spp., and the mayflies, Baetis spp. were most strongly affected, although benthos sampling did not indicate a decline in numbers.

Field studies in three sprayed streams suggested that early summer increases in fish biomass may be slowed by the spray program. In field and laboratory experiments, extreme doses were necessary to affect fish behaviour and survival. Evidence indicates that at worst the forest spray program in New Brunswick may sometimes reduce normal summer increases in fish biomass through a reduction in fish-food organisms.

It is concluded that in streams the consequences of spruce budworm spraying with fenitrothion by the methods and rates studied in New Brunswick are environmentally tolerable.

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