Mortality, recovery, and survival of larval Brachycentrus numerosus (Trichoptera) after exposure to the insecticide fenitrothion

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Abstract
The 72 and 96 h LC50 of the organophosphate insecticide fenitrothion to caddis larvae (B. numerosus Say) at 5.degree. C were 20 and 11 ppm, respectively. Within 30 days of 24 h exposures to 0.076 and 0.130 ppm fenitrothion, 66 and 100% of the larvae, respectively, were dead. Recovery of survivors from exposure to 0.076 ppm appeared complete within 19 days. The 24 h threshold concentration for case-leaving and mortality lay between 0.055-0.076 ppm. Case-leaving response increased with increasing concentrations, but the percentage leaving their cases varied in different experiments at similar concentrations. All larvae leaving their cases were incapacitated and died without signs of recovery. On the basis of the 72 and 96 h LC50 B. numerosus was insensitive to fenitrothion compared with other species, but on the basis of 30 day tests, its margin of safety during current forest spraying with fenitrothion to control forest pests appeared low.

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