Dispersal and toxicology of the insecticide fenitrothion; predicting hazards of forest spraying

Abstract

The organophosphate insecticide fenitrothion was used in attempts to control an outbreak of the spruce budworm in eastern Canada and Maine, U.S.A. since 1968. In the standard operation fenitrothion has been sprayed from aircraft in a single application at 210 g/ha or in two sequential applications at 140 g/ha separated by 4 days to a week. About 40% or 0.8 μg/cm2 reaches open ground or tree-tops. Beneath trees a further 50 to 60% appears to be removed so that medium peak concentrations in forest streams are about five ppb. Except in conifer needles where fenitrothion may be present at concentrations of 0.02 to 0.06 ppm of foliage (as sampled) a year after spraying, accumulating slowly with subsequent annual treatments, fenitrothion hydrolizes or is broken down biologically and disappears in a few days. Laboratory tests have shown that fenitrothion is highly toxic to arthropods, the median 24-hr LD50 among species of forest inset pests being 0.187 μg/cm2, and the median LC50 for aquatic insects being 0.066 ppm. Fish appear to be about 100 times less sensitive than the median aquatic insect species; laboratory data on birds and mammals are inadequate for generalizations. The severest ecological consequence of the standard spray program appears to have been the prolongation in time and extension in area of the high density spruce budworm population. Some nontarget terrestrial arthropods are killed, and since some of these are budworm predators this kill is undesirable. Sometimes small kills of aquatic insects with biomass reductions estimated at around 5% occur. Fish are apparently unaffected directly. Birds may suffer some mortality but population reduction is unusual. No deleterious effects have been reported for mammals. These effects on nontarget organisms have been termed 'acceptable' by biologists monitoring the spray program. Predicting the hazards of increased application rates is difficult because of inadequate knowledge of dispersal of fenitrothion at these higher rates and because of inadequacies in the number of species tested and in the treatment procedure of laboratory tests. Increased kills of birds and terrestrial insects, however, are to be expected at higher applications; the acute ecological ED50 for aquatic insects is estimated at a spray application rate of 450 g of fenitrothion/ha. The long-term effects of spraying on populations of all species except spruce budworm are unknown. Effects of sequential spraying in one season may be more severe than those following a single application; however, predictions of indirect and long-term effects of repeated annual spraying are, with few exceptions, impossible at the present time.