Effects of a C-47 Airplane Application of DDT on Fishfood Organisms in Two Pennsylvania Watersheds

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Abstract
Investigations on the effects of DDT on stream bottom insects that serve as food for fish were made on two watersheds totaling 52,000 acres near Wilkes Barre, Pennsylvania, which were sprayed aerially with one pound of DDT per acre during May and June to control the gypsy moth. Prespray and postspray bottom samples were taken with a square foot stream sampler at 14 stations in riffle areas of streams in the treated area for comparison with samples at 3 stations in untreated streams. Over 94,000 aquatic insects were identified generically. Though the insecticide rapidly diminished the numbers and volume of bottom insects in the streams, none of the stations in the treated areas were completely depopulated and at a few stations losses were small. The Megaloptera and Odonata appeared resistant to DDT whereas the Trichoptera were affected severely and were not taken in large numbers and variety until 16 months after spraying. Insect repopulation was evident 2 months after the spraying and by autumn, except for certain caddis flies, ordinary numbers of bottom insects were present. Factors that prevent the eradication of aquatic insects following a widespread DDT spraying of forest land include a great variety and abundance of insects having diverse life histories and habits, variation in susceptibility to poisoning, ecological differences in streams, variation in tree canopy, proximity of untreated streams that furnish migrants for repopulation and influences of weather on the application and distribution of the insecticide.

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