Large-scale insecticide treatments in the field are directed against mosquitoes. Many of the ecological situations frequented by mosquito larvae and pupae fortunately are not important as fish habitat. Mosquitoes generally utilize quiet, shallow waters, often of a temporary nature and not accessible to fish. In such places, the danger to fish is least when well-controlled methods of toxicant application are used. However, airplane spraying against mosquito larvae may be hazardous to fish in waters adjacent to the area being treated. In the San Joaquin Valley of California in 1950 toxaphene in kerosene solution, with an emulsifier, was deposited by airplane at rates up to 1 pound per acre of water, which was much in excess of the usual 0.2-0.3 pound per acre normally used. Biologists of the California Division of Fish and Game found numbers of black bass, sunfish, and catfish dead as a result of the insecticide treatment. Such instances are rare, however, and it is gratifying that the modern mosquito-abatement supervisor is aware of the ecological situation as he uses insecticides in the field.

Control of forest insects often entails the wide dispersal of potent toxicants over forest areas through which trout streams flow. In heavily forested places under airplane treatment, it becomes impractical to treat the woody areas without covering the streams. The problem is one of choosing the ideal insecticide in such strength that the forest insect pests will be destroyed, while harm to fish and fish food is kept at a minimum.

The control of black fly larvae and pupae in running streams has been attempted by chemical methods. The hazard then to other aquatic organisms is very obvious. Streams inhabited by black flies are almost always swiftly flowing and usually contain populations of trout or other prized fish, such as salmon, smallmouthed bass, or shad. Also in need of protection are the various minnows and other forage fish and the bottom-insect fauna so important as fish food. When chemicals are poured directly into the stream for action against the black flies, the problem again is principally one of choosing the most suitable insecticide--the one that will yield effective control while inflicting a minimum of damage on useful aquatic organisms.

Federal and State agencies have experimented with the new toxicants considered to be the most promising for field campaigns. Insecticides have been tested against many kinds of fishes and their foods, and toxicity thresholds have been established for various aquatic environments in the laboratory and in the field. Different formulations have been tested and different methods of insecticide dispersal have been tried, so that field biologists are now armed with facts that will guide them in the safe use of insecticides outdoors. Cooperative projects, in which entomologists and fishery biologists have worked together in the field on these problems, have been successful.

The new chemicals about which we know most in relation to their fish-killing properties are DDT, benzene hexachloride, chlordane, as well as toxaphene, TDE, methoxychlor, parathion, and tetraethyl pyrophosphate. Various formulations have been tested. The most popular mixtures have been xylene-triton emulsion, kerosene solution, fuel-oil solution, and acetone solution.

The choice of a field insecticide entails several considerations cost, ease of handling, safety to humans, effectiveness in destroying the pest, and safety to wildlife. On those bases, DDT seems to be one of our safest all-around insecticides. TDE has shown promise against certain insect pests and is only mildly toxic to fish. Chlordane, methoxychlor, and benzene hexachloride have been used experimentally against some kinds of fish and might be all right to use under carefully controlled conditions. Toxaphene is very toxic to fish, and should be used only with extreme caution. The type of formulation can often affect the toxicity of these chlorinated hydrocarbons. In most instances xylene-triton emulsions of these toxicants are most toxic to fish, oil solutions are intermediate, and acetone solutions exhibit the least potency.

Fish are not alone among the cold-blooded vertebrates in their vulnerability to insecticides. Wartime tests by E. S. Herald in Florida showed that DDT-kerosene solutions, as sprayed routinely from aircraft, killed green water snakes, banded water snakes, coachwhips, black snakes, pine snakes, and king snakes, and one of the four cottonmouth moccasins that were exposed. 'Typical DDT tremors were observed in frogs and toads, although they appeared to be moderately resistant to its action.

As we accumulate new facts concerning the tolerances of fish and other cold-blooded vertebrates to the newer insecticides, we shall arrive at a better understanding of the possibilities for safe field treatments. The complex aquatic environment binds all of its members so closely that careful management becomes strictly necessary if the desirable components are to be perpetuated.

In the give-and-take struggle in which all animals are engaged, it is obvious that many kinds of animals are decidedly dependent upon the six-legged arthropods; certainly the cold-blooded vertebrates would have a most difficult time in discovering a suitable substitute for the insect link in the food chains leading to their subsistence.

OLIVER B. COPE is chief of the Central Valley Fishery Investigations, Fish and Wildlife Service, Department of the Interior. He is a native of San Francisco and received a doctorate in biology at Stanford University. He has worked with the California and United States Departments of Agriculture, did mosquito abatement for the Navy in the southwest Pacific during the Second World War, and is now occupied with salmon and trout research in the Western States.