30 



Usinger: Introduction 



As regards invertebrates, Tarzwell (1947) reported 

 that DDT is less toxic applied as a dust than in oil. 

 He found that treatment at the rate of 1 to 2 pounds 

 per acre in fuel oil killed Hemiptera, Coleoptera, 

 Odonata, Ephemeroptera, and Chironomids. At 0.025 

 pound per acre in fuel oil Dytiscids, Gyrinids, Hydro- 

 philids, and Corixids were killed. Seasonal effects 

 after periodic treatment were: an increase in the 

 number of Oligochaetes, nematodes and copepods; 

 a decrease in the numbers of Chironomids, Hemiptera, 

 Coleoptera, and Ephemeroptera. Insects as a group 

 decreased, with the greatest effect of the treatment 

 on the Chironomids. 



Repopulation after treatment with DDT was studied 

 by Hoffmann, Townes, Sailer, and Swift (1946). As 

 might be expected, the insect fauna of ponds, which 

 is characterized by short life histories, came back to 

 normal within a few weeks. Streams, on the other 

 hand, required a year or more. 



Gnat control. — Gnat control is a peripheral activity 

 of mosquito abatement districts in a few places. 

 Chironomid gnats are the principal pests in Lake 

 Elsinore (southern California) (Miller, 1951) and in 

 Klamath Lake and nearby waters along the Oregon- 

 California border. By far the worst pest, however, is 

 the "Clear Lake gnat," Chaoborus astictopus D. and 

 S. In 1940 it was estimated that the total seasonal 

 emergence in the upper part of Clear Lake (44 sq. mi.) 

 was 712 billion gnats or 356 tons (Lindquist and 

 Deonier, 1942), and the "phantom larvae" were esti- 

 mated on the basis of adequate samples to number 

 800 billion. One light trap captured 88% pounds of 

 gnats in two hours. Eggs at a density of 10 million 

 per square foot occurred near shore in drifts 20 feet 

 wide and several miles in length. Fork-tail catfish, 

 square-tail catfish, and split-tail were important 

 feeders on all stages of the gnat; one 9-inch fish was 

 found with more than 1,000 larvae in its stomach. 



In former years control seemed to be impossible 

 but the advent of chlorinated hydrocarbon larvicides 

 opened up new possibilities. Experiments had shown 

 that DDD would kill larvae at a dilution of one part to 

 75 million parts of water whereas fish and other 

 aquatic life in the lake were not killed unless the 

 concentration was increased to 1 in 45 million. With 

 this margin of safety, and after a preliminary trial 

 in nearby Blue Lakes, Lake County organized a 

 mosquito abatement district and, with state and federal 

 help, treated the entire 41,600 acres of lake surface 

 on September 15 and 16, 1949 (Lindquist, Roth, and 

 Walker, 1950). The lake is eutrophic and relatively 

 shallow (27 to 50 feet) without a thermocline so the 

 wettable insecticide was thoroughly mixed by the 

 wind. Control was complete and no gnats were found 

 in the lake for several years, though they gradually 

 increased until 1954 when, on September 25 and 26, 

 a second treatment was carried out. It was a remark- 

 able fact that the removal of so much fish food had 

 no apparent effect on the over-all economy of the 

 lake, probably because Chaoborus larvae are carniv- 

 orous and are an intermediate link in the food chain 

 and hence were bypassed. 



LEGEND 



6 C«p*t 

 3 Copplc. 7 Floating M«l 



3 L.ol, Eract t F looting L.ol 



4 FI.kuow. 9 Sobmt.fl.il 



5 Nob.d Erect 10 Pl.o.tott 



Intro, fig. 43. Classification of plant types along the shore 

 line of a reservoir in relation to water level management (Hess 

 and Hall, 1945). 



Man-Made Impoundments 



Literally hundreds of dams have been built or are 

 planned for California. The resulting impoundments 

 vary in size from small stock ponds to local reservoirs 

 and enormous multipurpose lakes. They do not differ 

 fundamentally from naturally dammed lakes and ponds, 

 but economic considerations are more likely to arise 

 because of the effects on erosion, mosquito breeding, 

 fish production, and recreation. Therefore, limnological 

 studies have now become an important part of reser- 

 voir planning. 



Reservoirs. — Some important considerations in a 

 preimpoundage entomological survey {Malaria Control 

 on Impounded Water, 1947) are: 



1. Location in relation to known pest or disease- 

 bearing insects is a critical factor. For example, a 

 mosquito survey should be made to determine the 

 species present in the locality and their relative 

 abundance, the present and potential breeding areas, 

 and the flight range of potential pests. Periodic 

 density observations should be made throughout the 

 season and during a period of several seasons in 

 order to provide a basis for comparing mosquito pro- 

 duction before and after impoundage. 



2. Soil and vegetation have a significant bearing 

 on aquatic life. Therefore a reconnaissance survey 

 should include: a study of the timber in the basin 



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MEDIUM 

 Relative Inletiection Values' 



Intro, fig. 44. Anophe/es quadrimaculatus production potentials 

 of plant types (Hess and Hall, 1945). 



