7. When pond areas are used as cattle pasture 

 between seasons the land should be so prepared that 

 good and proper pasture irrigation practices can be 

 used. 



Ponds created or influenced by faulty engine eriny 

 practices. — Roadside ditches are found in many places 

 where highways and railroad beds interrupt normal 

 drainage and impound water because culverts are 

 improperly placed. Borrow pits and quarries almost 

 always hold water and provide breeding places for 

 mosquitoes and other aquatic insects. Such conditions 

 can be corrected only at considerable expense. 



Swimming pools. — Outdoor swimming pools are 

 becoming a feature of suburban life. Most of them are 

 easy to clean and are free from insect pests. How- 

 ever, biting insects such as backswimmers (Notonecta) 

 and "toe-biters" (Belostomatidae) sometimes fly in 

 and annoy swimmers, usually in the heat of the summer 

 when large flights of insects are attracted to nearby 

 electric lights. The most practical method of control 

 is to drain the pool and destroy the insects when they 

 are concentrated at the deepest point. The mosquitoes, 

 Culex pipiens and quinquef asciatus, occasionally breed 

 -in swimming pools. They may be controlled with 

 emulsifiable pyrethrum or kerosene sprays. 



Irrigation 



In California most of the precipitation occurs in the 

 north and only during the winter months. Therefore 

 'elaborate irrigation systems have been developed for 

 transporting water to the arid south, and the under- 

 ground water supply is tapped by thousands of irriga- 

 tion pumps. According to Henderson (1951) California 

 'leads the nation in area under irrigation with 6 million 

 "acres. With the huge volume and extensive surface 

 'jof water it is not surprising that insects intrude into 

 the picture at several points. Caddisworms, for exam- 

 ple, have been reported obstructing water in irrigation 

 'tunnels in southern California (Simmons, Barnes, 

 'Fisher, and Kaloostian, 1942) and many irrigation 

 'ditches and canals are inhabited by Simuliid larvae 

 and other stream insects. 



In irrigated fields the insect fauna is determined 

 "largely by the nature of the crop and by the water 

 .schedule. Any irrigated crop can produce mosquitoes 

 but in general, row crop irrigation is less troublesome 

 than sheet irrigation. Rice, for example, is constantly 

 flooded over wide areas and presents ideal conditions 

 for the development of aquatic insects. In 1954 more 

 than 453,000 acres of rice were harvested in Cali- 

 'fornia, and most of this area was under water from 

 April or May into September. Leaf- and stem-boring 

 aquatic flies (Cricotopus, Hydrellia) are pests in the 

 iSacramento Valley, and a host of pond-dwelling insects 

 commonly invade the fields. Hydrellia griseola var. 

 scapularis Loew, in particular, built up to epidemic 

 proportions in 1953, causing an estimated loss of 

 10 to 20 per cent of the crop (Lange, Ingebretsen, 

 and Davis, 1953). The rice leaf miner, as it is called, 

 belongs to the family Ephydridae. The eggs are laid 

 |on leaves lying prone in the water and hatch in about 



33 

 Usinger: Introduction 



four days. The larvae feed in the leaves. Control was 

 achieved by Lowering the water to a depth of about 

 two inches and spraying with dioldrin or heptachlor. 

 After forty-eight hours the water level wn- raised and 

 the checks wore blockod off so that no water was 

 spilled from the fields for two weeks. 



Mosquitoes have long been a problem in rice field-. 

 Culex tarsalis is the most important of theso. Ano- 

 pheles freeborni is the malaria vector that caused the 

 epidemics of former years (Herms, 1949) and was 

 responsible for the recent outbreak initialed by a 

 malaria carrier just returned from Korea (Brunetti, 

 Fritz and Hollister, 1954; Fontaine, Gray and Aarons, 

 1954). A. freeborni is mainly a breeder in rice fields 

 and outlying areas and can be reduced in numbers by 

 good irrigation practices. Aedes dorsalis breeds in 

 the rice fields early in the season in response to 

 initial flooding (Portman and Williams, 1952). The 

 eggs of this mosquito overwinter in the soil and may 

 remain viable for several years. Soon after flooding, 

 larvae appear in great numbers and produce myriads 

 of adults unless controlled by application of insecti- 

 cides such as wettable DDT powder mixed with the 

 seed rice at the rate of one and one-half to two pounds 

 of 50 per cent powder to the acre. 



Other aquatic pests in the rice fields (Portman and 

 Williams, 1952) include fairy shrimps, Apus spp., which 

 undergo a life history somewhat like Aedes dorsalis 

 and chew off the tender leaves and dislodge the soil 

 around the roots of the seedlings, and giant scavenger 

 beetles, Hydrous triangularis (Say), the larvae of 

 which dig in the bottom mud and uproot entire plants. 



Alfalfa fields and irrigated pastures provide exten- 

 sive aquatic habitats, with more than a million acres 

 of each in California. 



Because of the increased availability of water in 

 the past two decades, the practice of irrigating pasture 

 lands has spread over much of the Central Valley. 

 Coincident with this, Aedes nigromaculis and Culex 

 tarsalis have extended their ranges and the former 

 has now become the number one pest mosquito over 

 a wide area. Culex tarsalis is a known vector of 

 encephalitis. In irrigated pastures a succession of 

 generations is produced through the season, the 

 larvae breeding in shallow water choked with grass 

 and weeds at low points in the fields. 



Another method of using irrigation water that is 

 quite common in California is the spreading of water 

 for percolation purposes. In such cases individual 

 pumps are used to replenish the water supply. The 

 use of percolation beds creates mosquito problems. 

 Also, of course, mosquitoes breed in irrigation struc- 

 tures of all kinds including canals, ditches, stand- 

 pipes, and the like. 



From the above discussion it is obvious that mos- 

 quito breeding is intimately related to irrigation. The 

 solution to the problem should therefore be sought 

 in better irrigation practices. Henderson (1951) refers 

 to the problem as "conservation irrigation" or the 

 use of "irrigated soils and irrigation water in a way 

 that will insure high production without the waste of 

 either water or soil . . . Generally, the direct economic 

 benefits of conservation irrigation materially exceed 



