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OSQUITO PREVENTION

ON IRRIGATED FARMS

1'^I Agriculture Handbook No. Z\9)ll

é^Agricultural Research Service

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Subcommittee on Vector Control mes h o

Inter-Agency Committee on Water Resources a

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ACKNOWLEDGMENTS

This bulletin was prepared by members of a special task force of

the Subcommittee on Vector Control, Inter-Agency Committee on Water

Resources. The following individuals were assigned to the bulletin task

force: ^

U.S. Department of Agriculture:

Agricultural Research Service:

Soil and Water Conservation Research Division—Howard Haise

Entomology Research Division—W. C. McDuffie

Soil Conservation Service, Information Division—D. Harj>er Simms

Forest Service, Division of Watershed Management—Byron Beattie

Federal Extension Service, Division of Agricultural Science, Technology and Management—^L. C. Gibbs

U.S. Department of the Interior:

Bureau of Reclamation, Division of Irrigation and Land Use—

Stanton J. Ware

Fish and Wildlife Service, Bureau of Sport Fisheries and Wildlife—

Lee Yeager and Paul Springer

Bureau of Indian Affairs, Branch of Land Operations—Gerald

Keesee

U.S. Department of Health, Education, and Welfare:

U.S. Public Health Service, Bureau of State Services, Communicable

Disease Center—A. D. Hess and M. B. Rainey (chairman)

Several members of the task force made major contributions to the

bulletin. The introduction was prepared by M. B. Rainey and A. D. Hess.

Mr. Rainey also prepared the section entitled "Nature and Extent of Irrigation-Mosquito Problems," except the part dealing with the effects

of mosquitoes on farmworkers and livestock, which was written by

W. C. McDuiïie. Howard Haise prepared the section covering relationships of mosquito prevention to irrigation agriculture, except the part

on drainage, which was prepared by Tyler Quackenbush, Soil Conservation Service. The section on relationships of mosquito prevention to

aquatic wildlife conservation was developed by A. D. Hess and Lee

Yeager. The basic principles and practices for mosquito prevention

outlined in the bulletin were adapted by Mr. Rainey from a recommendation of the American Society of Agricultural Engineers entitled

"Principles and Practices for Prevention and Elimination of Mosquito

Sources Associated with Irrigation," which was prepared in 1958 by the

Committee on Irrigation System Design for Mosquito Control, Irrigation

Group, Soil and Water Division.

Other members of the Subcommittee on Vector Control contributed

their knowledge and experience to the preparation of the bulletin. Since

their number precludes separate mention, the members of the task

force express appreciation for the cooperation and assistance of numerous individuals, who provided constructive criticism and suggestions.

No claim is made for originality in much of the information presented

in the bulletin. It has been developed from the research and experience

of many individuals and agencies. The sources are far too numerous

to list individually, and this means is taken to thank all those who have

contributed directly or indirectly to the bulletin.
CONTENTS

Page

Nature and extent of irrigation-mosquito problems 2

Public-health and socioeconomic importance 2

Biology of irrigation mosquitoes 4

Mosquito sources on irrigated farms -- 6

On-field sources 6

Oif-field sources 9

Interrelationships of mosquito prevention and other interests 15

Relationships to irrigation agriculture 15

Water losses during storage, conveyance, and application.— 15

Soil and water management 17

Relationships to aquatic wildlife conservation 23

On-field relationships 23

Off-field relationships 23

Basic principles and practices for mosquito prevention 28

Impoundments 29

Irrigation-conveyance and distribution systems 29

Irrigated lands 30

Drainage systems 30

Selected references 31

BN-26977

Illustration on cover shows Boise, Idaho, irrigation project. Aerial view looking northwest toward Nampa-Meridian, irrigation district. In foreground is New York Canal

running toward Lake Lowell.

t? WashingtoniÍD.C. Issued Februaryr 1967:J^V

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402 - Price 15 cents
MOSQUIT(|fREVENTIOr^pN IRRIGATED FARMS

prepared by BULLETIN TASK JORCE^ Subcommittee on Vector Control, LQ A

^0^ Inter-Agency Committee on Heater Uesources^

In many areas, particularly in the West, the expansion in irrigation

agriculture has been accompanied by an increase in the production of

mosquitoes. These bloodsucking insects have a serious impact on the

health, comfort, and economic welfare of people. They also hinder

agricultural, industrial, and recreational activities and greatly reduce

the overall benefits of irrigation developments. The irrigation-mosquito

problem is being intensified by the continuing expansion of irrigation

agriculture, the increase in human population, the acceleration of

urbanization and industrialization, the development of insecticide resistance in mosquitoes, the growing concern over insecticide residues

in food and water, the rapid expansion in outdoor recreational activities, and the public's increasing demands for a more comfortable and

healthful environment.

Natural mosquito-producing habitats occur in most irrigated areas,

but experience has shown that manmade rather than natural habitats

are usually responsible for excessive mosquito populations. These

manmade mosquito sources are most often caused by faulty irrigation,

drainage, and management practices.

Mosquito sources are often associated with both the engineering and

agricultural phases of irrigation, including storage reservoirs, conveyance systems, drainage systems, and farm irrigation. The ultimate

solution to mosquito problems associated with irrigation must be based

primarily on source-reduction measures aimed at eliminating or modifying aquatic habitats. In recent years it has become increasingly evident that this requires close and continuous coordination between those

interested in mosquito control and those concerned with improving irrigation agriculture and related interests, such as fish and wildlife

conservation and outdoor recreation. Although conflicts sometimes develop between mosquito control and water users, there are often ways

to mutually accomplish the major objectives of all groups concerned.

Public interest demands that mosquito control be coordinated with

irrigation water uses to the end that maximum benefits may be provided for the greatest number of people. Although solutions may not be

readily apparent for certain types of problems, they can be solved

more rapidly through a coordinated attack based on a thorough knowledge of mutual interests.

Mosquito prevention on irrigated farms is only one part of the overall

irrigation-mosquito problem. This bulletin provides information on

(1) the overall importance of irrigation-mosquito problems, (2) manmade mosquito sources on irrigated farms, (3) relationships of mosquito prevention to good irrigation agriculture and aquatic wildlife
2 AGRICULTURE HANDBOOK 319, U.S. DEPT. OF AGRICULTURE

conservation, and (4) basic principles and practices for mosquito prevention on irrigated farms. It is intended as a basic reference and guide

for agricultural extension agents, managers of mosquito abatement

districts, soil conservation technicians, and other technical and semitechnical personnel in public health, agriculture, fish and wildlife, and

related fields concerned with the development and utilization of irrigation water on the farm.

NATURE AND EXTENT OF IRRIGATION-MOSQUITO PROBLEMS

Public-Health and Socioeconomic Importance

About a dozen species of mosquitoes associated with irrigation in the

United States are of public-health importance because of their ability to

transmit disease or because of their pestiferous biting habits. Encephalitis, commonly known as sleeping sickness or brain fever, is now the

most important mosquitoborne disease in the United States. Mosquitoes obtain the encejJialitis viruses from birds and other wild vertebrates and then transmit them to horses and humans. No effective

chemotherapeutic measures for preventing or treating human cases

are known; and some individuals, particularly children, who recover

from the disease suffer permanent mental disability.

Three principal types of mosquitoborne encephalitis occur in the United

States. Eastern encephalitis (EE) occurs principally in the Atlantic

and Gulf Coast States from New Hampshire to Texas, but sometimes extends as far inland as Wisconsin. St. Louis encephalitis (SLE) occurs

chiefly west of the Mississippi River and in several of the Central States

and Florida. The third type, western encephalitis (WE), is confined

primarily to the States west of the Mississippi River.

Mosquitoes such as Culiseta melanura and several sj>ecies of Aedes

may be involved in the transmission of EE. The principal vector of both

SLE and WE in the Far West is Culex tarscdis, which is abundant in

many western irrigated areas. In the Central States, Culex pipiens

and Culex quinquefasciatus are important in the transmission of SLE

in urban areas. Both of these species are produced in irrigated areas,

particularly where stagnant and foul water, such as sewage effluent,

is used for irrigation. Aedes and other irrigation mosquitoes may also

be secondary vectors of encephalitis. WE and SLE are endemic in

many western irrigated areas, and outbreaks of WE among horses and

of both WE and SLE among humans have been rather widespread. In

recent years encephalitis outbreaks have occurred in irrigated areas in

the Texas High Plains (1956 and 1963), in the Intermountain States

(1957), in the Lower Rio Grande Valley (1957), in Utah and New Mexico (1958), and in Wyoming (1960).

In past years serious outbreaks of malaria in the Southeastern States

were associated with improperly prepared reservoir basins. Malaria

has also been related to irrigation in several States, including California, New Mexico, and Texas, and in the rice-growing areas of the

Mississippi Delta. This disease has been almost eradicated from the

United States, and at present malaria transmission is not important

in irrigated or nonirrigated areas.
MOSQUITO PREVENTION ON IRRIGATED FARMS 3

The malaria vectors Anopheles quadrimaculatus in the East and

Anopheles freeborni in the West are still prevalent in some areas

where favorable habitats are present. These mosquitoes constitute a

potential hazard for establishing new foci of malaria transmission,

particularly where the disease may be reintroduced from foreign countries. This was well illustrated at Lake Vera in California during the

summer of 1952, when 35 cases of malaria occurred among Camp

tire Girls. The source of their infections was traced to a soldier who

had recently returned from foreign duty and had an attack of malaria

while camping near the lake.

Several vicious biting mosquitoes, including Aedes vexans, Aedes

dorsalis, and Aedes nigromaculis, occur in large numbers in many

irrigated areas. These mosquitoes often create public-health problems

m addition to transmitting specific diseases. For example, a study

made by the U.S. Public Health Service in irrigated areas in northern

Montana revealed that in three-fourths of the families surveyed, mosquitoes severely annoyed both adults and children and interfered with

their normal outdoor activities during the summer. Mosquito bites

caused some injurious reaction in 8 out of 10 people interviewed. In

one section, 40 percent of the individuals examined by a physician had

secojidary infection from mosquito bites. Some individuals, particularly

children, required medical attention and sometimes even hospitalization for treatment of secondary infections and occasional allergic reactions to mosquito bites.

In addition to their public-health importance, mosquitoes reduce the

production efficiency of irrigated farms. Their biting limits the efficiency of farm laborers and other outdoor workers and may depress

land values. Crops sometimes cannot be harvested properly or at the

optimum time because of mosquito annoyance. Dense populations of

blood-suckmg mosquitoes interfere with feeding activity and drain the

vitality of farm animals and poultry, resulting in reduced meat, milk,

and egg production.

Even greater losses may result from the diseases mosquitoes transmit to hvestock. Mosquitoes are vectors of encephalitis, anaplasmosis,

fowl pox, and several other serious diseases that kill large numbers of

farm animals and poultry every year. Thus, mosquito control is of

interest to the farmer from the standpoint of both his health and his

economic welfare.

Irrigation mosquitoes also cause economic losses in urban areas by reducing the efficiency of industrial workers, lessening the value of real

estate, reducing attendance at outdoor business establishments, such

as drive-m theaters and eating places, and restricting outdoor recreational activities.

The economic importance of irrigation mosquitoes is further illustrated by expenditures for their abatement. For example, in California

over $6 million is spent annually to control mosquitoes. Most of this

control activity is carried out in irrigated areas. It has been estimated

that at least twice the amount of present expenditures would be required

lorvf^^^^^-^ adequate control throughout the State. Approximately

$200,000 is spent each year for mosquito abatement in a few irrigated

areas in Utah. Many urban communities in other western irrigated
4 AGRICULTURE HANDBOOK 319, U.S. DEPT. OF AGRICULTURE

areas spend large sums of money each season for chemical control

measures to provide partial protection from mosquitoes. Individual

families also spend sizable amounts of money each summer for household sprays, mosquito repellents, livestock sprays, and medicine for

treating mosquito bites.

Biology of Irrigation Mosquitoes

Irrigation mosquitoes represent four important genera or groups:

Anopheles, Culex, Aedes, and Psorophora, AH mosquito species have

four distinct stages in their life cycle: Egg, larva ("wiggler"), pupa

("tumbler"), and adult (fig. 1). A characteristic common to all mosquitoes is that they live in water continuously from the time the eggs

hatch until the adults emerge. They are generally found in shallow

water with abundant vegetation and flotage and where they are protected from wave action. They do not occur in the deep open waters

of lakes, ponds, or streams.

On the basis of egg-laying habits, mosquitoes may be divided into

temporary-water and permanent-water species. The Aedes and Psorophora are temporary-water breeders, which deposit their eggs on the

saturated soil of areas from which surface water has receded. In

contrast, the Anopheles and Culex usually lay their eggs on the surface

of permanent and semipermanent bodies of water. Hatching of the

Aedes and Psorophora eggs is stimulated by subsequent floodings. The

eggs of these mosquitoes may remain dormant for long periods, sometimes for several years if conditions are unfavorable for hatching. Normally the eggs hatch more or less simultaneously when they are

flooded, and for some species a new brood may be produced by each

flooding during the summer. Anopheles and Culex eggs usually hatch

within a few days after oviposition. The time between hatching of

eggs and emergence of adult mosquitoes varies with species and environmental conditions, especially water temperature. Development

of the aquatic stages may be completed in 4 or 5 days in hot weather,

whereas several weeks may be required in cool weather. Aedes

and Psorophora mosquitoes generally develop more rapidly than

Anopheles and Culex,

Adult mosquitoes mate soon after emergence. The females begin

seeking blood meals, which most species require before laying eggs.

The biting habits of adult mosquitoes vary with the species. Anopheles

and Culex feed mainly at night, whereas Aedes and Psorophora feed

both at night and in the daytime. Most species exhibit a peak of biting

activity during a 1- or 2-hour period immediately after sundown. Aedes

and Psorophora are aggressive and vicious biters of both man and

livestock. Culex tarsalis, the common encephalitis mosquito, feeds

readily on a wide range of hosts, including man, wild and domestic

birds, and livestock.

The flight range of mosquitoes varies widely with the species and

environmental conditions. The direction and distance of travel are

greatly affected by availability of food and shelter and by wind conditions. Availability of food is probably the most important factor

affecting the movement of mosquitoes on irrigated farms. If an adequate food supply is close to the production sites, adult mosquitoes
MOSQUITO PREVENTION ON IRRIGATED FARMS

ANOPHELES CULEX

FIGURE 1.—Characteristics of three common mosquitoes.

BN-26995

probably will not travel far from their sources. On the other hand, mosquitoes may travel several miles when adequate food supplies are not

available near their larval habitats. The flight range of the Anopheles

and some Culex mosquitoes is usually considered to be about 1 mile,

although some species frequently travel several miles; for example

the encephalitis mosquito Culex tarsalis has flown as far as 10 miles.

Most Aedes and Psorophora species are strong fliers and are known to

range several miles from their larval habitats.
AGRICULTURE HANDBOOK 319, U.S. DEPT. OF AGRICULTURE

Mosquito Sources on Irrigated Farms

Studies in irrigated areas have shown that mosquitoes are often produced in both "on-field" and "off-field" aquatic habitats on irrigated

farms. Natural mosquito-producing habitats occur in most irrigated

areas, but manmade sources are usually far more important.

On-Field Sources

Prolific mosquito production often occurs in low areas on fields used

for pastures, hay meadows, and other close-growing forage crops when

irrigation water remains ponded long enough for the larvae to mature

(fig. 2). Surface water must be present for at least 4 or 5 days for the

aquatic stages to mature and produce adult mosquitoes. Rice is the

only important field crop in America that thrives when flooded this

long. Mosquitoes are sometimes produced on irrigated fields planted

to row crops, such as cotton, but usually ponded water is not present

on these fields long enough for the aquatic stages to complete their

development.

Observations in several areas have shown that when irrigation water

is ponded on pastures and hay meadows long enough to produce

mosquitoes, the desirable forage grasses and legumes are frequently

killed and replaced by wetland plants, which are less desirable from an

agricultural standpoint. Thus, the extended flooding that is favorable

for mosquito production is unfavorable for growth of common field

crops.

BN-26978

FIGURE 2.—Ponded areas at lower end of irrigated fields are important sources of

mosquitoes. In addition, such ponding often damages crops and interferes with

cultivation and harvesting.
MOSQUITO PREVENTION ON IRRIGATED FARMS 7

In California, tremendous numbers of Aedes nigromaculis are produced in ponded areas on irrigated pastures. Some of these pastures

are irrigated 10 to 15 times from April through October, and a brood

of mosquitoes may be produced in the ponded areas that remain after

each irrigation. Investigations in the Milk River Valley of northern

Montana showed that over two-thirds of all mosquito production (mostly

Aedes dorsalis and A. vexans) in a 5,000-acre study occurred in ponded

areas on irrigated fields, pastures, and western whe^tgrass meadows.

In this valley the irrigation water frequently remains ponded on the

hay meadows long enough to produce Culex tarsalis, the encephalitis

mosquito. Studies in western Nebraska showed that large numbers of

both Culex tarsalis and Aedes were produced in ponded areas on irrigated pastures and hay meadows along the North Platte River. The

use of irrigated mountain meadows for producing hay and grazing of livestock is widespread in the Rocky Mountain States. In many valleys the

bottom-land meadows are flooded almost continuously throughout the

spring and early summer. Ponding also occurs in low areas after irrigation later in the season. Such extensive aquatic habitats are very

favorable for producing mosquitoes in irrigated areas.

In addition to depressions on irrigated fields, water sometimes remains ponded in field laterals and drains long enough to produce the

encephalitis and other irrigation mosquitoes (fig. 3). This condition

occurs most often in areas having fine-textured soils and poor underdrainage. Field laterals and drains that contain excessive amounts O'f

vegetation are more favorable for mosquito production.

Mosquitoes may also be produced in the residual water in various

control structures, such as drops, particularly when they contain tum-

••^ ., " Í1V -. »* .

BN-26979

FIGURE 3.—Water ponded in field laterals may produce mosquitoes.
8 AGRICULTURE HANDBOOK 319, U.S. DEPT. OE AGRICULTURE

BN-26980

FIGURE 4.—Residual water in various control structures, such as drops, may produce

mosquitoes.

FIGURE 5.—Flooding ricefields often results in prolific mosquito production.
MOSQUITO PREVENTION ON IRRIGATED FARMS 9

bleweeds and other debris (fig. 4). Mosquito production in such

structures is usually of minor importance in comparison with that in

other types of on-field aquatic habitats.

Ricefields constitute a special type of on-field mosquito source, since

the type of flooding used for growing rice is favorable for mosquito production (fig. 5). In ricefields of the Mississippi Delta and gulf coast

areas, large numbers of Psorophora confinnis and Psorophora discolor

are produced by intermittent flooding, and Anopheles quadrimaculatus,

the malaria mosquito, is produced by constant flooding. The ricefields

in California are usually flooded throughout the growing season, and

they provide favorable aquatic habitats for Culex tarsalis and Anopheles freeborni. In addition, large numbers of Aedes nigramaculis

and Aedes dorsalis are frequently produced by the initial flooding of

the ricefields.

OfF-Field Sources

In many western irrigated areas, widespread and prolific mosquito

production occurs in a variety of aquatic habitats located outside the

boundaries of irrigated fields. These mosquito-producing areas include

roadside ditches, borrow pits, depressions on nonarable land, and

numerous other undrained areas that are flooded by waste irrigation

water from fields and other sources (figs. 6-8). Such aquatic habitats often contain dense vegetation, which makes them highly suitable for mosquito production. Both semipermanent and permanent water areas are

BN-26981

FIGURE 6.—Large numbers of mosquitoes are often produced in undrained borrow pits

and roadside ditches, which collect waste water from irrigated fields.
10 AGRICULTURE HANDBOOK 319, U.S. DEPT. OF AGRICULTURE

BK-26983

FIGURE 7.—Culverts placed too high result in ponding suitable for mosquito production.

BN-26984

FIGURE 8.—Improperly maintained drainage ditches with sluggish flow and ponding

are suitable for mosquito production.
MOSQUITO PREVENTION ON IRRIGATED FARMS 11

favorable for the production of Culex tarsalis. Areas with fluctuating

water levels also may produce large numbers of Aedes species.

These off-field habitats accounted for more than one-fourth of the

total mosquito production on a 5,000-acre study area in the Milk River

Valley of northern Montana. In the North Platte River Valley of western

Nebraska, depressions on nonarable land, roadside ditches, borrow pits,

and other undrained areas flooded by runoff from irrigated fields and

overflow from conveyance systems were important sources of both

Culex tarsalis and Aedes mosquitoes. Playas or land-locked depressions, flooded by runoff from irrigated fields, were responsible for

three-fourths of all mosquito production on study areas in the Texas

High. Plains (fig. 9). TÎiese playas produced large numbers of Culex

tarsalis and several other sjjecies of irrigation mosquitoes.

Reservoirs and ponds are used on some farms for storage and

regulation of irrigation water. The conditions associated with farm

impoundments that are conducive to mosquito production are basically

similar to those connected with large reservoirs. They include (1)

emergent or floating vegetation or both in shallow-water areas (fig. 10),

(2) accumulations of flotage and debris in shallow-water areas and

embayments protected from wave action, (3) undrained depressions

within the fluctuation zone (fig. 11), and (4) seepage areas below dams.

Aedes mosquitoes may be produced in reservoirs where water levels

fluctuate, alternately exposing and inundating shallow vegetated areas.

Several species of permanent-water mosquitoes may be produced in

reservoirs having shallow-water areas with emergent vegetation or flotage

protected from wave action (fig. 10). In the West, Culex tarsalis mosquitoes

BN-26985

FIGURE 9.- Major sources of mosquitoes are playas or land-locked depressions, which

collect runoff from irrigated fields.
12 AGRICULTURE HANDBOOK 319, U.S. DEPT. OF AGRICULTURE

BN-26986

FIGURE 10.—In irrigation reservoirs and ponds, conditions favorable for mosquito production are created by emergent or floating vegetation in shallow-water areas and

embayments protected from wave action.

BK-26987

FIGURE 11.—Mosquitoes also are produced in undrained depressions within fluctuation

zone of reservoirs and ponds.
MOSQUITO PREVENTION ON IRRIGATED FARMS 13

are often found in reservoirs and in the seepage areas that develop

below the dams. In the East, Anopheles mosquitoes are produced in

similar habitats.

Mosquito sources are sometimes associated with irrigation-distribution systems, which convey water to the farm ditches. Some of these

canals may pass through nonirrigable lands. A large amount of irrigation water is often lost by seepage from unlined canals and laterals

located in pervious material. In areas with poor underdrainage, seepage

losses alone or in combination with deep percolation from irrigated

fields often result in marshes and ponded water in roadside ditches,

borrow pits, and other low areas (fig. 12). In many western irrigated

areas, such aquatic habitats created by seepage produce large numbers

of mosquitoes. Permanent and semipermanent seepage areas are especially

favorable for the production of Culex tarsalis, the common encephalitis

vector. Aedes mosquitoes are also produced in seepage areas that have

fluctuating water levels.

Mosquito production sometimes occurs in various other off-field aquatic

habitats associated with farm irrigation-conveyance systems. These include

blocked natural drainageways (fig. 13), canals and laterals choked with

vegetation (fig. 14), areas flooded by overflow from canals and laterals,

and other surface pools created by leakage from or retained in conveyance

channels and structures during nonoperational periods. Although these

types of aquatic habitat contribute to the total mosquito problem,

they are generally minor factors compared to those caused by seepage

and the other off-field mosquito sources.

BN-26988

FiGUKE 12.—Borrow pits, roadside ditches, and numerous other depressions are often

flooded by seepage from unlined irrigation canals and laterals. Such aquatic

habitats are major sources of mosquitoes in many irrigated areas.
14 AGRICULTURE HANDBOOK 319, U.S. ÜEPT. OF AGRICULTURE

*****«-»„ iflU

BN-26989

FIGURE 13.—When irrigation canals block natural drainageways, resulting ponds are

often suitable for mosquito production.

BN-26990

FIGURE 14.—Mosquitoes may be produced in canals and laterals overgrown with

vegetation. Mosquito sources are sometimes created by overflow from improperly

maintained channels.
MOSQUITO PREVENTION ON IRRIGATED FARMS 15

INTERRELATIONSHIPS OF MOSQUITO PREVENTION

AND OTHER INTERESTS

Relationships to Irrigation Agriculture

Various structures and systems have been developed in connection

with good farm irrigation practices. Most of the on-field and off-field

mosquito sources previously described are associated with poor irrigation and drainage practices, including inadequate land preparation; use of farm layouts and irrigation methods that do not fit the

land, crops, or water supply; application of water in excess of crop

requirements; improper preparation and maintenance of reservoirs

and conveyance systems; and inadequate drainage systems for removal of excess water. Such practices also cause excessive water

losses, waterlogging of fields, salt and alkali accumulations in soils,

damage to soil structure, leaching of plant nutrients, and reduced crop

yields. These problems are intimately associated with losses of water

that occur when an irrigation project is developed and water is used

for crop production. Such water losses often contribute to conflicts of

interest between various agencies concerned with the use of water.

Competition and growing demands are demonstrating the importance

for all consumers to manage and to use water more efficiently. Irrigation waste water is a major factor in the development of habitats suitable for mosquito production. Research, experience, and technological

advances have provided many practical techniques for reducing irrigation water losses. In the following sections are described some of the

basic problems in soil and water management on irrigated lands and

how they are being solved.

Water Losses During Storage, Conveyance, and Application

It is not uncommon to find irrigation projects where only 25 percent

of the water actually diverted for irrigation purposes is used by the

crops grown. The remaining 75 percent is lost during storage, conveyance, or application. Ultimately a large part of this "lost water" returns

to streams or ground-water reservoirs and is available for other users.

Much of it collects in surface ponds and seep areas unless carried away

by natural or constructed drainage systems.

Reservoirs,—Water collected or diverted into storage reservoirs is

subjected to losses by evaporation, seepage, and consumption by noneconomic vegetation in and surrounding impoundments. Mosquito production is often directly associated with the last two losses. Considerable

research is underway to develop methods for controlling such water

Reservoirs often contribute to seepage problems if the underlying

material is porous or fissured. It is often possible to treat small reservoirs with bentonite or other sealing materials to reduce seepage losses.

If the reservoir involved is only a few acres in size, plastic liners

can be used to decrease seepage loss and to control vegetative growth

around the edges of a reservoir and thus advantageously reduce mosquito

production. The initial high cost of materials, their relatively short life.
16 AGRICULTURE HANDBOOK 319, U.S. DEPT. OF AGRICULTURE

and the damage resulting from rodents and birds are factors still to be

resolved before wide usage of such materials can be expected.

Many reservoirs have shallow embayments and marginal areas that

are flooded only when the water level is high. A system of dikes can

sometimes be used to confine the water to the main reservoir basin and

thereby reduce growth of vegetation in shallow-water areas and decrease use of water in évapotranspiration. Such a practice would also

eliminate potential mosquito sources.

Canals, Laterals, and Ditches.—Evaporation of water in transit and

consumption by vegetation on properly maintained ditchbanks are

negligible. In contrast, seepage losses from unlined irrigation canals

have been estimated at 23 to 50 percent of all the water diverted into the

canals. Seepage losses in 1949 totaled 3,900,000 acre-feet of water on

U.S. Bureau of Reclamation projects alone. Seepage losses also result

in waterlogging of adjacent lands, which generally makes farming

operations and crop production impossible without the added cost of

expensive drains. At the same time, larger canals must be built to

carry the additional water that will ultimately be lost to seepage before

reaching the land to be irrigated. The alternative, of course, is to line

or to place in closed conduits those reaches of canal that demonstrate

excessive permeability.

Considerable effort is being directed toward the development of

economical canal linings to reduce excessive seepage losses. Nonreinforced concrete canal linings and buried asphaltic membranes are the

most widely used materials at the present time, but costs are high.

Other materials used include asphaltic concrete, prefabricated asphaltic membranes, asphaltic emulsions, compacted earth, bentonite, and

soil cement. Considerable attention is currently being given to low-cost

liners utilizing plastic films and to laminated asphalt-jute liners for

laterals and on-farm ditches. Most of these developments are still in

the experimental stage. Nonetheless, they offer promise of eventually

controlling the major seepage losses and mosquito production associated

with them.

Operation of conveyance systems can also result in substantial losses

of water where demand is affected by such factors as weather and

holidays. Water must be wasted to protect the canal when the demand

is erratic. Unless adequate wasteways and drains are provided, such

water may periodically overflow wasteland and result in ponding ideally

suited to mosquito production.

Water Application,—Irrigation-application efficiency (percent of water

applied that remains in the crop-root zone) is affected by the farm-system

layout, degree of land preparation, and the skill of the irrigator.

The Bureau of Reclamation has reported farm efficiencies of 34 to 70

percent on Federal irrigation projects. In other words, 30 to 66 percent

of the water applied on farms is lost to deep percolation, evaporation,

and surface runoff. Some of this water is reused as return flow on

downstream projects, but some of the losses cannot be recovered. This

water must be carried away in surface or subsurface drains to avoid

crop damage due to ponding or high water table or both and to reduce

mosquito sources.
MOSQUITO PREVENTION ON IRRIGATED FARMS 17

Irrigation and cropping methods should fit the soil, slope, crop, and

water supply. Erosion, alkali damage, waterlogging, and undue water

loss should be avoided. An adequate inventory of soil and water resources

and selection of irrigation methods to fit these conditions are required.

Distribution systems must be developed to get enough water to all parts

of the farm when needed, and the land must be prepared so that the

water can be applied with high efficiency. When completed, the irrigation system must be operated properly to attain maximum application

efficiency. This operation requires adjustment of stream size to intake

rate, length of run, furrow size or width of border, and stage of crop

development. Also, one must have a knowledge of the water-holding

capacity of the soil and crop-rooting depth in order to apply the correct

amount of water.

For a detailed discussion on design and operation of irrigation systems,

refer to "Irrigation on Western Farms," prepared by the Bureau of

Reclamation and the U.S. Soil Conservation Service (see Selected References at the end of this bulletin). It is important to realize that low

application efficiency may occur with the best designed irrigation systems if the operator manages his water poorly. Conversely, the best

irrigator cannot achieve maximum efficiency if the irrigation system he

operates is not adequately designed and constructed.

Soil and Water Management

Intake Rate,—An efficient irrigator requires a knowledge of intake

rates to determine how long water should remain on the land to replenish the crop-root zone reservoir. Management of soils to maintain adequate intake rates is considered by some authorities to be the primary

problem associated with crop production on irrigated lands in arid and

semiarid regions.

Many irrigation projects have been developed that at best were marginal from the standpoint of inherent soil capability. In some areas,

topography has been ideally suited to land development and distribution of irrigation water, but the soils have been predominantly finetextured clay with jx)or internal drainage. In such instances, intake

rates, once the topsoil has become wet, are practically nil. If the clay

mineral is montmorillonitic, cracks develop upon drying. Irrigation

then becomes a matter of applying the amount of water required to fill

the cracks, which subsequently close by swelling.

Surface soil structures may be improved by alternate wetting and

drying, and freezing and thawing, but seldom are they adequate to circumvent the problems associated with water application and distribution. When farmers do not know how much water is required to replenish the crop-root zone reservoir, they tend to irrigate for excessive

periods. This results in considerable waste water collecting at the lower

end of fields. Without adequate surface drainage, water accumulates

in low areas and creates excellent mosquito sources.

Research conducted on clay soils in the Milk River Valley of Montana illustrates how mosquito production can be controlled by applying

good soil- and water-management practices. The traditional practice

of wild flooding western wheatgrass for 30 to 40 days in the spring pro-
18 AGRICULTURE HANDBOOK 319, U.S. DEPT. OF AGRICULTURE

duced ideal habitats for mosquito production. Moreover, the crop yield

averaged only about 0.5 ton of poor-quality hay per acre. In contrast,

installing a well-designed border irrigation system, utilizing irrigation

water as needed, and adopting good fertility practices resulted in a

tenfold increase in yield of high-quality hay and no mosquito production.

Ponding of water was eliminated by applying only that amount required

to refill the crop-root zone. Although this research demonstrated conclusively how the farmer could manage his water and soil to increase

crop yields and reduce mosquito production, adoption of such practices

on a project basis is often difficult in view of the economics of low water

cost and increased labor requirements.

In many instances, low intake rates of soils cannot be greatly changed.

Minimum tillage when the soil is at optimum moisture conditions, subsoiling to break up impervious layers near the surface, vertical mulching, and other practices can sometimes improve water intake and reduce

runoff. The choice of practice depends on the nature of the problem.

At the Owyhee project in Oregon, deep plowing (30 inches) of laminated,

slowly permeable silt layers within the soil profile increased intake rates

and crop yields manifold. In the Texas High Plains, a Pullman silty

clay loam responded to a deep-rooted crop of alfalfa. Here, a fivefold

increase in intake rate persisted over a 3-year period, illustrating the

benefits that can accrue from crop rotation. Minimum tillage practices

in the Salt River Valley of Arizona have been highly successful in facilitating water intake to depths of 6 feet. These examples illustrate adoption of practices by farmers to "live" with situations involving low intake rates and still make efficient use of available water supplies.

Intake rates can also be affected by the amounts and kinds of salt

that accumulate in the soil profile. These salts may be present in the

soil before it is ever irrigated or they may accumulate from application

of irrigation water containing large amounts of dissolved salt. For example, in some areas the 2 to 4 acre-feet of water normally applied in

a given year, much of which is evaporated or transpired, can add 20 tons

of salt to the soil profile. Salts must be leached periodically by applying additional water in order to maintain a favorable salt balance. If

natural drainage is inadequate, artificial drainage must be provided

to dispose of the excess water and salt.

Where alkali problems develop, soil and water management is more

difficult. Alkali soils have poor structure with low intake rates. Water

that ponds in alkali areas remains until lost by evaporation. Improvement of alkali soils requires lowering the water table and replacing

sodium in the soil with soluble calcium or other divalent cation (gypsum

is commonly used), removing salts by leaching, and rearranging and

aggregating soil particles to improve soil structure and intake rate.

Reclamation of alkali soils is usually slow, requiring application of

water for leaching. In general, about 1 acre-foot of water is required

for leaching purposes for every ton of gypsum applied. It is not uncommon to find a gypsum requirement of 10 tons or more per acre for some

soils.

Timing Irrigations.—The magnitude of water loss through deep percolation and runoff at the lower end of fields is intimately associated

with the farmer's uncertainty as to "when to irrigate a crop" and "how
MOSQUITO PREVENTION ON IRRIGATED FARMS 19

much water should be applied." Many farmers irrigate from experience.

Others may be restricted to a given irrigation regime determined by a

prearranged water-delivery schedule on a project rotation basis. The

farmer who can get water on demand has maximum flexibility and

opportunity to use water most efficiently.

More crops are overirrigated than underirrigated, especially in areas

where water is plentiful and cheap. In such areas the farmer will often

substitute water for the additional labor required to do a good job of

irrigating. When water is limited and its cost is high, good irrigation

principles are more often utilized. Not only does overirrigation waste

water but soluble plant nutrients may be leached to depths below the

rooting zone. Some crops, like alfalfa, are susceptible to damage by

prolonged flooding during hot periods and either die or yield poorly.

Ponding from overirrigation likewise produces mosquitoes.

Frequency of irrigation depends on soil-moisture storage capacity

in the crop-root zone and the évapotranspiration rate. It is important

for many agricultural crops to maintain continuous growth for optimum

yields. Most recommendations favor irrigating when about 50 percent

of the available moisture in the root zone has been used. During the peak

growing period, when évapotranspiration rates are highest, a sandy

soil might require an irrigation every 4 to 6 days, whereas a silty clay

loam would not require irrigation for 14 days or more. Hence, the farmer

needs a knowledge of the soil's water-holding capacity in order to

properly schedule irrigations.

The problem is further complicated, since évapotranspiration rates

for annual crops are not constant throughout the season, but generally

increase from a low value at seeding to a maximum value at peak growth,

folfowed by a gradual decrease as the crop matures. Moreover, periodic

harvest of perennial crops, like alfalfa, also affects water use; rates

are lower immediately after crop removal. In some areas climatic

conditions markedly affect the rate of water loss, which is lower during

wet, cool periods and higher during hot, dry periods.

Many soil and crop factors must be considered to properly schedule

irrigations. Research has provided the farmer with techniques and

information that can be used to do a better job of irrigating. Instruments

to measure soil moisture are available commercially. If properly used,

they show when a crop should be irrigated. Seasonal and periodic consumptive use of water (évapotranspiration) rates have been measured

for many crops and are being utilized in a "bookkeeping" approach to

scheduling irrigations. In the Columbia River Basin, évapotranspiration

rates are estimated from evaporation pans and are furnished daily to

the farmers by radio.

Experiments dealing with irrigation needs based on plant appearance and critical stages of plant development have practical application. For example, field beans growing on moist soils can be irrigated

5 days after the foliage changes from light green to dark green without

affecting yield. Limited wilting of corn prior to tasseling has little

effect on yield, but 4 to 6 days of wilt during the tasseling and silking

period can reduce yield as much as 40 percent. If available information

and techniques were applied to the timing of irrigations on farmers'
20 AGRICULTURE HANDBOOK 319, U.S. DEPT. OF AGRICULTURE

fields, much of the water now wasted could be used to irrigate additional land.

Determining Amount of Water Applied.—In many areas the farmer

irrigates on a 12- to 24-hour set or longer, depending on soil conditions

and replenishment water required. He seldom actually measures water

"on" and "off" his fields to determine how much he has applied. Several

water-measuring devices are available, but most of them are not simple

or rugged enough to be readily acceptable. The development of a vane

meter by Parshall, though not extremely accurate, comes close to

determining field requirements, but still requires standard channel

sections at several points on the farm. It has the advantage, however,

of portability and direct indication of flow measurement. Meters reading directly in acre-feet are now used in some areas, particularly where

irrigation water is pumped. Research on water measurement continues

to have a high priority in some hydraulic laboratories. Until the farmer

is supplied a simple tool for measuring water accurately, runoff from

irrigated fields will continue to be a problem.

Other developments may affect water-use efficiency. Low-gradient

or level-basin irrigation systems show promise of being highly efficient.

On Tripp fine sandy loam at Scottsbluff, Nebr., irrigation-application

efficiency on bench-leveled irrigation basins has reached 90 percent with

excellent distribution of water. With this system, the irrigator need only

know the size of the irrigation stream and the time required to apply

a given amount of water. In the Lower Rio Grande Valley, about 250,000

acres of irrigated land is now in level or low-gradient basins. Basin irrigation not only provides better control of the amount and distribution of

irrigation water but permits collection and retention of rainfall often during high-intensity storms. The additional water from rainfall facilitates

leaching of salts and supplements the limited irrigation water supply.

Some problems are still to be resolved before low-gradient or

level-basin systems will be widely accepted. Foremost among these

are management of escarpments between benches, difl&culties in maintaining the border level, susceptibility of some crops to flooding, and

large streams required, but as water for irrigation becomes more

scarce and more expensive, doubtless more low-gradient irrigation

systems will be installed.

Water-Use Efficiency in Relation to Crop Needs.—Any practice that

increases crop yields also increases water-use efficiency, expressed as

the units of marketable crop produced per inch of water. Hence, it is

important that all factors affecting production, such as soil-moisture

availability, plant density, and fertility, be maintained in proper balance for most efficient use of water. If any one factor becomes limiting, maximum production efficiency cannot be expected.

Research has shown that increasing crop growth and yield with fertilizer results in some increase in the water required per acre to produce the fertilized crop. However, doubling yields does not double the

water consumed. In fact, the increased évapotranspiration from fertilized crops seldom exceeds a 5-percent increase in water use, even

though yields may be increased 200 to 300 percent. Maximum water-use

efficiency comes with good soil and water management. Under such

conditions, mosquito production is seldom a problem. On the contrary,
MOSQUITO PREVENTION ON IRRIGATED FARMS 21

low water-use efficiency often means too much water and high mosquito

production.

Drainage.—Successful irrigation agriculture requires adequate natural

or artificial drainage. Historically, first attention has been given to

getting water to the land and drainage has been essentially ignored

until problems have developed. Unfortunately many highly productive

soils have become waterlogged and "salted out" through delays in providing adequate drainage. Of course it is not always possible to know in

advance where drains will be required. However, when the need for and

location of drains can be predicted, it is easier and less costly to provide

drainage initially than to correct severe waterlogging and alkali conditions when they develop.

The principal source of water contributing to drainage problems

arises from overirrigation and seepage from unlined canals. Such water

losses can be reduced, as previously described. The operation of most

irrigation systems results in some deep percolation and runoff losses

that are difficult to avoid. In such cases, drainage facilities may be

needed to prevent direct injury to crops, to minimize salt accumulation

by leaching, and to allow proper timing of farming operations.

Both surface and subsurface drains are required on most irrigated

land. Surface drains are necessary to remove excess irrigation water

and runoff from precipitation. Subsurface drains are necessary for the

removal of ground water that would result in a high water table. Surface drains to remove excess irrigation water usually are included as a

part of the modern irrigation system; however, drains for the disposal

of waste water were not included in many older systems. Runoff from

precipitation is usually relatively small in irrigated areas in arid and

semiarid regions because of low rainfall and because fields are small

tracts isolated topographically by irrigation laterals and canals. Ditches

for disposal of excess irrigation water are therefore often adequate for

disposing of surface runoff from precipitation.

Subsurface drainage is usually the primary objective in western irrigated areas. Often the subsurface system provides for the removal

of both surface and subsurface water. Drains may be either covered or

open. Open drains often serve to remove both surface and ground water.

Covered ones are seldom used as dual-purpose drains because of the

increased cost of larger tile needed to accommodate surface runoff in

addition to ground water. Subsurface drains are either relief or interception drains depending on their alinement with respect to groundwater flow. Relief drains are alined parallel to the direction of groundwater flow, and interception drains are installed about j>erpendicular

to ground-water flow and are used also under some conditions where

there is little or no slope of the water table.

The location of surface drains in irrigated areas is usually governed

by the pattern of irrigation. Field drains are located at the lower end

of rows or border strips and preferably adjacent to irrigation canals,

fence rows, or roads. Control structures are generally provided so that

drain water can be discharged into lower irrigation canals or conveyed

over or under the canals to an outlet. If the discharge from drains is

small in comparison to canal capacity, it may be possible to outlet into

irrigation canals without danger of exceeding their capacity. Where the
22 AGRICULTURE HANDBOOK 319, U.S. DEPT. OF AGRICULTURE

drain discharge is large in comparison to canal capacity, some type of

canal crossing must be provided. Structures generally used for this

purpose include culverts, inverted syphons or flumes, and bridges.

The pattern of subsurface drains in irrigated areas is similar to that

used to solve drainage problems in nonirrigated areas. Drains are

usually laid out on a parallel, gridiron, herringbone, or random pattern.

Interception-type drains are most frequently used and are randomly

laid out on a slight grade to maintain uniform depth. Main drain and

outlet conditions for subsurface drains must meet the same requirement as surface drains in irrigated areas. Subsurface drains usually

have greater depth ; therefore, there is less opportunity to discharge into irrigation canals for reuse of water. Culvert-type crossings are generally used

at irrigation canals. A high salt content of drainage water may prohibit

its discharge into irrigation-water supply.

Most of the drains constructed in western irrigated areas have been

the open type designed as dual-purpose subsurface drains. They have

many advantages, but because of their depth and relatively flat side

slopes, they may require rights-of-way from 60 to 100 feet wide. Such

a right-of-way may require as much as 10 acres per mile of drain.

Surface water is admitted to subsurface drains through grade-control

structures of various types. The present trend is toward the construction

of covered drains, since no land is removed from production, and if

properly installed, they require very little maintenance. Many existing

open drains are being rehabilitated as covered drains.

Open drains are often poorly maintained and become clogged with

weeds and vegetation. The slow-moving ponded water creates an ideal

habitat for mosquito production. Replacement of open drains with

covered drains is therefore often mutually beneficial to irrigation agriculture and mosquito control.

The "pump-back" systems, which are currently being used on sloping

lands in California, not only allow for more efficient use of water but

at the same time eliminate mosquito habitats. In these systems, runoff

from irrigated fields is collected in basins and is pumped back to the

head ditch for redistribution. The water is recirculated, so to speak,

until the amount required has entered the soil. In the Texas High Plains,

runoff water from rainfall and irrigation often collects in playa lakes.

Not only would the use of pump-back systems solve the mosquito problem associated with ponded water, but in an area where ground-water

irrigation supplies are rapidly becoming depleted, such a practice

might well conserve and extend available water supplies.

Pump-back systems should maximize application efficiency on graded

irrigation systems, particularly where restricted intake rate is a problem. This development also illustrates how a conservaltion practice

works to the advantage of several interested groups—those interested

in eliminating mosquito habitats and those interested in conserving

limited water supplies while at the same time improving irrigation

practices and reducing a water source that contributes to drainage problems.
MOSQUITO PREVENTION ON IRRIGATED FARMS 23

Relationships to Aquatic Wildlife Conservation

Some years ago mosquito control and aquatic wildlife conservation

interests were generally incompatible. During recent years a cooperative approach to the development and management of aquatic wildlife

resources often results in practices mutually beneficial to both mosquito prevention and wildlife interests.

Aquatic wildlife—fish, waterfowl, certain fur animals, and scores

of shore, wading, and diving birds—^must have a favorable aquatic environment in order to thrive. The following sections point up some of

the basic requirements of aquatic wildlife and suggest how these requirements may be integrated with those for mosquito prevention on irrigated farms.

On-Field Relationships

As previously mentioned, retention of ponded water on irrigated lands

long enough to produce mosquitoes is injurious to all field crops except

rice. If good irrigation and drainage practices are followed, on-field

relationships between mosquito prevention and aquatic wildlife will be

of concern only in rice-farming areas. Since rice culture does not

usually involve permanent year-round flooding, these relationships involve waterfowl and shore birds, but not fish. Top minnows are sometimes stocked in ricefields to aid in mosquito control, but as such they

do not constitute a fishery resource.

Ricefields flooded after harvest may provide excellent resting and

feeding grounds for waterfowl. If the flooding takes place at the end of

the mosquito-production season, it will not create a mosquito problem.

However, if the flooding occurs during the mosquito season, it may

result in prolific and extensive mosquito production.

Off-Field Relationships

The major relationships between mosquito prevention and fish and

waterfowl production on irrigated farms involve off-field habitats.

These habitats include (1) systems for the collection and disposal or

utilization of excess irrigation water, (2) small storage or regulating

reservoirs, and (3) irrigation-conveyance systems. All of them have

considerable potential as productive habitats for fish and waterfowl,

and can usually be managed in a manner compatible with mosquito prevention.

Irrigation runoff water is often wasted, and this waste water creates

some of the most important mosquito sources on irrigated farms. In

some areas, nonarable land is wild flooded with excess irrigation water

for use by ducks and other waterfowl. Such areas are often highly favorable for the production of Aedes mosquitoes. Abatement districts in

California and Utah spend large sums of money to control mosquitoes

produced on waterfowl areas of this type.

The rapidly growing shortage of our natural water resources demands

that excess irrigation water be utilized to the fullest extent possible

rather than wasted. One of the most valuable types of utilization is to

create impoundments for the production of fish and waterfowl (figs.

15 and 16). Correct design and construction, and proper management
24 AGRICULTURE HANDBOOK 319, U.S. DEPT. OF AGRICULTURE

BN-26991

FIGURE 15.—Farm impoundments are valuable source of fish and waterfowl production.

If properly designed, they may produce fewer mosquitoes than previous unmodified

habitats.

WÊÊgm BN-2G992

FIGURE 16.—Farm fishpond such as this with clean, abrupt shoreline and open water

exposed to wave action will produce very few mosquitoes. Adequate water-control

structures are necessary to prevent flooding of marginal vegetation for extended

periods during mosquito breeding season.
MOSQUITO PREVENTION ON IRRIGATED FARMS 25

and control of water levels and aquatic vegetation in these impoundments, are essential for both mosquito prevention and eflScient production of fish and waterfowl. In general, a properly constructed and

managed wildlife impoundment will produce only a fraction of the mosquitoes that would be produced in the same area if it were unmodified

and subjected to haphazard flooding with excess or salty irrigation

water.

An important construction feature for small farm fishponds

is to eliminate shallow-water areas insofar as possible. Water less than

2 feet deep encourages the growth of both submerged and emergent

aquatic plants. These plants are undesirable in a fishpond because (1)

they shelter small fish so that too many survive and exceed their food

supply, resulting in a stunted population; (2) they may absorb valuable

nutrients from the water and the bottom, tying them up on compounds

the fish cannot use; and (3) they interfere with fishing, boating, and

swimming. These same plants also add to the problem of mosquito

control.

In impoundments used primarily for waterfowl, both submerged and

marginal emergent aquatic plants are desirable. The submerged aquatics may grow in water with depths up to 6 or more feet, depending on

water transparency. Such areas should comprise about 80 percent of the

total water surface for an ideal waterfowl habitat. These plants not

only serve as food for waterfowl but also harbor small aquatic animals,

such as insects, crustaceans, and moUusks, which are highly valuable,

if not essential, in the diet of young ducks because of their high protein,

phosphorus, and calcium content. Some of the most valuable submerged waterfowl food plants, such as sago pondweed {Potamogetón

pectinatus), may be produced in such open-water areas without creating a significant mosquito source if the open areas are 2 feet or more

deep and at least 30 feet wide.

Marginal, terrestrial, and emergent aquatic plants are necessary to

provide nesting cover for waterfowl, and some of them are also utilized

as food. Some of the tall emergent species, such as hardstem bulrush

and cattail, provide excellent nesting cover for geese and ducks and do

not serve as important sources for mosquito production (fig. 17). The

shorter, more flexuous species, such as saltgrass (Distichlis stricto),

are less satisfactory as nesting cover and are among the most prolific

sources of mosquito production, particularly when growing in shallow

water. However, they may provide feeding areas for water birds. Steepening of pond margins is of considerable value in limiting mosquito

production. Although steepening of pond margins may reduce the total

amount of wildlife habitat, it can increase the availability to waterfowl

of "ditchbank" types of marginal vegetation. This vegetation is often

used as nesting cover, and some species, such as rice cut-grass, millet,

and smartweed, are also excellent waterfowl food plants. In larger

ponds, small islands may be constructed to provide this type of cover.

The installation of adequate water-control structures is required in

the construction of small wildlife impoundments for collecting irrigation

runoff. This is necessary for both wildlife management and mosquito

prevention. These structures should hold the water at constant levels
26 AGRICULTURE HANDBOOK 319, U.S. DEPT. OF AGRICULTURE

BN-26993

ÍICURE 17.—lall, dense, emergent vegetation, such as bulrushes in background, provides

food and nesting cover for ducks and geese and good habitats for muskrats, with

minimum of mosquito production. Open grassy vegetation to left of muskrat house

IS likely to produce mosquitoes.

or raise or lower it to any desired level. Where feasible, they should

also provide for bypassing excessive flows to prevent undesirable flood

surcharges.

The main phases of water-level control in permanent ponds are constant pool, surcharge, recession, and cyclical fluctuation. The effect of

each of these phases on mosquito prevention and fish and wildlife management may be beneficial or detrimental, depending on timing and magniConstant pool levels in fishponds are particularly important during

the spring spawning period of game fish to prevent stranding of the eggs

before they hatch. A constant pool level during this same period usually

benefits mosquito prevention by limiting invasion of undesirable marginal vegetation. Surcharge flooding of pond margins during the waterfowl breeding season may inundate nesting areas and cause catastrophic

losses of eggs and young birds. A brief flood surcharge may benefit mosquito prevention by stranding marginal debris and flotage, but it should

be carried out in the early spring before the beginning of the waterfowl

nesting season.

Employed without discretion, seasonal recession and cyclical fluctuation may be deleterious to wildlife and fish, but if properly applied

these measures may be one of the most effective tools in wildlife management. For example, a fall drawdown restricts the amount of water

and food available to migrating waterfowl. On the other hand, a fall

drawdown or cycle of fluctuation is often beneficial in permitting predatory game fish to better utilize small rough fish, which would interfere

with reproduction by the desirable game species. Seasonal recession

and cyclical fluctuation are also important in mosquito prevention, but here
MOSQUITO PREVENTION ON IRRIGATED FARMS 27

again they must be used properly. For example, early and excessive seasonal drawdown is very favorable to mosquito production,

in that it increases the invasion of undesirable marginal plants, which

will be sources of mosquito production during the following year.

It is important for both mosquito prevention and fish management

that any marginal depressions be connected with the main pond by

ditches so that they will be self-draining. The normal seasonal pattern

of water-level fluctuation in natural ponds is high water in late winter

and early spring, a relatively constant level in late spring, and a gradual

recession during summer and early fall. The most effective water-level

schedules for both wildlife management and mosquito prevention are

usually adaptations of this natural cycle.

In situations where there is no year-round source of water, it is impossible to develop ptermanent fishponds, and waterfowl impoundments

must be managed on the basis of seasonal or intermittent flooding. Where

the water supply is variable from year to year, it is sometimes possible

to construct a series of small ponds and fill only those for which water

is available. In some situations, pond basins dewatered during the summer may be planted with cultivated crops or seeded and allowed to produce natural waterfowl food plants, such as wild millet {Echinochloa

crusgalli), and then reflooded in late fall after mosquito breeding has

ended to provide resting and feeding grounds for migrating waterfowl

(fig. 18).

Small farm impoundments are often damaged by muskrats, which

burrow through the dams and cause them to leak. Ideal habitats for

muskrats are shallow marsh areas with only about 20-percent open

water. Therefore, fish and waterfowl ponds with steep banks and limited

Bir-26994

FIGURE 18.—Wetland types of vegetation, such as this wild millet, may provide valuable

nesting £ind feeding areas for waterfowl if located adjacent to farm impoundments.
28 AGRICULTURE HANDBOOK 319, U.S. DEPT. OF AGRICULTURE

marginal vegetation will discourage muskrats. Deliberate use of irrigation runoff to create muskrat habitats usually creates many more mosquito problems than the use of this water for fish or waterfowl impoundments.

Irrigation canals and drainage ditches frequently provide valuable

habitats for fish and waterfowl, particularly in the Western States.

If properly maintained, these waterways do not create important mosquito problems and their use in fish and waterfowl management is compatible with mosquito prevention. Irrigation canals make excellent habitats for trout, especially brown trout, if the flow is maintained year round.

A year-round or only a growing-season flow prevents the establishment

of terrestrial and emergent aquatic plants in the canal bed. A bypass

structure provides for return of water to the main stream during the

off-irrigation season. When it is necessary to drain canals for repair

or cleaning operations, it is often possible to salvage most of the fish

stranded in the canal by seining or shocking and transplanting them to

other habitats.

Constant-flow canals and drainage ditches are often used by waterfowl that feed on submerged vegetation and aquatic insects. They make

excellent areas for "jump hunting." Such use does not interfere with

mosquito prevention, which is primarily concerned with the control of

marginal and emergent vegetation rather than submerged aquatics.

Seeped areas from irrigation canals are a common source of mosquito production. They may be used by waterfowl, but they do not provide full potential for waterfowl unless properly developed. Where control of canal seepage is not practicable, it may be desirable to develop

fish or Waterfowl impoundments to utilize the seepage water. This will

not increase the mosquito problem if the same principles are followed

as previously outlined for collection and disposal of irrigation runoff.

BASIC PRINCIPLES AND PRACTICES

FOR MOSQUITO PREVENTION

As indicated previously, the ultimate solution to mosquito problems

associated with irrigation should be based on source-reduction measures

aimed at preventing, eliminating, reducing, or suitably modifying manmade aquatic habitats on both irrigated and nonirrigated lands. On

irrigated farms, this can be accomplished largely through the use of good

irrigation and drainage practices that will insure high crop yields without excessive water losses or decreased soil productivity. The full utilization of source-reduction measures will greatly minimize the need for

repeatedly applying chemicals for mosquito control.

Both on-field and off-field mosquito sources commonly found on

irrigated farms can be prevented, reduced, or eliminated through applying the principles and practices outlined in the following sections. The

agricultural benefits to be derived from these practices will usually

greatly exceed the cost of applying them. Some of the practices will

also be mutually beneficial to aquatic wildlife conservation. Thus, in

many situations, the prevention and control of mosquitoes and mosquitobome diseases may be accomplished cost-free if these basic principles and practices are followed.
MOSQUITO PREVENTION ON IRRIGATED FARMS 29

Impoundments

(1) Prior to impoundage, the reservoir basin should be prepared as

follows:

(a) The normal summer fluctuation zone of the permanent pool

should be completely cleared, except for isolated trees and sparse

vegetation along abrupt shorelines or in large open areas that will

be exposed to wave action.

(b) Timber rooted below the normal summer minimum pool level

but extending above that elevation should be cleared. In some situations, such timber may be felled and securely tied down in lieu of disposal. This practice sometimes has advantages for fisheries management.

(c) All depressions, marshes, and sloughs that will be flooded by the

reservoir at maximum pool level and that will retain water at lower

pool levels should be connected with the main reservoir by drains to

insure complete drainage or fluctuation of water within them.

(d) If the summer fluctuation zone of the permanent pool is limited

to a few feet, consideration should be given to "building out" mosquito-producing areas by deepening or filling the more extensive

shallow-water areas. This would minimize the need for repetitive

measures for controlling vegetation and mosquito production.

(e) Provision should be made for utilizing water-level management

to minimize conditions favorable for mosquito production to the maximum degree permitted by the primary functions of the reservoir.

(2) After impoundage, dense vegetation should be removed periodically from flat, protected areas within the normal summer fluctuation

zone of the permanent pool in all potential mosquito-producing areas

located within mosquito flight range of human populations or recreational areas frequented by large numbers of persons. Vegetation, debris,

and flotage should be removed periodically from all drains to insure free

flow.

Irrigation-Conveyance and Distribution Systems

(1) Lining, closed pipe, or other satisfactory seepage control measures

should be provided for all sections of canals and laterals located in porous

material where excessive leakage would result in waterlogged areas,

seeps, and ponds.

(2) Drains should be installed to prevent ponding of excess irrigation

water and natural runoff along the upper side of canals and laterals.

All drainage crossing or inlet structures should be placed on grade to

avoid ponding.

(3) All borrow areas should be self-draining to keep them from retaining ponded water.

(4) Where possible, provision should be made to prevent the bottom

of canals and laterals as well as turnouts and other control structures

from retaining residual water when they are not being used.

(5) Effective measures should be provided to prevent ponding of

leakage from water-control structures.
30 AGRICULTURE HANDBOOK 319, U.S. DEPT. OF AGRICULTURE

(6) Every effort should be made to establish delivery schedules that

will provide adequate but not excessive amounts of water at proper intervals to insure eflBcient irrigation of the crops concerned.

(7) Vegetation and debris that retard normal flow should be periodically removed from all conveyance channels, water-control structures,

and drains.

Irrigated Lands

(1) The farm-supply system, drainage system, and field layouts should

be properly fitted to the topography, soil, water supply, crops to be

grown, and irrigation methods to be used.

(2) AH surface-irrigated fields should be properly leveled or graded

to provide for efficient water application and removal of excess water

without ponding.

(3) Adequate drainage systems should be provided for removal of

excess water from all irrigated fields.

(4) Irrigation methods should be used that will provide optimum

irrigation efficiency.

(5) Application of irrigation water should be limited to that required

to fill the crop-root zone plus unavoidable losses and excess leaching

water to prevent damaging accumulation of salts in the root zone.

(6) Cultural and soil-management practices that will develop and

maintain good soil structure and infiltration rates should be utilized to

the fullest extent possible.

Drainage Systems

(1) Trunk drainage systems should be installed to insure complete

removal and proper disposal of excess irrigation water, natural runoff,

and seepage from both irrigable and nonirrigable lands affected by the

distribution and use of irrigation water on the farm.

(2) Drainage ditches should be designed, constructed, and maintained

so as to minimize ponding in the channels and to insure free flow at

all times.

(3) Provision should be made to prevent water from ponding behind

spoil banks.

(4) Underdrains, culverts, inlets, and similar structures should be

placed on grade to prevent ponding.
MOSQUITO PREVENTION ON IRRIGATED FARMS 31

SELECTED REFERENCES

(1) AMERICAN SOCIETY OF AGRICULTURAL ENGINEERS, COMMITTEE ON IRRIGATION SYSTEM DESIGN FOR MOSQUITO CONTROL, IRRIGATION GROUP, SOIL AND WATER

DIVISION.

1965. PRINCIPLES AND PRACTICES FOR PREVENTION AND ELIMINATION OF MOSQUITO

SOURCES ASSOCIATED WITH IRRIGATION. Agr. Engin. Ybk. 1965: 350.

(2) BAILEY, S. F., BOHART, R. M., and BOOHER, L. J.

1954. MOSQUITO CONTROL ON THE FARM. Calif. Agr. Expt. Sta. Ext. Cir 439

27 pp.

(3) CONFERENCE FOR CO-ORDINATED PROGRAM ON WILDLIFE MANAGEMENT AND

MOSQUITO SUPPRESSION, YOSEMITE NATIONAL PARK, CALIFORNIA.

1963. PROCEEDINGS OCTOBER 15-18, 1962. 120 pp. California Mosquito Control

Association.

(4) DAY, A. M.

1959. NORTH AMERICAN WATERFOWL. 363 pp. Stackpole Co., Harrisburg, Pa.

(5) EARL, J. P.

1950. PRODUCTION OF MALLARDS ON IRRIGATED LANDS IN THE SACRAMENTO

VALLEY, CALIFORNIA. Jour. Wildlife Mangt. 14: 332-342.

(6) EGAN, J. P.

1955. HOW THE STATE GAME PROGRAM CAN BE CORRELATED WITH MOSQUITO

CONTROL WORK. Utah Mosquito Abatement Assoc. Abs. and Proc. Ann.

Meeting 8, p. 13.

(7) ERRINGTON, P. L.

1961. MUSKRATS AND MARSH MANAGEMENT. 183 pp. Stackpole Co., Harrisburg. Pa., and Wildlife Management Institute, Washington, D.C.

(8) GiRARD, G. L.

1941. THE MALLARD: ITS MANAGEMENT IN WESTERN MONTANA. Jour. Wildlife

Mangt. 5: 233-259.

(9) HESS, A.D.

1958. VECTOR PROBLEMS ASSOCIATED WITH THE DEVELOPMENT AND UTILIZATION

OF WATER RESOURCES IN THE UNITED STATES. 10th Intematl. Cong. Ent

Proc. 3 (1956), pp. 595-601.

(10) IsRAELSEN, O. W., and HANSEN, V. E.

1962. IRRIGATION PRINCIPLES AND PRACTICES. Ed. 3, 447 pp. John Wiley &

Sons, New York.

(11) KORTRIGHT, F. H.

1942. DUCKS, GEESE, AND SWANS OF NORTH AMERICA. 476 pp. American Wildlife Institute, Washington, D.C.

(12) MARTIN, A. C, and UHLER, F. M .

1939. FOOD OF GAME DUCKS IN THE UNITED STATES AND CANADA. U.S. DcDt.

Agr. Tech. Bui. 634, 156 pp.

(13) MYERS, L. E., JR.

1956. GLOSSARY OF TERMS USED IN IRRIGATION PRACTICE. Calif. MosquitO

Control Assoc. 23d Ann. Conf. and Amer. Mosquito Control Assoc.

11th Ann. Meeting Proc. and Papers, Dec. 7, 1955, pp. 64-70.

(14) PARTAIN, L. E.

1963. RECREATION BRINGS NEW OPPORTUNITIES TO RURAL AMERICA. Soil Conscrv.

28: 171-174.

(15) PETERSON, M. L., OSTERLI, V. P., and BERRY, L. J.

1959. MANAGING IRRIGATED PASTURES. Calif. Agr. Expt. Sta. Ext. Serv. Cir.

476, 31 pp.

(16) RAINEY, M. B.

1955. GOOD IRRIGATION VITAL IN MOSQUITO CONTROL. Agr. Engin. 35: 185-

187, 191.

(17) STATE COLLEGE OF WASHINGTON, INSTITUTE OF AGRICULTURAL SCIENCES, EXTENSION SERVICE.

1959. MOSQUITO PREVENTION AND CONTROL. Ext. ScrV. Bul. 534, 11 pp.
32 AGRICULTURE HANDBOOK 319, U.S. DEPT. OF AGRICULTURE

(18) SYMPOSIUM ON COORDINATION OF MOSQUITO CONTROL AND WILDLIFE MANAGEMENT, WASHINGTON, D.C.

1960. PROCEEDINGS APRIL 1-2, 1959. 154 pp. Morris County Mosquito Extermination Commission, Morris Plains, N.J. [Processed.]

(19) TEXAS AGRICULTURAL EXPERIMENT STATION in cooperation with the U.S. DEPARTMENT OF AGRICULTURE.

1952. USE OF IRRIGATION WATER ON THE HIGH PLAINS. Bul. 756, 43 pp.

(20) THORNE, D. W., and PETERSON, H. B.

1949. IRRIGATED SOILS: THEIR FERTILITY AND MANAGEMENT. 288 pp. The

Blakiston Co., Philadelphia and Toronto.

(21) U.S. BUREAU OF RECLAMATION.

1951. IRRIGATION ADVISERS' GUIDE. 216 pp. Washington, D.C.

(22)

1962. BATTLING THE MOSQUITOES. Rcclam. Era 4S: 35-37.

(23) and U. S. SOIL CONSERVATION SERVICE.

(23) and U.S. SOIL CONSERVATION SERVICE.

(24) U.S. DEPARTMENT OF AGRICULTURE.

1955. WATER. U. S. Dept. Agr. Ybk. 1955: 1-751.

(25) U.S. PUBLIC HEALTH SERVICE, COMMUNICABLE DISEASE CENTER.

1953. MOSQUITO PROBLEMS IN IRRIGATED AREAS AND THEIR PREVENTION. 18 pp.

Atlanta, Ga.

(26) U.S. PUBLIC HEALTH SERVICE and TENNESSEE VALLEY AUTHORITY.

1947. MALARIA CONTROL ON IMPOUNDED WATER. 422 pp. Washington, D.C.

(27) U.S. SOIL CONSERVATION SERVICE.

1959. NATIONAL CATALOG OF PRACTICES AND MEASURES USED IN SOIL AND WATER

CONSERVATION. 11 pp. Washington, D.C.

(28) WiEBE, A. H., and HESS, A. D.

1944. MUTUAL INTERESTS OF WILDLIFE CONSERVATION AND MALARIA CONTROL

ON IMPOUNDED WATERS. Jour. Wildlife Mangt. 8: 275-283.

(29) YEAGER, L. E., and RUTHERFORD, W. H.

1957. AN ECOLOGICAL BASIS FOR BEAVER MANAGEMENT IN THE ROCKY MOUNTAIN

REGION. N. Amer. Wildlife Conf. Transcript 22, pp. 269-299.

"tfr U.S. GOVERNMENT PRINTING OFFICE: 1967