How to Manage Pests

Mosquitoes

Managing Mosquitoes on the Farm Section 7: Irrigated Fields for Upland Crops Published 2005 Sections of this publication:

Why You Should Care About Mosquito Control West Nile Virus You May Be Raising the Mosquito That Is Biting You! Mosquito Prevention Three Basic Principles of Mosquito Prevention Natural Waters Associated With Farms Managing Stagnant Waters Created by Agricultural Activities Irrigated Fields for Upland Crops Mosquito-Free Irrigated Pastures Rice Dairy Operations Biological and Chemical Mosquito Control Common Mosquitoes and Their Life Cycles Acknowledgments For More Information

Good agriculture results from the efficient use of water, and efficient use of water reduces mosquito populations. The larval and pupal stages of all mosquitoes require water for their development. It is here that you can break their life cycle most effectively.

Fortunately, the irrigation practices that are best for upland crops do not generally produce mosquitoes. Most crops thrive when water remains on the surface for no more than 12 hours, but even the most rapidly developing mosquito species need 4 days in water to reach the adult stage. Fields may be irrigated without creating mosquito breeding sites by using either flooding or sprinklers as long as the fields are properly graded and do not hold water longer than 4 days. Water standing on fields for excessive periods of time reduces crop yields, encourages the growth of weeds, presents an unhealthy environment for livestock, and may decrease soil quality, in addition to producing mosquitoes.

Increase the efficiency of irrigation on your farm and at the same time decrease your mosquito problem by following three rules of water management:

• Prepare the land so that slopes are uniform and drainage is good.
• Inspect and correct areas with poor drainage.
• Do not overirrigate, as this can create standing water. For example, if you irrigate by flooding it may not be necessary to run the water until it comes out of the bottom check. Running the water until two-thirds of a field is covered may supply enough water to spread to the remaining one-third of the field, although some trial and error may be needed.

Land Preparation

Proper land preparation for both irrigation and drainage is the key to successful water management. Correctly leveled fields help control mosquitoes and also produce higher yields at less cost. On good slopes you can apply irrigation water more efficiently, lowering costs for water and irrigation labor. On these slopes you can grow more uniform crop stands that can be harvested more easily. Laser planing is currently the most effective way of leveling fields.

A field that is properly leveled will have a continuous slope in the direction of the irrigation runs. Slopes of at least 0.2 foot per 100 feet (6 cm per 30 m) are desirable for most crops. Where flatter slopes must be used on soils that do not take water readily, extreme care should be taken that no low spots occur in the field. When available, a slope of 0.3 to 0.5 foot per 100 feet (9 to 15 cm per 30 m) is preferable on pastures. If possible, level the land so there is a slight side fall across the field. This facilitates the delivery of irrigation water through the field ditches and helps collect wastewater in the lower corner of the field.

It is the practice in some areas to make the lower sections of the irrigation run perfectly flat, which spreads out excess irrigation water that reaches the end of the field. This is a desirable practice only in areas where the soil takes water readily, and it should not be used on soils where water penetration is slow. Standing water not only causes mosquito problems, it can "drown out" crops in the lower ends of fields. In such cases it is better to increase the slope rather than reduce it and to include a small drainage ditch at the ends of the irrigation runs.

Strip (Border) Checks Versus Contour Checks

Strip, or border, checks are small levees that create rectangular bays on fields with good slopes. Contour checks are levees that follow the contours on relatively flat land, producing irregularly shaped bays. In general, fields that are irrigated with contour checks are a greater source of mosquitoes than those irrigated with strip checks, because contour checks are used on flatter grounds that are prone to ponding. Also, contour checks are often flooded sequentially, and flows between checks are slow. This allows mosquitoes to continue developing as they move with the irrigation water from check to check. Either kind of check can provide mosquito habitat if water is left in the bay too long, and waterlogging can be a problem if bays are too large relative to the water flow rate.

Strip checks

Strip checks are usually preferred when minimum slopes are available of 0.25 foot per 100 feet (7.5 cm per 30 m) on soils with good vertical drainage, or 0.5 foot per 100 feet (15 cm per 30 m) on more impervious soils. The land between the levees should be perfectly cross-leveled so water will spread uniformly as it moves down the field. The slope must be continuous, although it doesn't have to be perfectly uniform.

Contour checks

Contour checks are most often used where the general slope of the land is less than 0.1 foot per 100 feet (3 cm per 30 m). If the slope is greater than this amount, growers generally prefer the strip check method. Contour checks can be used if relatively large heads of water are available, the soil takes water slowly, and the land is fairly flat.

The contour method of irrigation consists of constructing levees on contour lines at vertical intervals of generally 0.2 or 0.3 feet (6 to 9 cm). Control gates are placed in the levees to hold the water until the contour basins are irrigated and to serve as a spillway to prevent water from overtopping the levee. The usual practice is to hold the water in each basin until the area is completely flooded and then open the control gates and release the water to the next lower check.

The main difficulty in using this method, for both crop production and mosquito control, is removing the excess water from the field following each irrigation. A small amount of land leveling may be needed to fill in the natural depressions that occur in the field. Corrugations made in the direction of the slope of the land before the levees are constructed help drain the water into the borrow trench. A float is often used to smooth and partially fill these trenches.

To minimize mosquito problems created by wastewater standing in contour checks, many growers use a broad, shallow ditch down the center of the field. This serves as a delivery ditch for the irrigation water and as a drainage ditch for wastewater. Constructing this ditch with a bucket scraper and removing the excavated soil leaves no spillbanks to dam the water. The fields are generally laid out so that ditches occur at intervals of not over 660 feet (200 m), or so that the drainage water from the field will not have to travel over 330 feet (100 m) to enter a ditch. Control gates are constructed at the points where the ditch crosses the levees. The crest of the control gates is on the same grade as the bottom of the ditch. Flash boards in the gates are used to check the flow of water during irrigations. When the flash boards are removed, the ditch carries off most of the excess water.

Field ditches

Ditches that hold water for more than 4 days can be a source of mosquitoes. Water can remain in a ditch constructed in heavy soil if the ditch has no drainage outlet, if it is not constructed on a continuous grade, or if it is not properly maintained. Clumps of debris and footprints can create puddles.

A properly constructed head ditch can be maintained more easily than one that is built poorly. You may want to place "ditch pads" along the line where the ditch is to be constructed. This should be done while the land is being leveled. A ditch pad is a mound of earth 8 to 12 inches (20 to 30.5 cm) high and 10 to 16 feet (3 to 5 m) wide. The excess earth permits building strong ditches with well-shaped banks. The bottom of the ditch can be maintained near the elevation of the normal ground surface, which permits easy draining of the excess water from the ditch after the irrigation is completed.

You may also try the practice of "plowing in" the field ditches following each irrigation. Plowing in allows cultivation equipment to turn more easliy. You can rebuild the ditches prior to each irrigation. This practice prevents field ditches from becoming mosquito habitat.

Leaky head gate and turnout structures create small pools of standing water. These pools often contribute greatly to mosquito problems. Take care during construction to make sure that water cannot escape around or under these structures and that the gates fit tightly enough to hold the water. Such precautions can often save sufficient water to pay for the cost of the structures.

Drainage

In many areas, irrigated farms with inadequate drainage produce most of the mosquitoes. "No till" systems typically have poor drainage due to low percolation and soil compaction and need extra care in water management. Surface drains can remove excess water that collects at the lower ends of irrigated fields. It may be impractical to prevent the escape of some irrigation water from the fields. The amount of water wasted may be small, but if ignored it can produce many mosquitoes.

Drainage systems installed in some areas collect wastewater from farms and remove stormwater during rainy periods. The wastewater can often be diverted into canals at lower elevations and reused for irrigating other lands. In areas where drainage outlets are not available, growers may wish to jointly construct a drainage system.

Return flow systems

Reusing wastewater is one of the most effective means of controlling mosquitoes. It eliminates standing water from the lower end of a field, which is frequently a trouble spot, particularly on irrigated pastures, and it also conserves water. Excess irrigation water is collected in a sump at the low point in the field. A low-lift pump lifts the water into a pipeline that delivers the water back into an irrigation system. The water can be used for irrigating other land close to the sump.

The main expense of a return flow system is for the pump and pipelines. But because of the low lift, power costs to operate the pump are low. This "waste" water is usually good enough for irrigating crops. It often contains fertilizers that can be saved by reusing the water. Also, its warm temperature is favorable to growing crops.

Managing Mosquitoes on the Farm, UC ANR Publication 8158 Sharon P. Lawler and Gregory C. Lanzaro, Department of Entomology, University of California, Davis

Copyright © 2005 The Regents of the University of California, Division of Agriculture and Natural Resources. All rights reserved.