UC Mosquito Research Program: Managing Mosquitoes on the Farm

University of California Division of Agriculture and Natural Resources
Publication 8158
By Sharon P. Lawler and Gregory C. Lanzaro, Department of Entomology, UC Davis

Mosquitoes on the farm can cause a variety of problems: they carry illness to humans, harm livestock, and reduce your property's value. Learn the basics of agricultural mosquito control.
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Why You Should Care About Mosquito Control?

A bumper crop.
(Photo by Chet K. Fukushima)

Most people dislike mosquitoes and are aware of the diseases and discomfort that they cause. While a minority of growers accepts mosquito problems as a part of rural life, there are many excellent reasons why everyone should pitch in to minimize mosquito problems in their area.

First, mosquitoes carry diseases to people. Your family, neighbors, and employees are particularly susceptible to mosquito-borne diseases because farming operations may create places where mosquitoes can breed and live. West Nile virus and other types of mosquito-borne viruses are present in California. These viruses can cause infections that may be fatal or lead to permanent brain damage. Other serious mosquito-borne diseases such as malaria and dengue fever are introduced into the state periodically by travelers. Keeping mosquito populations under control helps keep these diseases from becoming established.

Second, mosquitoes threaten livestock and pets. Animals tormented by mosquitoes do not feed properly. If attacked by mosquitoes, cows give less milk, and beef cattle, sheep, and poultry may suffer losses in weight. Mosquitoes may transmit viruses that cause encephalitis in horses. Horses infected by West Nile virus have over a 30 percent chance of dying or becoming disabled. Mosquitoes also transmit dog heartworm and can also carry the virus that causes myxomytosis in rabbits. See your veterinarian to get your animals protected against these diseases.

Third, mosquito problems can decrease property values and cause labor problems. Areas infested with mosquitoes are less desirable places to live and work. Farmworkers may refuse to work if a serious mosquito problem exists. Homeowners and their families and guests find it difficult to enjoy barbecues, ball games, gardening, or other outdoor activities when plagued by mosquitoes.

West Nile Virus

What Is It?

West Nile virus is a virus carried by mosquitoes. Viruses are tiny microorganisms (smaller than bacteria) that inject their genes into cells. The infected cells produce more viruses. Antibiotics are not effective against viruses.

About 80 percent of human West Nile virus infections are so mild that there are no symptoms. About 20 percent of infected people experience West Nile fever, which produces flu-like symptoms (headache, nausea, fever, and sometimes a rash or swollen glands). However, one-half percent of those infected may experience life-threatening or disabling consequences including severe fever and headache, disorientation, paralysis, numbness, convulsions, vision loss, coma, or death. People with severe symptoms should seek medical care immediately. Persons over 50 years of age or those with compromised immune systems are at higher risk of complications.

How Is It Transmitted?

The virus is mainly transmitted by mosquitoes in the genus Culex, but other kinds of mosquitoes may transmit it as well. The primary life cycle of this virus is between birds and mosquitoes. Birds can produce high levels of the virus in their blood, and mosquitoes pick up the virus when they feed. Mosquitoes that feed on infected birds can transmit the virus to humans, horses, and other mammals.

During the brief period that the virus is circulating in the human bloodstream, it is possible (but rare) for it to be transmitted from human to human through blood transfusions, organ transplants, or even breast feeding of infants. In California, the blood supply and organ donations are now screened for West Nile virus, and the risk of infection from these sources is very low. Nursing mothers who think they have symptoms of West Nile fever should talk to their physicians. The virus is not transmitted through casual contact, such as touching, kissing, and coughing.

It is unlikely that mosquitoes would carry the virus from horses to humans or between horses, because the level of virus in mammal blood is too low. Therefore, euthanizing infected horses does not prevent the spread of the disease.

Who Is at Risk?

Anyone suffering mosquito bites is at risk of this disease.

What Animals Are at Risk?

Horses and wild birds are the animals that are most susceptible to the virus. Fortunately, domestic poultry and livestock other than horses very rarely show symptoms of infection. One-third of infected horses show severe symptoms and either die or are euthanized. A vaccine is available for horses. Vaccines against other encephalitis viruses do not work against West Nile virus.

You May Be Raising the Mosquito That Is Biting You! Mosquito Prevention

The mosquito life cycle. Egg rafts are typically dark and smaller than a pinhead. Larvae and pupae are about the size of a grain of rice, or slightly larger.

If you minimize mosquito production on your farm, you protect your family, riends, and employees from disease and keep your livestock healthy. You can't grow crops or livestock without water. You can't grow mosquitoes without water, either—young mosquitoes hatch and develop in water before they mature into flying insects. Although some areas experience problems with mosquitoes flying in from longer distances, locally grown mosquitoes usually cause the most problems. Many mosquitoes stay in the general area where they matured, especially if there is water and animals to feed upon.

Water

Mosquitoes breed in standing water, that is, any body of water without turbulent flow. They do best in weedy ponds and stagnant water that collects in ditches, fields, or containers, and they require water for just 4 to 14 days to mature (but sometimes longer in cold weather).

Three Basic Principles of Mosquito Prevention

Prevent or eliminate any wastewater that stands for longer than 4 days. This includes water in ruts or unnecessary containers.
Keep weeds down around ponds, in ditches, and in shallow wetlands.
Irrigate properly so that all surface water is gone within 4 days.
Biological and chemical control of mosquitoes can supplement these essential preventative measures.

You can gain some personal protection by wearing long trousers, long sleeves, and using repellant. DEET-based repellants are very effective but it is important to follow the label directions to ensure safe and successful use. To protect the home, make sure that screens are intact and doors are kept closed. Minimize outdoor activity during dawn and dusk, the peak periods of mosquito biting.

The remainder of this publication gives advice for specific situations, discusses biological and chemical control of mosquitoes, and describes common mosquito species. Your local mosquito abatement district or mosquito and vector control district (MVCD) can also help you address specific problems. Consult them on which methods of supplemental control are most effective against the mosquitoes in your area.

Natural Waters Associated with Farms

Growers and dairy operators often serve as stewards of natural waters because ponds, streams, and other wetlands can occur on farmland. Natural waters support a variety of animals including ducks, fishes, amphibians, dragonflies, and fairy shrimp. Managing mosquitoes in extensive wetlands is beyond the scope of this publication (see Managing Mosquitoes in Surface-Flow Constructed Treatment Wetlands, (PDF* 373K), ANR Publication 8117). This section supplies a few principles of mosquito prevention that are useful in most natural waters.

Permanent, clean natural waters produce relatively few mosquitoes because these have good populations of predatory insects and/or fish. However, significant mosquito problems can occur in waters that are very weedy and shallow, especially if they receive fertilizers or manure. Therefore, such runoff should be prevented through proper irrigation drainage, conservative use of fertilizers, and creating buffer areas between fields and wetlands.

Unlike stagnant waters created by farming activities, it is undesirable and usually illegal to drain, fill, or substantially reconfigure natural wetlands, and some wetland vegetation is also protected. Private citizens may not introduce fish into natural waters. Changes can sometimes be made under state or federal permits. Consult your local MVCD about what changes are needed and permitted. Several kinds of alterations reduce mosquito problems. Weed control allows natural predators to hunt mosquito larvae more effectively. For example, thin rushes and cattails annually and remove old leaves. Shallow, temporary wetlands can be mowed when dry. If you have a stream on your property that forms isolated, stagnant pools, connect the pools to the main channel with small ditches. This allows predatory insects and fish to find and eat mosquitoes and can aid stream flow, which drowns mosquitoes.

Some coastal farms have tidal areas that produce mosquitoes when flooded by the tides or rain. Manage this problem by constructing and maintaining ditches that drain off the water when the tide falls. You may need to add a levee with a tide gate to reduce or prevent salt water from entering during high tide, while allowing fresh water to flow out at low tide. MVCDs may use insecticides to control marsh mosquitoes, but this can become costly in extensive areas of marsh. In such cases, physical control is preferable unless the presence of endangered species or other concerns prevent land reconfiguration.

Managing Stagant Waters Created by Agricultural Activities

Impoundments

Impounded water can be a source of mosquitoes, but a few simple approaches will help reduce mosquito breeding sites. For example, a reservoir will not produce mosquitoes if its sides and bottom are properly graded, there is little emergent vegetation, and it is stocked with insect-eating fish. In general, a steep-sided pond of at least 3 feet (1 m) in depth (to cover bottom-growing weeds) offers the least problem. Destroy weeds around the margin of the pond to help eliminate hiding places for larvae. Maintain a stable water level to minimize the area where floodwater mosquitoes can lay their eggs. Equip water sources with float valves to maintain the water level.

Miscellaneous Breeding Habitats

Many items create areas where mosquitoes can breed: leaky irrigation piping, old tires, birdbaths, tarps that form puddles, clogged gutters, paint cans, boats or wheelbarrows stored upright, open garbage cans and lids, and pet food bowls. Inspect your property and fix, clean up, invert, puncture, or regularly flush out any of these potential problem areas. Because they typically contain few or no predators, each of these small habitats can produce hundreds of biting mosquitoes.

Poorly Maintained Cesspools and Septic Tanks

Septic systems can produce droves of mosquitoes. A properly constructed and maintained cesspool or septic tank should have an adequate soakage pit, cover, vent, and overflow outlet. Check your local health department requirements.

Road Ruts and Potholes

Even relatively small road ruts and potholes can produce mosquitoes if water stands in them during in warmer weather, because these waters contain few predators.

Treeholes

Treeholes like this one may provide habitat for mosquitoes. (Photo by Jamesina Scott)

Rain water that accumulates in treeholes can provide an ideal place for the western treehole mosquito to develop its larvae, usually in winter and spring. This mosquito carries dog heartworm, and dogs should be protected against this serious disease. Treeholes can occur in old orchard and landscape trees as well as in wild trees. Holes may be caused by rot, fire injury, or from pruning that leaves a hollow at the center of the tree. Fill treeholes with cement or other substances. Keep in mind that treeholes may provide bird nesting habitat during the drier months; it may be preferable to drill drains from the base of the treehole to the outside and remove debris that clogs the holes. Consult an arborist about treeholes in valuable or potentially hazardous trees. Methoprene pellets added to holes prevent mosquitoes from maturing (see the section Chemical Control); add these before the holes fill or anytime before the mosquitoes pupate and renew them annually.

Livestock Watering Ponds

Ponds and natural pools that livestock drink from may become serious mosquito-breeding areas. Animal waste adds nutrients to the water that benefit mosquitoes, and hoofprints create puddles where mosquitoes may be isolated from predators. If feasible, fence ponds along the edge to create a single access area. This keeps the animals from trampling the entire edge, although if too much room is left between the pond and fence, animals may still create ruts. Paving the access point can reduce puddles, but be sure to provide good footing for the animals. Weed control reduces hiding places for mosquitoes. It may be necessary to treat these ponds regularly with a biological larvicide or to introduce insect-eating fish if they are not already present. However, do not introduce fish unless the pond is man-made and isolated from natural waters.

Watering Troughs

Troughs and other containers that provide water for animals can easily become a source of mosquitoes. The ground surface around the troughs often becomes roughened by animal hooves, producing hundreds of small water pockets. Permanently correct this potential source of mosquitoes by providing drainage or paving the area. Temporary improvement can be had by flushing out the water every week if drainage outside the trough area is good, or by treating the water with a biological larvicide. Do not add chemical insecticide to water that livestock may drink. Clean cattle tanks and watering troughs regularly to prevent mosquitoes from multiplying. Mosquitofish may also be added to provide control. Remove or destroy troughs or tanks that are not in active use. -more-