Mosquitos and Disease: Halt the Assault

In 2003, a total of 5,087 horses and more than 9,000 humans were infected with West Nile virus (WNV), according to the USDA's Animal and Plant Health Inspection Service, the Center for Disease Control, and individual state statistics. The statistics are sobering. Yet WNV isn't the only threat posed by the common mosquito. All forms of arboviral encephalitis (arthropod-borne neurologic disease)--some of which, like WNV, can afflict both horses and humans--are mosquito-borne, as are malaria, dengue fever, and deadly canine heartworm disease. Mosquitoes can also cause severe allergic bite reactions in some people and animals. And, as everyone knows, they are annoying pests.

For most of us, defending ourselves against mosquitoes means keeping ourselves and our horses coated in bug repellent from spring to fall. But mosquito control experts say that's a bit like putting the dog to sleep to cure a case of fleas: The dog isn't suffering, but the fleas aren't gone, either.

To really tackle a mosquito problem, you need what people in the bug world call "integrated pest management." It's a holistic approach that encompasses biological and chemical tactics, environmental management, population control, and, finally, bite reduction. In a nutshell, it's a strategy that helps you reduce the overall number of mosquitoes on your property, so there are fewer to harass you, your horses, and the other people and animals on your farm.

Shut Down Breeding Grounds

Eliminating mosquito breeding areas everywhere you can is the first and arguably most important step in reducing your farm's mosquito population. In effect, it's source reduction: You're stopping the problem before it ever gets started.

Since mosquito larvae feed on microbes and decaying organic matter, and they need to breathe air, female mosquitoes lay their eggs in shallow, standing water, often preferring spots with plenty of vegetation in and around the water. According to the University of Illinois cooperative extension service, mosquitoes rarely lay eggs in deep lakes or ponds, on flowing water, or in areas with little vegetation. Thus, the core of your mosquito-management program is to eliminate all the standing water you can.

In some cases, that's as simple as draining and/or turning over things like empty wheelbarrows, unused wading pools, and other containers where water collects. Check tarps covering your trailer, shavings pile, and so forth to make sure no water collects in the folds of plastic. Dispose of or cover old/spare tires, which are notorious mosquito breeding spots.

For large containers you can't turn over or do away with, try drilling holes in the bottom so water will drain away. Make sure the water drains thoroughly and doesn't collect in a puddle. Mosquitoes can and do squirm to life in puddles and other equally small bodies of water that exist for more than four days, according to the Alabama cooperative extension service.

Similarly, keep roof gutters clean and free of debris so water can flow freely through them and out the downspout. Again, make sure there's sufficient drainage at the downspout to prevent puddles.

Of course, some bodies of water you simply have to live with, like drinking water for your horses. Luckily, you probably empty and refill water buckets often enough to prevent mosquitoes from breeding in them. Likewise, if you use automatic waterers, they generally have enough flow to make an inhospitable egg-laying environment. Water troughs, though, can be a prime breeding spot unless you dump and clean them on a regular basis; most mosquito-control experts recommend at least once a week.

Horse waterers aren't the only permanent bodies of water around the farm. If you have bird baths, clean them at least once a week, too. You can aerate ornamental pools to keep the water from stagnating, and you can make swimming pools uninviting to mosquitoes by keeping them clean and chlorinated.

Lawn Care and Landscaping

Ponds that you can't chlorinate or aerate pose a greater challenge. Focus on keeping the water's surface and the area immediately surrounding the water free of vegetation and debris. Since puddles, as noted earlier, are also a problem, inspect your property for low spots where water tends to collect and stay. You can eliminate smaller low areas--such as potholes in the driveway and urine holes in horse pens--by filling in the low areas so they're level with surrounding ground.

You might find a swale useful for draining more extensive depressions. A swale is similar to a ditch, about three feet wide and six inches deep, with a downhill slope that helps move water where you want it to go--such as to a flowing creek, drainage easement, or well-drained street, according to a fact sheet from the University of Georgia cooperative extension service that's based on their publication Best Management Practice for New Homeowners. You can accomplish the same goal with an underground drain pipe that has grated inlet holes to drain water from the surface. And in some situations, you might consider grading portions of your property so that water drains away. A professional landscape architect can help you with any of these options.

Another issue pops up if you irrigate pastures or cropland on your farm. Irrigation pipe leaks, standing water in control structures or fields, and clogged drainage ditches provide plenty of hospitable environments for egg-laying mosquitoes, notes the Colorado State University (CSU) cooperative extension service.

Belinda Thompson, DVM, senior extension veterinarian for the New York State Animal Health Diagnostic Laboratory at Cornell University, says, "We've learned that WNV transmission is (often) by the prairie mosquito (Culex tarsalis). This mosquito can breed in irrigation ditches. Its transmission is really intense, and it put a lot of horses at risk in Nebraska, Colorado, and the Dakotas in 2003, and will almost certainly be a factor in California next year."

Wetlands, such as marsh and swamp areas, pose a separate problem. By their very nature, they are ideal places for mosquitoes to breed and develop. In some cases, you might solve the problem by draining these areas. However, not only can this be an expensive and time-consuming task, but local wetland-protection regulations might prohibit it. If you have swamps or marshes on your property, talk with your local cooperative extension service, department of natural resources, or department of environmental protection for advice pertinent to your area.

Put Biology on Your Side

Even the most diligent attention to habitat control won't eliminate all mosquito breeding spots; some eggs will still be laid and turn into larvae. Your next stage of offense centers on attacking the larvae before they can turn into full-grown mosquitoes. Again, your efforts will focus on areas of standing water--particularly places that you cannot easily drain, aerate, or keep clean.

Several biological controls exist to help in this task. These include microbial larvicides, the most common of which is probably the bacteria BTI (Bacillus thuringiensis israelensis), available in solid or granular form in such products as Bactimos, Teknar, Vectobac, and Healthy Ponds Mosquito Control. "Donut" forms are also available and are often called mosquito dunks. According to a CSU extension bulletin, "BTI is toxic only to mosquito and midge larvae. It is not hazardous to non-target organisms, but can reduce midge populations that serve as fish food."

Other common biological controls include natural larvae eaters, such as fish, dragonfly nymphs, and diving beetles. The so-called mosquito fish (particularly Gambusia affinis, native to the southeastern United States) and killifish (of the family Cyprinodontidae) are most often suggested for mosquito control. In addition, the Environmental Protection Agency (EPA) notes that Mesocyclops longisetus, a predacious copepod (tiny crustacean), also preys on mosquito larvae.

These predators might not eat just mosquito larvae, but could attack some you want around. According to the EPA, for instance, mosquito fish eat tadpoles, other fish eggs, aquatic insects, and zooplankton. Realize, too, that if the fish you select are not native to your area, they might not thrive or survive cold, requiring annual repopulation. Other concerns are introducing new species that destroy the habitat of current fish species. The University of Illinois cooperative extension service recommends contacting your state department of natural resources for assistance in selecting fish and other natural predators that are suitable to release in your area.

The Environmental Protection Agency notes that other biological controls are being explored. These include Lagenidium giganteum, an entomopathogenic (causing disease in insects) fungus that is registered under the trade name Liginex, but is not readily available to the public; Nosema algerae, a pathogenic parasite of mosquito larvae whose use is still being researched; and entomoparasitic (parasites of insects) nematodes such as Romanomermis culicivorax and R. iyengari are available and effective but, according to the EPA, "are not easily produced and have storage viability limitations."

Chemical Warfare

Besides biological controls, you have several chemical options for battling mosquito larvae. These include what the CSU extension service refers to as "soft chemical insecticides," such as methoprene. Available in products such as Altosid and Pre-Strike, methoprene is an insect growth regulator. It works by creating hormonal imbalances that inhibit larval growth. According to CSU, the products are toxic to insects and other arthropods, but won't harm fish or other wildlife--except for potentially limiting the amount of food (insects) available to them.

Chemical larvicides, which kill larvae, are common weapons. Cornell's Thompson says typical bacterial BTI dunk-type larvicides (which float on your pond) are believed to be safe for horses, and according to the Environmental Protection Agency they are okay to use in water that is likely to be drunk by horses or humans. And while many of these products are available at hardware, garden, and pet stores, Thompson recommends talking with your local department of environmental protection for advice on appropriate larvicides. Likewise, CSU advises working with a licensed pest control expert when using larvicides on your property.

Of course, you should always carefully read product labels before purchasing any chemical or biological agents to make sure they are non-toxic for horses and other animals. Then closely follow label directions for maximum safety and effectiveness.

Square Off Against the Adults

As with habitat management, your anti-larvae efforts probably won't eliminate 100% of your larvae, which means you'll still have to deal with adult mosquitoes. As with your larvae-control tactics, your efforts to reduce the number of grown mosquitoes on your farm can start with biological allies.

Bats, frogs, dragonflies, and some bird species enjoy dining on mosquitoes. Not everyone is gung-ho about bats because they are a public health hazard. For one thing, these nocturnal insect predators can carry rabies, which could be transmitted to your horse or yourself through a bite. (An equine rabies vaccination is available--but no vaccine is 100% effective.) Plus, mosquitoes won't be the only bugs that bats will eat; they could snack on beneficial insects, too. In fact, some experts say that mosquitoes make up only a minor portion of a bat's daily diet.

Birds also eat good bugs along with mosquitoes. In addition, more than 130 species of birds have been shown to carry West Nile virus (WNV). While a bird can't pass the virus directly to your horse, a mosquito can pick it up from an infected bird and subsequently infect your horse.

Another control tool is simple yard care. Since mosquitoes enjoy resting in dense vegetation, keep your grass, weeds, trees, and other flora neatly trimmed to make your property less inviting to adult mosquitoes.

Bring Out the Big Guns

Most experts agree that biological controls work best when used in conjunction with other anti-mosquito efforts. Primarily, that means as a complement to chemical insecticides (sometimes called adulticides, since they kill adult insects rather than larvae). In general, chemical insecticides fall into two categories--the kind you spray in the air and the kind you spray directly on your horse. In both cases, notes Thompson, "It's only safe to use those products that are labeled as safe for horses."

Products sprayed into the air--which include pyrethrins and malathion--can be applied with ultra-low volume (ULV) foggers, either hand-held or attached to a tractor or riding lawn mower. However, experts generally agree that the application of these insecticides is best left to licensed operators and/or to city or county officials. That's partly because of the potential health hazards stemming from improper use and partly because of the complexities involved with effective application. For instance, North Country Pest Control notes that temperature, wind, and time of day can have a major influence on the product's effectiveness.

On the other hand, you can easily find insecticides to spray on your horse at most tack shops. Many products that we think of as "fly spray" are effective in not only repelling, but also killing mosquitoes. Just read the labels to learn which make this promise, then follow directions for proper use. Thompson notes that you can also spray these insecticides on the walls of your stalls, barns, and sheds.

"Once a mosquito bites, it sits on walls to digest its blood meal, and it can't fly far when it's full," she explains. If they alight on an insecticide-coated wall, she adds, these sprays are effective for helping reduce their population.

The mosquito trap is another tool invented to kill adult mosquitoes. It's designed to attract insects--using light, carbon dioxide, or a combination of the two--then trap or kill the bugs on landing. Most experts discourage the use of traps because studies have shown they don't reduce mosquito landing or biting, and they kill at least as many good bugs as bad. Furthermore, says Thompson, "There have been instances where traps designed to attract insects were shown to raise the population of insects (in the area). If an attractant or trap is used, I advise people to place it as far from the barn as possible."


The last stage of your integrated mosquito control program involves barn and farm management tactics. These are designed to reduce the chances that you and your horses will get bitten by whatever mosquitoes survive your other attack strategies.

Timing--Avoid the outdoors at dawn and dusk, which tend to be the busiest hours for mosquito activity, recommend experts. Furthermore, according to the Alabama cooperative extension service, "A recently completed study of WNV suggests that keeping horses in stalls at night may be helpful in reducing their risk of infection."

Lights--Light attracts mosquitoes, so the less artificial lighting you have around your horses, the safer they'll be from mosquitoes. In fact, an Ohio State University (OSU) extension bulletin suggests placing lights around the perimeter of your stable area to draw mosquitoes away from your horses. Another option might be to use a different type of bulb. The OSU bulletin notes that mosquitoes are less attracted to black lighting than regular "white" light. And Thompson says some publications claim that fluorescent lights attract fewer mosquitoes than incandescent bulbs because they give off less heat. But, she adds, that finding has been questioned.

Screens--Thompson says that screens "are a very effective means of controlling mosquitoes if they're applied before mosquitoes begin to breed in the barn." (Along with screens, you'll need to seal up cracks and crevices in the barn to keep mosquitoes out.) Of course, you'll need to open doors to let horses and people in and out of the barn, so some mosquitoes can sneak in. To handle these invaders, consider adding a misting system that shoots insecticide into the barn at timed intervals.

Fans--Mosquitoes don't fly well in a breeze. So, says Thompson, "Fans actually do help." In fact, she adds, fans can keep mosquitoes and other types of flying insects off your horse.

Repellents--Insect repellents that you apply directly to your horse are perhaps the defense most familiar to horse owners. Be aware that not all repellents on the market are effective against mosquitoes; read the label closely to learn which insects it's designed to repel and how to safely use the product. Repellents containing a pyrethroid compound--most commonly permethrin--are noted by the USDA's Animal and Plant Health Inspection Service as offering superior safety and repellency. This chemical also works as an insecticide. You can also find numerous biological (herbal)
repellents on the market. Citronella is one common herbal repellent, and a botanical extract called geranoil has proven in some studies to have an effectiveness close to that of DEET (N, N-diethyl-m-toluamide), the gold-standard of mosquito repellents for humans. (For more information, see "The Death Squad" on page 59.)

Protective apparel--Fly masks, sheets, and leg wraps provide a barrier between mosquitoes and your horse's skin. (Light colors are less attractive to mosquitoes, notes the CSU extension service.) You can increase the effectiveness of anti-fly horse clothing by spraying insecticides or repellents directly onto the fabric, states Thompson.

However, she cautions, "Remember that your horse needs protection over his entire body. Using a fly mask and fly sheet that leaves his legs and belly (and neck) bare may not offer adequate protection." She recommends using spray- or wipe-on repellents or insecticides as a complement to the clothing.

Look Beyond the Home Borders

By now, you're probably thinking you have the mosquito problem under control. And you would--if mosquitoes didn't travel. Unfortunately, while mosquitoes don't fly great distances, their mile-or-less roaming pattern can be enough to get them from neighboring farms onto yours, which means you could be doing a bang-up job of mosquito control and still suffer from a neighbor's negligence.

"If you share a fence line and your neighbor has a pile of tires along the fence with mosquitoes breeding there, your own actions may not protect your horses," says Thompson. "It's reasonable in a situation like that to offer to help your neighbor clean up." As long as you remain diplomatic with your mosquito-management suggestions and/or offers of assistance, the results should be a win-win for all horses and humans involved.

Similarly, any time you take your horses off your property, whether for a trail ride or a show, you put them (and yourself) at the mercy of someone else's mosquito control strategies. Because of that, it's always a good idea to remember anti-insect apparel as well as mosquito repellents for yourself and your horse.

Just as important, plan ahead by making sure your horse is vaccinated against WNV as well as the Eastern and Western strains of equine encephalitis. Your veterinarian can recommend the most appropriate vaccination schedule for your locale and your horses.

Remember that vaccinations, while important, are just one tool for protecting your horse. It's the whole picture--your comprehensive, integrated management strategy--that will ultimately win the mosquito war and make your farm a safer, more comfortable place for humans and horses alike.


The following resources offer additional information on mosquito management and mosquito-borne diseases:

About the Author

Sushil Dulai Wenholz



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