10 Cryptic Mosquito Breeding Hotspots

During the inspection of a property for mosquito breeding sites, we are taught to “look for anything that will hold water.” To do this properly takes a trained eye and patience. However, sometimes the sources of standing water are not obvious. This article describes 10 such sources and briefly discusses what can be done about each of them.

Depending on where in the United States you live and operate, the primary mosquito species that will be in these types of breeding sites are the Asian tiger mosquito (Aedes albopictus), the Asian bush mosquito (Ae. japonicus), the yellow fever mosquito (Ae. aegypti) and the house mosquito (Culex pipiens/quinquefasciatus).

TIRES. Any tire holding a small amount of water as well as some organic matter, which provides a food source for mosquito larvae, will breed mosquitoes in large numbers (Fig. 1). This is particularly true for the Aedes species listed above. Use a plastic turkey baster (Fig. 2, right, and Fig. 3) to sample the tires for the presence of immature mosquitoes. Also, a slight kick on the tires may cause adult mosquitoes to fly out. The best practice is to remove the tires completely. Other measures include: (1) Keeping them covered or stored so they do not collect water; (2) Treating them with a mosquito larvicide; (3) or drilling holes in the tires large enough to allow all the water to regularly drain out.

PLANT DRIP SAUCERS. Customers often use drip saucers under plants, indoors and outdoors, to protect surfaces such as decks from water stains. These saucers will accumulate water, either from rain or watering. Mosquitoes may lay fresh eggs in these saucers or the saucers may contain drought-resistant eggs that will hatch just a few minutes after being flooded. Simple fixes include: (1) Emptying the saucers completely once a week; (2) replacing the saucers with small rocks or bamboo sticks under the pot so the water drains out of the plant completely; or (3) putting a small amount of mosquito larvicide in the saucer.

CLOGGED GUTTERS. Leaves and other debris can result in gutters and drain pipes becoming clogged. Even though these sources may be high off the ground, mosquitoes will still lay eggs in them. It is a myth that mosquitoes will rarely fly more than 8-10 feet off the ground. Regular cleaning of gutters, ensuring proper flow of water, is the best remedy. If this cannot be done, a mosquito larvicide can be applied according to label directions. It is important that the gutters be level so the larvicide is equally distributed. Some industrious companies offer an add-on service to clean gutters, usually just on the first story of a structure. Proper training on the use of ladders is essential.

LIPS ON VARIOUS ITEMS. Sometimes we can create our own problems by telling a customer to simply turn over an object, thereby draining the water. This includes things such as wheelbarrows, buckets, recycling containers, kiddie pools and trash cans. However, some of these have a lip that will collect water when the item is flipped over. These lips may not hold a great deal of water, but remember that mosquitoes can develop in a bottle cap! Storing these items in ways that they will not collect water will minimize this risk.

TARPS. Tarps are used to cover and protect many things such as boats, swimming pools, motorcycles and outdoor grills. They can produce lots of hungry mosquitoes in two different ways. First, tarps in use usually sag a bit and can then hold water and organic debris. Second, when tarps are removed, rarely are they folded up neatly and put in the storage shed! No, usually they are hastily balled up and thrown somewhere nearby. Remember that water collecting on a tarp will become superheated, which will make the mosquito life cycle speed up. Proper use and storage of tarps will prevent them from becoming a public health nuisance!

IN-GROUND DRAINS. Some customers, especially those with well-manicured lawns, may have in-ground drains present on the property. These can be prolific mosquito breeding sites, especially for the house mosquito, because they almost always hold some water and they regularly accumulate organic debris. One easy way to determine if a drain is breeding mosquitoes is to simply cover it for a night or two with a collecting device or a modified glueboard. Any emerging adults from the water in the drain will be captured. Drains positive for mosquito breeding can be covered with hardware cloth or similar material that will prevent female mosquitoes from entering and laying eggs. It will also prevent them from emerging from the drain. If this is not feasible, the drains can be treated with a mosquito larvicide, per label directions.

CORRUGATED PLASTIC TUBES. We encourage customers to use corrugated plastic tubes (Fig. 4) or similar methods attached to downspouts to pull water away from a structure’s foundation. Invariably, the ends of such tubes curl up, thereby collecting and holding water and organic matter. Voila! You now have not only a wonderful mosquito breeding site but also a great resting site for adult mosquitoes because inside it is cool, shaded, moist and out of the wind. Best practices here include regularly removing and draining the tubes or drilling holes in the bottom of them, out near the end, to allow standing water to drain. Alternatively, they can be treated with a small amount of larvicide if it is a persistent problem.

FLAT ROOFS. Flat roofs, especially on commercial structures, can collect and hold enough water to produce mosquito breeding. Examples include parking structures, utility buildings and maintenance areas. This is particularly true in areas that receive regular rainfall. Often, the water will collect in one corner or on one side of the roof. If the roof cannot be reengineered or modified to eliminate the standing water, it can be treated with a mosquito larvicide. There are long-lasting (up to 90 days) formulations that are well-suited for such a service.

PLANTS. Water-holding plants, such as bromeliads, can have many small pockets of water that will breed mosquitoes. The author once removed, using my handy-dandy turkey baster, an average of 150 mosquito larvae per sample from plants at a residence in Beverly Hills (Calif.)! Water-holding plants are often found at facilities such as botanical gardens, museum grounds, zoos, golf courses, etc. Removal of the plants, which can be expensive, is usually not an option. Treatment can consist of simply power spraying the plants once a week to flush out the larvae or treating them with a small-granule larvicide that can be dispersed using a hand fertilizer.

TREE HOLES. Tree branches and trunks can break off or rot out, leaving a hole that easily accumulates water. These tree holes, which are often hard to see, are attractive to several different species of mosquitoes. They can hold lots of water and organic debris, and they are almost always free of any predators. Tree holes can be filled with suitable expanding foam, which will prevent water collection. Alternatively, they can be treated with larvicide if they are accessible. Do not fill the holes with concrete or a similar material, and do not drill drainage holes in them, as this will damage the tree.

SUMMARY. Cryptic (hidden) mosquito breeding sites are common, especially on properties that are not well-maintained. During inspection, great care should be taken to look for them and remember that they may not be obvious. As with many of our pest issues, proper customer communication and cooperation can go a long way toward solving mosquito problems. Look hard, look often and happy hunting!

Stanton Cope is director of technical services for Rose Pest Solutions, Chicago. He is past president of the American Mosquito Control Association (AMCA), spent almost 24 years on active duty in the United States Navy as an entomologist and has been researching mosquitoes for more than 45 years.