Bothersome Bloodsuckers

Mosquitoes are a maddeningly common and widespread pest, which can bother people from all walks of life. They cause irritation, allergic reactions, and can transmit many dangerous diseases. When people have issues with this pest, they would like to have a pest management professional (PMP) come out and do something to stop the biting. However, this is not a simple task. Mosquitoes can establish a population anywhere, and this should be kept in mind when attempting to control them. When a PMP sets out to control mosquitoes, it should be expected that there will be a great deal of searching involved in the treatment. This is because the PMP must locate the source of the mosquitoes. As with many pests, mosquito control succeeds mostly through the treatment and removal of harborages. As a PMP, there are several questions you should consider when you are treating for mosquitoes.
 

Where do they come from? Mosquitoes spend a majority of their lives underwater. Mosquito eggs, larvae, and pupae are all aquatic, requiring access to water in order to survive. Adult female mosquitoes lay eggs in or around standing water during warm and moist times of the year, usually right after a rain. After about one week these eggs will hatch into larvae and feed on the particulate matter floating in the water. Another week later the larvae have grown large enough to become pupae. Pupae don’t feed, and instead float around in the water for around 24 hours until they become adults. Adults emerge on the surface of the water, and fly away to mate once their wings have hardened.

Anything that has had water standing for more than one day is likely to contain mosquitoes. This is especially true if the water has become stagnant and clouded. Any source of standing water could become a source of mosquitoes, no matter how small. They could nest in both naturally occurring areas and those left by humans. Examples include tree holes, abandoned tires and bird baths.

Due to the recent decline in the housing market, many houses have become foreclosed and abandoned. These houses can often become breeding grounds for mosquitoes, especially if they have a pool. Green pools are telltale signs of mosquito problems. When a pool is abandoned for some time, the chlorine within the water breaks down, and algae begins to grow in the water, making it appear green. This water is then rapidly colonized by insects, including mosquitoes. When called upon to look for mosquitoes, consider searching the Internet for recently foreclosed upon homes within your customer service area. It is unnecessary to enter these houses, just look into the backyard. If there is an abundance of litter or a green pool, this could indicate that this house is a source of mosquitoes. If this is the case, it is important to contact the bank and request permission to treat that area. Otherwise you will be coming back repeatedly, since you have not been able to treat the source of the mosquitoes.
 

How do I control them? Proper mosquito control depends on detection and treatment of mosquito larval habitats. The PMP should attempt to find and treat all water sources which contain or could contain mosquito larvae. First the PMP should search all around the home, emptying any small containers of water and making note of any permanent sources of stagnant water, like ponds or pools.

All permanent water sources should then be evaluated in order to note what stages of mosquitoes, if any, are within the water. This can be accomplished easily by bringing a small cup or aquarium net and dipping it in the water. Raise this sample up to the light and look inside. Mosquito larvae are small, slender insects which float on the top of the water. Mature larvae are larger and darker than the others. When they are disturbed they thrash around in the water, diving deeper to hide from predators. Pupae are larger, appear round and tumble rapidly through the water. Once identified, there are several different pesticides which the PMP can use in order to control the mosquito larvae.

There is no single pesticide which works best against all stages and species of mosquitoes. Each pesticide functions best at different levels of infestation. In order to achieve complete control, it is often best to coordinate the application of two or more pesticides at one time. One common example of this is the combination of Bti and IGRs applied simultaneously. This will cause high levels of initial mortality from the Bti, and the IGRs will assure that few, if any, of the survivors will be able to survive to adulthood. If there are many pupae in the water at the time of treatment, it would also be a good idea to treat the water with oil in addition to the larval treatment. This would drown the pupae, which are unaffected by larval treatments.
 

Management. Mosquito control options include:

Draining – The simplest and most rapid way to eliminate any mosquito infestation is to drain the water source that is harboring the mosquito larvae. Mosquito larvae cannot survive outside of water.

Bacillus thuringiensis isrealensis (Bti) – This is a naturally occurring bacterium that causes rapid, high mortality in mosquito larvae which consume it. Bti can be purchased in many forms, but all function in the same way. Once Bti is applied within mosquito-infested water, Bti bacteria are released into the water, and fed upon by second and third instar larvae. First instar larvae don’t feed on the bacteria, as they are too small. Fourth instar larvae don’t feed, and instead prepare for metamorphosis. Once inside of mosquito larvae, the bacteria produce a toxin which pierces the walls of the gut. This causes rapid, high mortality among the larvae.

Insect Growth Regulators (IGRs) – Alternatively, IGRs can be used in water sources to achieve long-term control of larval mosquitoes. Mosquitoes consume IGR particles in the water, and once this chemical is within the body of the larva, it functions in the same manner as a naturally occurring insect hormone, juvenile hormone (JH). While this hormone is present within the body of the larval mosquito, it is incapable of molting to adulthood. The IGR assures that JH levels never reduce within the mosquito, and the mosquito dies as a pupa. Commercially available IGRs typically come with one of two active ingredients, methoprene and pyroproxyfen. These chemicals can come in solid granules, pellets and briquets which are placed within water for slow release and control over long periods of time, or a liquid spray which breaks down in one week.
 

Mosquito Control Oils – Oils are used to control older, fourth instar larvae and pupae. Once the mosquitoes reach this stage they don’t feed, and cannot be killed by IGRs or Bti. Oils function by creating a thin film on top of the water, which suffocates all of the larvae contained within. While mosquito larvae and pupae are aquatic, they still breathe atmospheric oxygen, and must be able to float on the surface in order to breath. Oil is less dense than water, so when it is applied to the surface of the water, it spreads out evenly, and forms a barrier between the mosquito and the air it is trying to breathe. This method works well to rapidly kill larvae and pupae in water with little emergent vegetation or litter. If the water surface is broken in too many places, it could lower the efficacy of the treatment. Oils specially made for mosquito abatement are commercially available in safe, biodegradable sprays which will readily break down in time after application.

Ultra Low Volume Sprays (ULV) – This treatment is commonly used to control mosquito adults, and is the fastest and simplest way to “stop biting.” These sprays are applied to bushes, dark corners and other places where adult mosquitoes hide during the day. These sprays distribute a pyrethroid-based insecticide rapidly across a large area, killing any adult mosquitoes the spray comes in contact with. Care should be taken that the sprays not reach large natural water sources, as the insecticide is toxic to aquatic wildlife. This spray is highly effective when adult mosquito populations reach high densities. If the adult mosquito densities have not yet reached high densities, larval mosquito control is both easier and more effective. It should also be noted that this method will not provide long lasting mosquito control, and you will likely be called back to this location if the mosquito breeding site is not located.

Gambusia affinis (Mosquito Fish) – Mosquito fish are biological control agents which are commonly available at local mosquito control districts and some pet stores for control of mosquitoes. These fish are used in the cases in which chemical controls are not acceptable by the standards of the customer. For example, if the customer has a horse trough, home pond or other permanent water source which has become infested with mosquito larvae. In these cases, the PMP may leave a small number of the fish within the water source. Mosquito fish are hardy, and will be able to propagate themselves within the water. The fish are best used in smaller, independent water sources and could grow out of hand if used in streams, lakes or other natural water sources. Treatment with mosquito fish is a poor choice to treat large bodies of water.

Customer Education. There are many ways in which a homeowner could influence the numbers of mosquitoes found around their home. Most of these methods entail the elimination of larval harborage. This means the removal or cleaning of any place which can contain mosquito larvae. Bird baths should be cleaned weekly and the water replaced often. Gutters need to be cleaned out before and after any hard rains. Pools should be regularly treated with chlorine and cleaned. Potted plants should be drained of any excess water. The homeowner should also walk around outside their house and empty, pick up or cover anything which could possibly fill with water. In respect to insect repellents, DEET has been shown to work well, and is an active ingredient found in many products. Citronella oil and eucalyptus oil are natural, albeit less effective repellents which could also be used.

Homeowners can lower their interactions with mosquitoes by preventing the adult mosquitoes from entering the house. Homeowners should put screens on all their windows and seal any cracks which lead outside. If the homeowner likes to keep the door open at night, screen doors are a must. Any opening to the outside can be exploited by the mosquito in order to come inside.

Not all methods work, however. Sonic repellents do nothing to affect mosquitoes. Mosquitoes do not navigate towards hosts using sound; it is only used for mating. Electrical Discharge Insect Control (EDIC) or “bug-zapper” systems are likewise ineffective. Mosquitoes do not use light to direct their flight; they fly towards sources of carbon dioxide, the chemical produced when someone breathes. Also, the light from the EDIC systems can actually attract increased numbers of insects within the area which the homeowner is attempting to protect. Pesticides do work, however homeowners do not have the expertise, products and equipment needed to effectively combat adult mosquitoes, and should leave any adulticide treatments to the technician.

The opinions expressed here are those of the author and do not necessarily reflect the positions of the University of Kentucky. The author is a graduate student in the University of Kentucky Entomology Department. E-mail him at pbrabant@gie.net.