Mosquitoes breed in a variety of aquatic habitats with stagnant water. Some species use human-made containers associated with human habitats such as tires, bins, gutters, rain barrels, or birdbaths as breeding sites. Other floodwater species hatch in great numbers at the same time after heavy rain or flooding.
Control strategies depend on species and the area of infestation. The control of mosquitoes aims to preserve human health, comfort, and the environment.
Larvicides are probably the best way to tackle a mosquito problem, but are they safe? Do they harm the environment? What sort is best to use in your backyard?
You’ll find the answers to these questions and more in this article.
What is a Larvicide?
A larvicide is a product that kills insects when they are in the larval stage of the life cycle. Often used against mosquitoes, larvicides kill the larvae before they develop into adults and fly off to mate, bite and reproduce. They are available in different dunks, briquettes, granules, pellets, tablets, or liquid.
Why Use a Mosquito Larvicide?
Larvicides are applied to mosquito breeding grounds, areas of water large or small where there are larvae. They are more successful than using adulticides as the areas (ponds, rain barrels, wetlands) are more easily defined and usually relatively small.
Larvicides are also less toxic than using sprays to kill adult mosquitoes and result in less exposure to humans.
If the breeding areas are successfully treated, the need for adult control programs will be significantly reduced.
In some regions, mosquitoes may be just a nuisance, but they can cause serious diseases such as malaria, dengue, West Nile virus, or yellow fever in other places.
Forms of Larvicides
Larvicides are found in several different forms, dunks, briquets, granules, pellets, tablets, or liquid. Many forms can be used by individuals, while other forms, notably liquids, can be used in large-scale mosquito control efforts. Liquid forms can be sprayed from backpacks, trucks, or planes.
For most homeowners, dunks, briquets, granules, pellets, or tablets will be the preferred form of treatment for relatively small areas and volumes of water.
Types of Larvicide
There are several types of larvicide. Some are available to the public, and others are used by governmental mosquito control programs and professional pest control operators.
The larvicides mentioned below are registered by the Environmental Protection Agency, which means they are effective and safe to use.
The most common larvicides used by homeowners are bacterial larvicides.
Bacterial Larvicides – can affect the gut or the nervous system of the mosquito, and they die while still in the larval stage.
An insect growth regulator or inhibitor stops mosquitoes from completing their larval stage, so they don’t become adults.
Organophosphate insecticides attack the central nervous system of insects. Resulting in death before they mature into adults.
Mineral oils are surfactants to prevent larvae and pupae from breaking through the water’s surface tension; this blocks their breathing tubes, and they eventually drown.
Monomolecular films are surfactants that reduce the surface tension of the water, making it difficult for the mosquito larvae and pupae to attach. If they can’t attach, they won’t be able to breathe. They will slowly tire and drown.
Bacillus sphaericus (Bs) is a naturally occurring bacterium found on soil. Discovered in 1965, it was registered as an active ingredient in 1991. Bs is also now known as Lysinibacillus sphaericus. Bs is added to water with mosquito larvae; they ingest the bacteria, which paralyzes the mosquito’s gut. The larvae die of starvation. Bs is not harmful to non-target organisms.
Bs is available in granule, powder forms, or a water-soluble pouch.
Bacillus thuringiensis subspecies israelensis (Bti) is a bacteria found naturally in the soil. It was discovered in Israël in 1976. The Bti is added to standing water, where mosquitoes lay their eggs. The mosquito larva ingests the Bti, which perforates the inner walls of the digestive tract, and the larva dies. Bti is not harmful to other insects or animals.
Bti is available in granules, briquets, dunks, and liquid. Dunks or bits(granules) will be the preferred treatment method for most homeowners.
Some granules, dissolvable pouches and briquets combine Bti and Bs. The combination of the two bacteria may give better results in certain situations.
- Bti provides broad-spectrum activity against mosquitoes, rapid control, and the risk of resistance is low.
- Bs can give extended control, has greater efficacy in polluted water, but has a higher risk of resistance, and is less effective against certain species.
Saccharopolyspora Spinosa (Spinosad)
Saccharopolyspora Spinosa is a bacterial organism found in the soil. Spinosad is an insecticide composed of spinosyns A and D. Spinosad attacks the insects’ nervous system, causing muscle contractions, paralysis, and death. Spinosad is toxic to aquatic invertebrates.
It is used to control many pests, including thrips, mosquitoes, ants, fruit flies, and leaf miners. Hence, Spinosad is found in many pesticide products for gardens, crops, and aquatic settings.
For mosquito control, it’s available in liquid, granules, and tablets. The tablet form is available for use in your yard; other forms are often restricted for use in governmental mosquito control programs and by professional pest control operators.
Insect Growth Regulators
Methoprene stops mosquitoes and other insects from becoming adults. A target-specific insecticide that doesn’t affect fish, waterfowl, mammals, or beneficial predatory insects. Available in briquets, pellets, granules, and liquid. It is sold under the name Altosid.
Although Altosid is meant to be safe for fish and many other insects, there is some controversy. According to the National Pesticide Information Center, methoprene is moderately toxic to some fish and low in toxicity to others. It is also slightly harmful to crustaceans and very highly toxic to freshwater invertebrates.
However, if used in the concentrations necessary to kill mosquito larvae, there shouldn’t be any adverse effects on other non-target organisms.
When highly refined mineral oil is applied to the water’s surface, it creates a film that stops the larvae from coming to the surface to breathe, resulting in suffocation. Licensed professionals often apply oils and films.
Licensed professionals often apply oils and films, and they have the advantage of suffocating larvae that are at the 4th molting stage (4th instar) and pupae. At these stages, they don’t feed, which makes other larvicides ineffective.
Monomolecular films can be made from plant oils and are biodegradable. A spray can be used to install an invisible film on the standing water, the habitat for mosquito larvae and pupae. The film reduces the surface tension of the water, making it difficult for the mosquito larvae and pupae to attach to the surface, which they need to do to breathe. Overtime (48 to 96 hours depending on species and water temperature) will cause them to drown. The film will also make it difficult for the mosquito to lay her eggs on the water surface.
The advantages of this form of action are that mosquitoes can’t develop resistance as it’s a physical mode of action, with very little or no chemical toxicity, meaning it can be used around humans, fish, and pets.
What Larvicides Can You Use in Your Yard?
In your garden, Bacillus thuringiensis israelensis (Bti) is the larvicide that’s easiest to find, available online or in garden centers. It comes in different forms for homeowners – dunks, granules, powder, liquid, soluble pouches, or briquets.
Bacillus Sphaericus is harder to find than Bti, and generally, the quantities available make it prohibitive. You can find some briquets and granules that combine Bs and Bti.
Methoprene is often available in granules or liquid under the trade name of Altosid, although there are other makes. It can be found online and in garden centers.
Spinosad is available for individuals in tablet form; other forms such as granules or liquid are available for professionals. The tablet form is available online and in garden centers.
Several natural or silicone-based tension eliminators are available online or from garden centers. These usually stop mosquitoes from breeding for around four weeks.
Are Larvicides Inoffensive for the Environment?
Using most of the larvicides mentioned above is generally considered safe and environmentally friendly. But recently, there has been a report from Europe that claims using larvicides could have a detrimental effect on the biodiversity in natural wetlands and conservation areas. The report focused on the use of Bti.
Preserving human health, stopping the nuisance mosquitoes bring, and protecting the environment are challenging. With the discovery of Bti and other chemicals that cause very little toxicity in other insects and animals, it seemed mosquitoes could be controlled with minor damage to other fauna.
However, eliminating mosquitoes and often midges that are a significant food source for amphibians, fish, insects, birds, bats, and spiders has a negative effect on the numbers of these mosquito predators. The report suggests considering other mosquito control methods in conservation areas.
If you use mosquito larvicides around your home, the impact may be less critical than treating large areas of water or swamps, but nonetheless, there will be less food for fish, birds, bats, amphibians, and other insects. Over time, this may affect the animal diversity you find in your garden.
Other Types of Mosquito Control to Consider
To preserve the biodiversity in your garden, you might want to consider mosquito control without the use of bacteria or chemicals.
- The first step is to remove any standing water, even small quantities, that give mosquitoes breeding grounds. Keep your garden tidy, remove plastic sheeting, childrens toys, empty containers… anything that can collect water. Any water that can’t be emptied, such as rain barrels, protect with a tight cover.
- Add fish (Goldfish, Mosquito Fish) to ponds, water features, or lakes; they will eat mosquito larvae.
- Adding dish detergent or shampoo to water will reduce the surface tension to stop larvae from breathing, and adults may drown if they land on the water. 1/16th teaspoon of liquid soap per gallon of water should work!
- Use an ovitrap, a mosquito larvae trap. This creates an artificial breeding site that attracts pregnant mosquitoes, who lay their eggs. The eggs fall through a mesh grid where they hatch and develop. The adult mosquitoes are trapped beneath the mesh and can’t escape. Another form of ovitrap is the lethal ovitrap that attracts the female mosquitoes and either kills them using a pesticide or kills the larvae that hatch from the eggs with a larvicide. Ovitraps are available online, or you can fabricate your own.
- Other ways of controlling mosquitoes are mosquito traps that emit CO2. They can be expensive to buy and may have operating costs.
Although the larvicides or surfactants may slightly affect other non-target species, they are very effective and a great way to deal with a mosquito problem.