Mosquito Biology
Larvae
All mosquitoes start their life cycle in the water. Some species prefer salt or brackish water as found in our local coastal salt marshes while others prefer fresh water derived from rainfall or agricultural irrigation. Mosquitoes do not develop in running water or in deep waters such as our Indian River, lakes or ponds.
The mosquito life cycle begins with an egg. After maturing for a few days, a mosquito egg may enter a “waiting period” where it is metabolically inactive. Some eggs may wait for a year or more for ideal hatching conditions, though most hatch within days or weeks. This “reserve” of unhatched eggs means that simply killing all the mosquito larvae in a breeding area does not guarentee that it will not breed again. This is particularly true of our local salt-marsh species.
Some fresh-water mosquitoes lay their eggs in a floating mass called a “raft” onto the surface of the water. Others may place their eggs singly at the water line in small containers. Our salt-marsh mosquitoes lay their eggs one at a time into the damp soil of the marsh, not into the water itself. By keeping these marshes covered with a thin layer of water, the breeding cycle can be interrupted. See our section on salt marshes for more about “source reduction”.
Mosquito eggs hatch when they are exposed to water. In citrus groves, different species may lay their eggs at different levels with the irrigation swales. A little rain which floods only the bottom of the swale will hatch one species while a heavy rain or flood irrigation event which completely fills the swales will hatch all species. The Psorophera howardii mosquito lays its eggs high in the swales. Once they hatch, the larvae actually eat those of other mosquito species. This is not a mosquito-control advantage though since P.howardii is itself a very large and hard-biting mosquito.
In the high salt marshes, rainfall or an unusually high tide will flood the area. The mosquito eggs waiting in the soil will hatch within 24 hours or less.
Larval Development
All mosquitoes have a six-part development in the water. After the egg hatches, the larvae begin to feed on suspended organic matter. Each larva will go through four distinct moltings called “instars” where it sheds its outer covering and emerges larger and more mature. During the summer months, each instar takes about one day. Cool weather can stretch out larval development though it will take near freezing temperatures to actually kill the larvae once they have hatched. The final in-water stage is a pupae. It is in this stage that the aquatic form of the mosquito metamorphizes into the adult, flying form. The pupal stage normally takes two or three days.
The threat to mosquito larvae is that their water puddle will dry out before they can complete this part of their life cycle. It is assumed that female mosquitoes have some way of detecting which spots are likely to stay wet for a week or so, probably by smell.
Mosquito larvae breath air. Some, as in the image above, have a special breathing tube which they thrust through the surface layer of the water. Other species lack the specialized tube and must lie parallel to the water surface to breath. When disturbed or when seeking food, the larvae may sink briefly to the bottom of the water, but they must soon rise again to get air. By spraying a thin film of oil onto the waters surface, the mosquitoes can be quickly suffocated. In the past, diesel oil formulations were employed for this purpose but ecological concerns have forced a change to less toxic oils or the abandonment of this technique entirely.
The larval development cycle of several salt-marsh mosquito species can be interrupted by applying methoprene. This is a synthetic version of a natural growth hormone found in insects. Left alone, this hormone is produced only during specific times by the larvae. By applying methoprene to the water, the larvae get a steady dose. This prevents them from entering the pupal stage and they die a few days later. See our section on mosquito larvicides for more information.
Adult Mosquitoes
After a couple of days as pupae, the pupal case splits open and fully developed adult mosquitoes emerge. The males typically appear a little ahead of the females. Newly emerged mosquitoes will rest for several hours, allowing their wings to inflate into shape and harden. Then, at dusk, they will immediately begin their mating flight. Once mated, the female mosquito is capable of producing hundreds of fertile eggs. In most species, the female will then go out to seek blood protein with which to increase her egg-production capacity, though most can lay a limited number of eggs without needing blood. This is called otogeny and allows the species to continue even under adverse conditions.
Only female mosquitoes bite. The males are not as large or hardy and in the wild will die within a few days to a week. During their brief life, they subsist on nectar. Female mosquitoes can also subsist strictly on nectar, though as mentioned before, their egg-laying capacity will be greatly reduced.
How Mosquitoes Find You
Mosquitoes find blood by several mechanisms. The primary attractants for mosquitoes are the carbon dioxide and lactic acid exhaled by animals. It appears that at least some species can also identify various animals by visual means. Other chemical vapors also interest mosquitoes, such as octanol found in the breath of cows.
There have been rumors that eating particular foods may make humans more attractive to mosquitoes. Bananna and peanut products were implicated. Testing done by the USDA did not prove this out. Similarly, there seems to be nothing people can eat which will make them less attractive to mosquitoes, so hold the garlic …
Sounds do not seem to have any relevance when it comes to seeking a host for blood-feeding … regardless what the purveyors of so-called “ultrasonic mosquito repeller” devices may wish us to believe. In a similar vein, only some species of mosquito are attracted to light, such as found in the common “bug zapper”, while others avoid light entirely. Bug zappers, on the whole, do not catch many mosquitoes.
Mosquito Repellants
The chemical-detection apparatus of mosquitoes can be confused by repellants containing the compound “DEET”. Most commercial mosquito repellants contain DEET in various concentrations. While effective, DEET can cause a skin rash in some people, is typically a bit oily and has an unpleasant smell. DEET may be toxic to dogs and other animals so think twice about using this compound on anything but humans. Some repellant products contain very high concentrations of DEET, far beyond what is required. While high concentrations may somewhat extend the period the sprays are effective, they also increase the chances of a skin reaction.
Current recomendations are for using repellants containing no more than 6% DEET on children under 12 and no more than 25% DEET for older individuals.
Some skin lotions claim to repel mosquitoes, Avons’ “Skin So Soft” for example. This effect exists, though not to the degree exhibited by DEET. There is a new product called “SkinTastic” which has greater repellant effect. Products containing citronella have also proven effective. Those who have a reaction to DEET, or worry about repeated applications of DEET, may consider these alternatives. Be aware that while DEET may last an hour or more, these other products may have to be re-applied every 15-20 minutes.
Of late, there has been a claim that common fabric-softener sheets, such as are added to clothes dryers, will help repel mosquitoes if rubbed on skin and clothing. This effect is as yet unconfirmed … and this author has NO idea as to what chemicals are used in these softener strips or WHAT they may do to people in concentrated form. Some things may be worse than mosquitoes … use caution.