January 2005

Mosquitoes - The Ultimate Survivors

Mosquitoes belong to the order Diptera (two wings), in the fly family Culicidae. Mosquitoes have diversified into over 3,000 species and inhabit every continent on Earth except Antarctica. Mosquitoes have evolved and adapted to just about every available environmental situation that presents itself. Most mosquitoes are parasitic as adults, ecto-parasites to be specific, requiring blood for protein to produce eggs. Only female mosquitoes take blood, males, as well as females, feed on nectar for energy. Some mosquito species are indiscriminate feeders, taking blood from whatever animal is available. Most, however, are very specific in regard to what animals they will take blood from. Some feed only on birds or amphibians or reptiles or mammals and some feed only on a specific specie. This specificity has created a tremendous diversity within the family of mosquitoes.

            Habitat has also stimulated diversity. For their larval stage (i.e. egg laying), some species prefer polluted water, some species lay eggs only in relatively clean water, some like brackish water, some pond environments, marshes, floodplains of rivers, treeholes, containers, etc. All of these “required” habitats are situations mosquitoes have adapted to over the eons of time.

            The seasonality of weather also exerts survival pressure on mosquitoes, again stimulating diversity. Some mosquito species exist only in temperate areas, some only in tropical areas, and many have adapted to both.

            All of these factors; weather, available hosts, various habitats, all factor into why there is such a large number of different species of mosquitoes. In nature, the more diversity and numerous the species within that family, the greater the chances are that that family will survive.

            Mosquitoes are an opportunistic organism. They are usually the first or among the first organisms to colonize a “newly” available aquatic habitat... they love an ecologically unstable situation. Examples of such are inundation of a river’s floodplain, an extra high tide in a tidal marsh, a treehole that collects and holds water, a polluted situation, woodland pools that appear after a rain event, or potholes that fill with water from melting ice or snow. Mosquitoes will utilize these “new” aquatic habitats by laying eggs directly on the water’s surface or by laying their eggs in these flood prone areas after the areas have dried up. These eggs will lay dormant for up to 4 years and then hatch when inundated by water from a flood, rain or tidal event. The viability of such eggs, for many years in a dry condition, ensures the survival of the species through severe droughts. As initial colonizers, mosquitoes avoid predation to a large degree, there just isn’t any predators in the “new” aquatic environment. By the time predators start showing up the mosquito larva have developed and emerged from their aquatic environment into their new terrestrial environment as adults, leaving behind the danger of predation and/or the drying up of its habitat. Mosquitoes that lay their eggs directly on the water surface usually hatch in 24 to 48 hours. Larva that develop in these semi-permanent aquatic habitats usually stay close to the shoreline in very shallow water, minimizing exposure to predators such as fish, crayfish, and a host of other predatory aquatic insects,... a very tenuous state of survival.

            As adults, mosquitoes are preyed upon by frogs, toads, birds, spiders, dragon flies, damsel flies, bats, lizards, and parasites such as mites and nematodes. Despite all the creatures that prey on them, mosquitoes still manage to survive quite well.

             A misconception a lot of people have is that cold, freezing weather kills mosquitoes...wishful thinking. When temperatures fall below freezing, adult mosquitoes go into a state of hibernation. Their metabolism slows and they stay in a protected area, usually near the ground under leaf litter, in sewer systems, or covered by snow in more northern areas. The larval stage can survive in water, close to freezing, because they too go into a state of hibernation, requiring very little oxygen. What little oxygen they do need they can absorb from the water through their skin. Other species survive the winter in the egg stage, totally unaffected by freezing temperatures. The only thing low temperatures do to mosquitoes is slow them down to the point of inactivity, once it warms up, they become active again.

            Another survival pressure mosquitoes must endure is man. We’ve eliminated their habitat by draining and filling, we use screens and repellents to keep them away, we’ve used biological controls and cultural practices to reduce their numbers, and we’ve used pesticides in both their aquatic and terrestrial habitats, still they survive.

            Mosquitoes have the genetic ability to adapt to all the survival pressures that have been thrown at them, by both man and nature. They have evolved to cope with environmental conditions such as drought and temperature extremes. Their reproductive fecundity allows them to survive the predatory pressure they are constantly under. But the greatest asset mosquitoes have is their genetic diversity and the ability of their gene pool to adapt to change. This is why there are over 3,000 species and why they are able to overcome environmental pressures that threaten their survival, such as exposure to pesticides. With all of our intelligence, knowledge, and ability to apply that knowledge we still have not won our war against the mosquito. Advancements in genetic research, a greater understanding of what makes a mosquito tick, and what their weaknesses are offers hope for the future. But for now, mosquitoes are winning the war, inflicting misery and disease on hundreds of millions of humans annually and causing the death of over 3 million people every year.