February 22, 2015 Kathleen Sayce
Insects are by far the largest of animal groups on the planet, with a staggering diversity of life forms and life styles. We tend to reserve a special loathing for insects that feed on blood, and mosquitos are probably at the top of that list, perhaps because they descend on their prey in clouds, or bite sleeping bodies, making a distinctive high-pitched sound that involuntarily triggers a faster heart beat and higher blood pressure.
That feeding cloud of mosquitos is composed of females, sometimes with males hanging around the edges. Successful feeders will depart with a full stomach of blood, take a day to digest it, then within a few more days lay eggs in suitable wet habitats. A week later, they repeat the cycle. The blood provides proteins to make the eggs, which mosquitoes cannot get from their other food source, flower nectar. In many species, females live five or six months, and overwinter in a sort of dormancy. The blood-borne diseases are picked up by females as they feed on infected hosts, and then spread to those hosts that they later feed on. The most dangerous mosquito, most likely to carry a disease, is the older female who has lived a few months and fed many times on a variety of animals and humans in areas where suitable diseases are found.
When not foraging for a meal of blood, both males and females behave more like flies––which mosquitos are close relatives to, the word 'mosquito' means “little fly”––congregating in their preferred habitats, drinking nectar for food, and hanging out. Where do they hang? It depends on the species. We have more than forty species of mosquitos in the state, and fourteen species in Pacific County. Some like salt marshes, and others freshwater marshes. Some like ponds with dense vegetation on the edges, others seek clean open water. Some like water-filled holes in trees. Others prefer manure-rich standing water, including sewage ponds and cattle yards. Still others seek out tiny containers, gutters, water in tires, or water-filled hoofprints in mud. Some look for sunny water sources, others for shade. Many live in lowland areas, but some prefer higher elevations, living in snowmelt ponds. As for time of day, that also varies. Some fly at dawn and dusk, others after dark, others in full daylight, some only in shade.
As for blood sources, all mosquitos do not prefer the same choices. Some only feed on amphibian or reptile blood––in our area, this includes salamanders and frogs, garter snakes and the occasional lizard. Others prefer bird blood. Many prefer large mammals, and those are the ones that we interact with most often. Some are generalists, mixing up meals between elk, deer, horses, cattle and people. Those that feed most often on large mammals and move between species are more likely to carry diseases.
In the 19th century, malaria was endemic in the Pacific Northwest. Anopheles mosquitoes carry malaria, and were already here, disease-free, when humans carrying malaria arrived as settlers; and so for some decades, malaria was a chronic disease east of the Coast Range from Olympia south to the bottom of the Willamette Valley. This shows a typical pattern of disease transmission: Infected animals or humans enter a previously disease-free area, and mosquitos that carry that disease are already present, and begin moving the disease to new hosts.
The list of mosquito-carried diseases that have been found in North America is impressive: Western and Eastern Equine Encephalitis (WEE, EEE), St Louis Encephalitis (SLE), Japanese B Encephalitis (JBE), California Encephalitis (CE), Venezuelan Encephalitis (VE), West Nile Virus (WNV), Dengue virus, Malaria, Avian Malaria, Yellow Fever. There are also several localized diseases of specific areas. The actual disease organism may be a virus or protozoan; Plasmodium species, which are protozoans, are often disease-causing organisms, including multiple forms of malaria.
While many have railed that mosquitos fill no useful purpose, the fact is, they are here. Widespread use of DDT in the 20th century demonstrated that attempting to wipe out mosquito populations with chemicals has disastrous unintended consequences on local ecosystems. Reducing their numbers, rather than waging all out war, is a better strategy.
First, know which species live in your community. It's very likely that there are several species, not one. Know the species, and you will know the larval habitat that species prefers. This is very important––without knowing what species you have, you may well spend your time and money in the wrong activities in the wrong places. Communities with mosquito control programs have staff that spend their time collecting larval and adult mosquitos and identifying which species live where; once they know the species, then they can work on the next step, below.
Second, reduce available larval habitat, or improve predation on larvae in those habitats. This is a good strategy for salt and freshwater pond and marsh mosquitos. Reduce and eliminate small sources of water too. These include old tires, bird baths, buckets and toys filled with water, gutters with standing water. Mosquitos can go from freshly laid eggs to adults in less than a week, so if you have bird baths, change the water at least twice a week. In ponds and ditches, Bti, a bacterial disease that kills larvae, helps with some species. Gambusia, a tiny guppy, eats mosquito larvae in freshwater ponds and lakes. There are other strategies too, these are just starting points to reduce larvae numbers.
Third, protect your home so that mosquitoes do not live indoors with you. Screened doors and windows are the first line of defense. Some species like to live around and in buildings. Keep your screens in good shape, and they will help keep mosquitos out of homes. Bed nets are also good, especially where night flying, malaria-carrying species are common. Currently this is not a local problem, but is very important where Anopheles mosquitoes and Plasmodium malarial species both live.
Fourth, cover your skin when outside. Mosquito hats (hats with fine netting from the brim to the shoulders), long sleeves, long pants, and good repellants all help. One summer Frank and I measured shore pine trees at Leadbetter Point in late June. Local populations of a native freshwater marsh mosquito were at their annual peak. We wore mesh hats, long sleeves and pants, used DEET repellant. Frank cored trees; I counted cores. The mosquito clouds were so dense that it was difficult to see the growth rings to count them, or my notes as I wrote down figures. We had to reapply DEET every 30 minutes to our hands, because after 25 minutes, the mosquitoes stopped hovering an inch or two above the skin, and started landing to feed. About day three, preparing for that day's work took sheer nerve!
As the weather warms up, after this unseasonably warm winter, expect hungry female mosquitoes to fly soon, and be ready for a long mosquito season.