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.
