Why can't we just kill all mosquitoes?
Story highlights
- It's impossible to wipe out all mosquitoes, but there are ways to reduce levels regionally
- Researchers are working to single out species of mosquitoes that spread human disease
(CNN)The mosquito was never going to win any popularity contests, but as the "deadliest animal in the world" spreads the Zika virus, it's become the target of tough talk and powerful pesticides.
"The
mosquito kills more people than any other animal on earth," Dr. Tom
Frieden, director of the Centers for Disease Control and Prevention, told CNN's Dr. Sanjay Gupta, this week.
"Is there any redeeming feature? ... Well, they do provide food for
birds and other insects, but I think the world would be a lot better off
without them."
Indeed, mosquitoes can spread diseases such as malaria, yellow fever, dengue and Zika virus. Even the ones that aren't carrying diseases are buzzing in ears and threatening everyone's outdoor fun.
Really, why don't we just kill all of them?
That solution, it turns out, is the stuff of science fiction.
Aside
from the potential impacts on the ecosystem when a species disappears,
"it is absolutely impossible to kill all the mosquitoes; it's just not
going to happen," said Roger S. Nasci, executive director of the North
Shore Mosquito Abatement District, a public mosquito control program
outside Chicago. "No one in the scientific field today has any illusions
of being able to eradicate the mosquito."
The
reality is no matter how big of a mosquito-killing effort cities and
countries unleash on the pests, "there will always be a remnant
population somewhere that will repopulate," Nasci said.
It's been tried before
In the not-so-distant past, there were notions of mosquito eradication, but history quickly taught us otherwise.
Nasci
points to the Herculean job that Latin America undertook in the 1950s
and 1960s to eradicate Aedes aegypti, the mosquito that transmits yellow
fever -- and also dengue, chikungunya
and Zika. The Pan American Health Organization rolled out all the most
effective measures -- spraying insecticide such as DDT and discarding
standing water containers -- on a massive scale. But after efforts let
up, the mosquitoes came buzzing right back, possibly hitching a ride on shipping vessels from Asia and Africa.
The
United States would probably be hard-pressed to achieve the same
success if it borrowed Latin America's strategy to wipe out mosquito
populations. For starters, the Environmental Protection Agency banned
the use of DDT, still the best pesticide to kill mosquitoes,
in 1972 because of its devastating environmental effects and possible
risks to human health. There is also the challenge that mosquitoes
develop resistance to DDT and other agents such as pyrethrin that limit
their effectiveness.
The best hope is
to limit the number of mosquitoes in an area to reduce the risk of
disease transmission, Nasci said. There are effective tools for doing
so. Nasci and his program turn to them when surveillance data show the
number of mosquitoes carrying West Nile virus -- the major
mosquito-borne disease in his area, carried by the Culex mosquitoes --
is on the rise. His team gets called into action, too, when a fed-up
resident can't enjoy his deck because of all the backyard biters.
The future of mosquito control
The
good news is that tools used for controlling mosquito populations wreak
less environmental havoc than in the days of DDT -- and at least so far
have not been associated with resistance in mosquitoes.
One approach is to spray a formula of bacteria around mosquito
habitats. Mosquitoes eat up the bacteria, which then kill them and only
them (and related insects) by destroying the lining of their guts.
Another
strategy could be to develop tools that only kill the few mosquito
species that transmit diseases to people and animals and spare the other
3,000 or so species that don't do any harm, said Laura Harrington,
professor and chair of entomology at Cornell University.
This
plan would involve using genetically modified mosquitoes. It could take
several shapes, but one would be to breed male mosquitoes to contain a
gene toxic to their offspring. These tweaked males would be released
into the wild to mate with females, but their larval babies would
quickly die off. Studies testing the effect of releases in Brazil and
the Cayman Islands have found 80% drops in Aedes aegypti levels. Harrington is also developing genetically modified males that would effectively kill all the females with which they mate.
The
downsides of this approach are that genetically modified mosquitoes
could be expensive, and they would probably have to be released at least
once a year to keep populations in check, Harrington said. It is
probably still years before these genetically modified mating menaces
are ready to join the armamentarium of mosquito control tools, she
added.
Learning to live with mosquitoes
In
the meantime, it is important to remember that "mosquitoes can play a
very important role in the ecosystem," Harrington said. They are
particularly key in their larval stages when, just because of their
sheer mass, they are major food sources for aquatic predators. "If you
removed all mosquito larvae in the Everglades, you'd probably have a
huge impact," Harrington said.
For his
part, Nasci said his team does not see a "wholesale ecological collapse"
when it does mosquito control in the Chicago area, which he would
expect to see if the insects played a clutch role in the food chain.
Just
because mosquitoes don't appear to be going anywhere anytime soon does
not mean that we must resign ourselves to their biting and buzzing.
There are a number of ways to keep mosquitoes at bay.
At the most basic level, make sure you don't have pools of standing
water, such as in plant pots or bird feeders, around your home and that
your window screens are in good shape.
If
you are outside and insects are really making a nuisance of themselves,
you can wear bug spray and a long-sleeve shirt, Nasci said. It can also
be a good idea to keep an eye any reports of increases in the level of
mosquitoes in your area harboring disease, such as West Nile. Many local
public health agencies have monitoring programs, Nasci said.
Even
if better mosquito control measures develop -- and perhaps one day
there are genetically modified mosquitoes -- it will still be important
to mix up approaches, Harrington said.
Even
if levels of Aedes aegypti can temporarily be brought down, people
should still take personal precautions, such as wearing repellent. And
experts need to work on developing anti-viral drugs and vaccines against
the diseases that that handle of mosquito species carry.
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