THE MOST DANGEROUS INSECT

The mosquito is the most dangerous animal in
the world, carrying diseases that kill one
million people a year. Now the Zika virus, which
is carried by mosquitoes, has been linked with
thousands of babies born with brain defects in
South America. Should the insects be wiped
out?
There are 3,500 known species of mosquito but
most of those don't bother humans at all,
living off plant and fruit nectar.
It's only the females from just 6% of species
that draw blood from humans - to help them
develop their eggs. Of these just half carry
parasites that cause human diseases. But the
impact of these 100 species is devastating.
"Half of the global population is at risk of a
mosquito-borne disease," says Frances Hawkes
from the Natural Resources Institute at the
University of Greenwich. "They have had an
untold impact on human misery."
Deadly mosquitoes
Aedes aegypti - spreads diseases including
Zika, yellow fever and dengue fever;
originated in Africa but is found in
tropical and subtropical regions throughout
the world
Aedes albopictus - spreads diseases
including yellow fever and dengue fever
and West Nile virus; originated in
Southeast Asia but is now found in tropical
and subtropical regions throughout the
world
Anopheles gambiae (pictured above) - also
known as the African malaria mosquito,
the species is one of the most efficient
transmitters for the spread of the disease
More than a million people, mostly from poorer
nations, die each year from mosquito-borne
diseases including malaria, dengue fever and
yellow fever.
Some mosquitoes also carry the Zika virus,
which was first thought to cause only mild
fever and rashes. However, scientists are now
worried it can damage babies in the womb. The
Zika virus has been linked with a spike in
microcephaly - where babies are born with
smaller heads - in Brazil.
There's a constant effort to educate people
to use treated nets and other tactics to avoid
being bitten. But would it just be simpler to
make an entire species of disease-carrying
mosquito extinct?
Biologist Olivia Judson has supported
"specicide" of 30 types of mosquito. She said
doing this would save one million lives and only
decrease the genetic diversity of the mosquito
family by 1%. "We should consider the
ultimate swatting," she told the New York
Times.
In Britain, scientists at Oxford University and
the biotech firm Oxitec have genetically
modified (GM) the males of Aedes aegypti - a
mosquito species that carries both the Zika
virus and dengue fever. These GM males carry
a gene that stops their offspring developing
properly. This second generation of mosquitoes
then die before they can reproduce and
become carriers of disease themselves.
About three million of these modified
mosquitoes were released on to a site on the
Cayman Islands between 2009 and 2010. Oxitec
reported a 96% reduction in mosquitoes
compared with nearby areas. A trial currently
taking place on a site in Brazil has reduced
the numbers by 92%.
So are there any downsides to removing
mosquitoes? According to Phil Lounibos, an
entomologist at Florida University, mosquito
eradication "is fraught with undesirable side
effects".
He says mosquitoes, which mostly feed on
plant nectar, are important pollinators. They
are also a food source for birds and bats while
their young - as larvae - are consumed by fish
and frogs. This could have an effect further
up and down the food chain.
However, some say that the role of mosquito
species as food and pollinators would quickly
be filled by other insects. "We're not left with
a wasteland every time a species vanishes,"
Judson said.
But for Lounibos, the fact this niche would be
filled by another insect is part of the problem.
He warns that mosquitoes could be replaced by
an insect "equally, or more, undesirable from a
public health viewpoint". Its replacement could
even conceivably spread diseases further and
faster than mosquitoes today.
Science writer David Quammen has argued that
mosquitoes have limited the destructive impact
of humanity on nature. "Mosquitoes make
tropical rainforests, for humans, virtually
uninhabitable," he said.
Rainforests, home to a large share of our total
plant and animal species, are under serious
threat from man-made destruction. "Nothing
has done more to delay this catastrophe over
the past 10,000 years, than the mosquito,"
Quammen said.
But destroying a species isn't just a scientific
issue, it's also a philosophical one. There would
be some who would say it is utterly
unacceptable to deliberately wipe out a species
that is a danger to humans when it is humans
that are a danger to so many species.
"One argument against is that it would be
morally wrong to remove an entire species,"
says Jonathan Pugh, from Oxford University's
Uehiro Centre for Practical Ethics.
And yet that's not an argument we apply to all
species, says Pugh. "When we eradicated the
Variola virus, which caused smallpox, we rightly
celebrated.
"We need to ask ourselves, does it have any
valuable capacities? For instance, is it sentient
and therefore has the capacity to suffer
pain? Scientists say mosquitoes don't have an
emotional response to pain like we do.
"Also do we have a good reason for getting rid
of them? With mosquitoes, they are the main
carriers for many diseases."
The question is likely to remain hypothetical,
whatever the level of concern over Zika,
malaria and dengue. Despite the success of
reducing mosquito numbers in smaller areas,
many scientists say knocking out an entire
species would be impossible.
"There's no silver bullet," says Hawkes. "Field
trials using GM mosquitoes have been a
moderate success but involved releasing
millions of modified insects to cover just a
small area.
"Getting every female mosquito to breed with
sterile males in a large area would be very
difficult. Instead we should be looking to
combine this with other techniques."
Innovative ways of tackling mosquitoes are
being developed across the world. Scientists at
Kew Gardens in London are developing a sensor
that can detect each different species of
mosquito from its distinctive wing beat. They
plan to equip villagers in rural Indonesia with
wearable acoustic detectors to track disease-
bearing mosquitoes. This would help them
manage future outbreaks.
Meanwhile, scientists at the London School of
Hygiene & Tropical Medicine have worked out
how female mosquitoes are attracted to
certain body odours, raising hope for more
effective repellents .
Another promising avenue is to make
mosquitoes resistant to the parasites that
cause the diseases. In Australia, the Eliminate
Dengue programme is using naturally occurring
bacteria to reduce the ability of mosquitoes to
pass dengue between people.
"This is a more realistic approach for
mitigating mosquito-borne disease," says
Lounibos.
Meanwhile, scientists in the US have bred a GM
mosquito with a new gene in the laboratory
that makes it resistant to the malaria
parasite.
"We are playing an evolutionary game with
mosquitoes," says Hawkes. "Hopefully it's one
we can get on top of over the next 10 to 15
years."