UC Riverside (US) research shows house-dwelling mosquitoes require minute changes in concentrations of exhaled carbon dioxide to trigger landing on human skin.
Females of the malaria-spreading mosquito tend to obtain their blood meals within human dwellings. Indeed, this mosquito, Anopheles gambiae,
spends much of its adult life indoors where it is constantly exposed to
human odor – from used clothing, bedding, etc. – even when people are
absent.
But is human odor enough as a reliable cue for the mosquitoes in finding humans to bite?
Not quite, reports a team of entomologists at the University of California, Riverside in a research paper published online earlier this month in the Journal of Chemical Ecology. The researchers’ experiments with female Anopheles gambiae
show that the mosquitoes respond very weakly to human skin odor
alone. The researchers found that the mosquitoes’ landing on a source
of skin odor was dramatically increased when carbon dioxide was also
present, even at levels that barely exceed its background level. The
researchers suggest, too, was that the mosquitoes use a “sit-and-wait”
ambush strategy during which they ignore persistent human odor until a
living human is present.
“Responding strongly to human skin odor alone once inside a dwelling
where human odor is ubiquitous is a highly inefficient means for the
mosquito of locating a feeding site,” said Ring Cardé, a distinguished professor of entomology,
whose lab conducted the research. “We already know that mosquitoes
will readily fly upwind towards human skin odor but landing, the final
stage of host location, which typically takes place indoors, does not
occur unless a fluctuating concentration of carbon dioxide indicates
that a human host is present. It may be that upwind flight towards human
odor has more to do with locating a human dwelling, which emits human
odor even when its occupants are absent, than locating a feeding site per se.”
Cardé explained that mosquitoes, once indoors, conserve their energy
by ignoring omnipresent human odor in an unoccupied room. Small
increases in carbon dioxide indicate to the mosquitoes the probable
presence of a human. This then triggers the mosquitoes to land on human
skin.
The findings could help in the design on new types of mosquito
control. One take-home message from this work is that studies defining
which human odors mediate host finding and which compounds are good
repellents need to precisely control exposure to above ambient carbon
dioxide – an experimenter entering an assay room quickly elevates the
level of carbon dioxide and thereby alters the mosquitoes’ behavior.
The research shows that when it comes to feeding on humans indoors,
malaria mosquitoes have developed a striking adaptation to how carbon
dioxide affects their landing on human targets in response to skin odor.
“It also would be useful next to see if mosquitoes’ response to skin
odor is similarly affected by carbon dioxide in outdoor situations and
how these interactions play out in human dwellings,” Cardé said.
Larvae of Anopheles gambiae can breed in diverse habitats.
This mosquito has evolved to search in human dwellings for blood meals
to carry out egg production. The mosquito enters houses throughout the
night, peaking around midnight and continuing at a high rate until the
early morning hours. Following a blood meal, the mosquito often remains
in dwellings until it is ready to lay eggs. Mosquitoes also seek refuge
inside human dwellings during the day, taking shelter from high daytime
temperatures outside.
Cardé, who occupies the A. M. Boyce Chair in the Department of
Entomology, was joined in the study by Ben Webster (first author of the
research paper) and Emerson S. Lacey.
To conduct their experiments, the researchers used Anopheles gambiae
originating from mosquitoes collected in Cameroon. They collected skin
odor by using pieces of white polyester gauze worn by Webster in a
cotton sock for 4-6 hours before the experiments began. The landing
behavior of the mosquitoes in the experiments was recorded with a video
camera equipped with night vision.
The research was supported by a grant from the National Institute of
Allergy and Infectious Diseases awarded to Cardé and Anandasankar Ray, a
colleague at UCR who was not involved in the research.