Lure, retain, and catch malaria mosquitoes. How heat and humidity improve odour-baited trap performance

Background: When seeking a human for a blood meal, mosquitoes use several
cues to detect and find their hosts. From this knowledge, counter-flow
odour-baited traps have been developed that use a combination of CO2,
odour-blend, visual cues and circulating airflow to attract and capture
mosquitoes. Initially developed for monitoring, these traps are now also
being considered as promising vector control tools. These traps are
attractive to host-seeking mosquitoes, but their capture efficiency is
low. It has been hypothesized that the lack of short-range host cues such
as heat and increased local humidity often prevents mosquitoes from
getting close enough to get caught; this lack might even trigger avoidance
manoeuvres near the capture region. Methods: We tested how close-range
host cues affect the flight behaviour of Anopheles female malaria
mosquitoes around odour-baited traps, and how this affects trap capture
performance. For this, a novel counter-flow odour-baited trap was
developed, the M-Tego. In addition to the usual CO2 and odour-blend, this
trap can provide the short-range host cues heat and humidity. By
systematically adding or removing these two cues, we tested how this
affected the trap capture percentages and flight behaviour. We first
compared capture percentages of the M-Tego with and without short-range
host cues to the BG-Suna trap, in both laboratory and semi-field testing.
Then, we used machine-vision techniques to track the three-dimensional
flight movements of mosquitoes around the M-Tego. Results: With heat and
humidity present, the M-Tego captured significantly more mosquitoes as
capture percentages almost doubled. Comparing the flight behaviour around
the M-Tego with variable close-range host cues showed that when these cues
were present, flying mosquitoes were more attracted to the trap and spent
more time there. In addition, we found that the M-Tego has a better
capture mechanism than the BG-Suna, most likely because it does not elicit
previously-observed upward avoiding manoeuvres. Conclusions: Our results
suggest that adding heat and humidity to an odour-baited trap lures more
mosquitoes close to the trap and retains them there longer, resulting in
higher capture performance. These findings support the development of
control tools for fighting mosquito-borne diseases such as malaria.