Chapter 4 Flight Patterns of Host-Seeking Anopheles gambiae s.s During
4.1.1 Height of Flight, Navigating Barriers
The majority of Anopheles gambiae caught in open, un-vegetated land will fly at less than 1m above the ground (Snow, 1979). Vertical barriers of up to 1.72m in height placed around a volunteer reduced the number of mosquitoes attacking a human host to less than 60% of that approaching an unprotected host (Snow, 1987). Another study found that An. gambiae and An. funestus were capable of flying over a 6m tall fence when responding to a human or cow bait, and that such a barrier did not reduce the number of mosquitoes caught within a circular fenced enclosure (Gillies & Wilkes, 1978). In that study, analysis of the flight elevation of Mansonia sp. mosquitoes, suggested that a mosquito’s movement upwards must occur very close to the barrier: even within 25cm of the fence there was no detectable increase in mosquito elevation. Catches inside a smaller (2.9m high, 3m radius) fenced
enclosure found that whilst some mosquitoes reaching the centre of the ringed circle had returned to ground level (less than 1m), the number still flying at elevations of 1- 3m had proportionally increased.
It is not known whether mosquitoes navigate up a barrier by contacting its surface during flight. Results of a study of passage over short insecticide treated fences around cattle enclosures suggested that culicine mosquitoes were contacting the fence during navigation over it (Maia et al., 2012). However little other information is available on how mosquitoes navigate barriers in flight.
Detailed knowledge of how the main mosquito vectors and nuisance species enter houses could be useful to guide house design or modification to reduce exposure to mosquitoes inside the home without, or at least reducing the reliance on,
insecticides (Lindsay et al., 2002; Ogoma et al., 2009). There is potential to exploit the house entry behaviour of mosquitoes for distribution of insecticide or bio-control agents such as fungi using treated curtains on eaves or windows (Sexton et al., 1990; Fanello et al., 2003; Farenhorst et al., 2011; Mnyone et al., 2012). These methods place treated materials across house openings, on the assumption that insects will contact them as they enter the house. Little is currently known about how mosquitoes move through such openings/ apertures/ spaces: whether they exhibit spatial patterns or preferences, e.g. preferring the boundary or the centre, or
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move randomly, when entering via a window.4.1.2 3D Tracking Methods in Entomology
Single camera 3D imaging has yet to be fully explored in mosquito tracking. The majority of systems that track insects in three dimensions use stereoscopy, in which insect activity is viewed from two perspectives using two cameras, and the data from each camera are coordinated to generate a 3D track (Reynolds & Riley, 2002; Lacey & Cardé, 2011). Using retroreflective screening (RRS) it is possible to
generate a 3D track using a single camera. This is achieved by placing the RRS at the rear of the field of view; using light from an infrared LED positioned adjacent to the camera lens, an image is obtained showing stereo positions of the insect and its shadow on the RRS. With calibration, the distance between the insect and its shadow is used to estimate the mosquito’s proximity to the RRS. Using the sun as a light source, this approach has been applied to obtain 3D flight data on diurnal insects including bees, wasps and midges, in studies examining a variety of insect behaviours including nest approach, swarming and landing (Okubo et al., 1981; Zeil et al., 1993; Srinivasan et al., 2000). Though the majority of those studies tracked flight of single insects, this method has also been used successfully to study interactions of multiple insects in swarms (Okubo & Chiang, 1974).
Single viewpoint imaging has many advantages in that it requires only one camera – hence it is cheaper, easier to transport and considerably simpler to calibrate, and has a faster tracking procedure following recording, as only one video file requires processing. Set-ups must be recalibrated according to the moving position of the sun (Zeil et al., 1993; Srinivasan et al., 2000), although this does not affect tracking of nocturnal insects where an infrared light can be installed in a fixed position to illuminate the entire test set-up. Clearly, single viewpoint 3D tracking offers many advantages for studying flight patterns of nocturnal insects such as mosquitoes in the laboratory and in the field.
This chapter reports on a set of studies utilising a newly developed single camera 3D tracking system, which explored the movement of An. gambiae s.s. during passage through an aperture or ‘window’ fitted between two experimental rooms. This study aimed to evaluate the capabilities of this novel tracking system for use in studies of mosquito behaviour, through experimental proof-of-principle of the use of retro-reflective screens in 3D flight tracking. The objective of this study was to test the viability of this tracking method for use in research into house entry behaviour of mosquitoes. The secondary research objective of the study was to characterise
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flight patterns of mosquitoes entering a room, and to determine whether insects exhibited spatial preferences or patterns in their flight paths. The study held the null hypothesis that mosquitoes would enter houses through random paths, and that no trends in spatial activity would be observed.90
4.2 Methods
4.2.1 Mosquitoes
All tests were carried out using 3-5 day old female An. gambiae s.s. Kisumu strain, reared in the LSTM insectaries (conditions described in chapter 2, section 2.2.5). Behavioural recordings were made in the initial 1-6 hours of the scotophase (13:00- 18:00). On the morning of the test day, individual mosquitoes were selected based on their attraction to an arm placed against the side of their cage. The selected mosquitoes were sugar starved for 4-6 hours before testing.