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Description
Many insect species engage in high-altitude, wind-borne migration, often several hundred meters above the ground. At these heights they can use the wind to travel tens or hundreds of kilometers in a single flight, and hence a knowledge of their movements is essential to understanding their ecology and population dynamics. Direct observation of high-flying insect migrants is very difficult, especially at night, but the remote sensing capabilities of entomological radar provide a solution to this seemingly intractable problem. We describe a novel, nutating- beam, vertical-looking radar with autonomous data analysis software. This system routinely extracts data on size, shape, alignment, and displacement vectors from individual targets, allowing long-term monitoring of migrant insect populations. We discuss the capabilities and limitations of this system and describe some of its applications in the study of insect migration behaviour.
Keywords: insect monitoring, flight, radar entomology, orientation, diamondback moth
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Millions of metric tons of insects are aloft in Earth's atmosphere at any given moment. Much of this multitude comprises insects engaged in high-altitude, wind-borne migration, often at heights several hundred meters (m) above ground level (agl), where they can take advantage of strong winds to fly considerable distances, frequently tens or even hundreds of kilometers (kin; Johnson 1969, Drake and Gatehouse 1995). The evolution of this behavior has allowed migratory insects to exploit resources that vary in space and time (Southwood 1977), providing them with adaptive benefits relative to more sedentary species. This enormous aerial "bioflow" has important implications for ecological, physiological, and genetic studies of insects, with applications in pest management, conservation, and environmental change programs (Woiwod and Harrington 1994, Drake and Gatehouse 1995).
The high-altitude flight of migratory insects, together with their relatively small body size and the fact that many species are nocturnal, means that it is very difficult to observe their movements. The study of insect migration has, therefore, relied primarily on the interpretation of indirect evidence of long-distance flights, such as catches in light traps and other ground-based observations. However, this approach has significant limitations: For example, light-trap catches are strongly influenced by the weather and the lunar cycle (Yela and Holyoak 1997), and there may be an interval of several days between an immigration and the resultant peak in lighttrap catches (Wada et al. 1987). Moreover, ground-based observations give no indication of the height of the migrants' flight, which is critical in determining the origin of migrant populations because wind speed and direction vary with height (Chapman et al. 2002a). Evidently, what is required is a means of monitoring high-altitude insect migration while it is in progress, but this is a technically challenging enterprise. Maintaining sampling platforms in the air (e.g., aircraft, tethered balloons; Reynolds et al. 1997) is expensive and impracticable over long periods. Also, mechanical sampling devices lack the necessary sampling volume to catch less numerous species and the necessary spatial and temporal sensitivity (i.e., they cannot sample simultaneously from many heights over a large altitude range) to adequately study high-altitude migration.
It seems obvious, at least in hindsight, that there should be a role for a remote sensing technology such as radar, and since G. W. Schaefer's pioneering study in 1968 (Schaefer 1976), groups in several countries have developed entomological scanning radar for observing insect migration at high altitude (see the Radar Entomology Web site, www.ph.adfa.edu.au/a-drake/trews). The great advantages of radar are (a) its unique capacity to detect insects simultaneously at a range of altitudes that can reach more than 1 km agi and (b) the large sampling volume that it provides relative to traditional sampling methodologies (Chapman et al. 2002b). Furthermore, because insects are unaffected by flying through the radar beam, the method provides an unparalleled opportunity to investigate aspects of insect migration behavior, such as orientation in high-flying migrants (Riley and Reynolds 1986, Reynolds and Riley 1997). Early studies used inexpensive Xband scanning radar for case studies of mass migrations of pest insect s and revealed many interesting and sometimes spectacular phenomena (Riley and Reynolds 1986, Drake and Farrow 1988, Beerwinlde et al. 1994, Reynolds and Riley 1997). However, these devices are labor-intensive to operate and so are intrinsically unsuitable for long-term monitoring tasks. Vertical-looking radar (VLR) systems allow continuous and autonomous monitoring of pest migrations (Beerwinkle et al. 1995), but early versions of these systems had very limited target identification capabilities. The inclusion of beam nutation in the 1990s (Drake 1993, Smith et al. 1993) has allowed more information to be derived from the returned signals, resulting in improved identification facilities (Smith et al. 1993, Chapman et al. 2002b). This makes nutating VLR amenable to long-term ecological studies in habitats with high insect biodiversity (Smith et al. 2000, Drake et al. 2001), and within the last few years VLR systems have been incorporated in the first long-term research programs monitoring high-altitude insect migration. Separate studies are being carried out by two groups. One group based at the... |
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