Radar-Based Studies of the Migratory Flight of Grasshoppers in the Middle Niger Area of Mali

ABSTRACT Some grasshopper species are pests of subsistence agriculture in the Sahelian zone of West Africa. Formulation of effective control strategies against these pests requires some knowledge of their migratory ability. In this paper a study is described in which radar was used to observe aspects of the nocturnal migratory behaviour of grasshoppers in the middle Niger delta. Mass take-off at dusk, layering and common orientation were regularly observed. Layering appeared to be related to air temperature. Mean orientation was often downwind but at other times crosswind headings occurred which added to the southerly component of the insects' displacement. Probable source areas of insects overflying the radar were identified by calculations of the insects' back-trajectories.

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    • "Radar echoes must be identified as birds, if the aim is to quantify bird movements. Insects can make up an overwhelming proportion of targets depending on time, location (Riley & Reynolds 1979, 1983) and radar sensitivity (Eastwood 1967), and present the most significant interference with bird targets. Although radar for studying insect movements is widely accepted (Glover et al . "
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    ABSTRACT: Besides the scientific interest in the quantification of bird migration, there is an increasing need to quantify bird movements for the assessment of bird collision risk with artificial structures. In many environmental impact studies, the radar method is used in an inappropriate manner. The processing of echoes consists often of counting blips within defined screen fields, and the surveyed volume is estimated without reference to the detection probabilities of different ‘target sizes’ (radar cross-sections). The aim of this paper is to present a procedure to quantify bird migration reliably using radar by stating the theoretical requirements of every single step of this procedure and presenting methodological solutions using our own radar data from extensive field studies. Our methodological solutions can be applied to various radar systems, including widely used ship radar. The procedure presented involves discriminating the echoes of birds and insects and estimating the different detection probabilities of differently ‘sized’ birds (radar cross-sections). By ignoring the different detection probabilities, density estimations may be wrong by as much as 400%. We fear that quantification of bird migration and predicted bird numbers affected by collisions with artificial structures are in many cases based on unreliable estimates.
    Ibis 03/2008; 150(2):342 - 355. DOI:10.1111/j.1474-919X.2007.00797.x · 1.92 Impact Factor
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    • "Selection of the warmest air by migrants appears to be most likely to occur in relatively cool conditions, and in taxa that have high optimum temperatures for migratory flight. For example, migratory acridoid insects (grasshoppers and locusts) have optimum temperature values for sustained flight of above 20°C (Clark 1969; Riley and Reynolds 1979), which are much higher than for instance the noctuid moths studied here (see also Taylor and Carter, 1961). On the other hand, there are many references in the literature in which insects, particularly moths, have ascended above the altitude of the temperature maximum. "
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    ABSTRACT: Insects migrating at high altitude over southern Britain have been continuously monitored by automatically operating, vertical-looking radars over a period of several years. During some occasions in the summer months, the migrants were observed to form well-defined layer concentrations, typically at heights of 200-400 m, in the stable night-time atmosphere. Under these conditions, insects are likely to have control over their vertical movements and are selecting flight heights that are favourable for long-range migration. We therefore investigated the factors influencing the formation of these insect layers by comparing radar measurements of the vertical distribution of insect density with meteorological profiles generated by the UK Meteorological Office's (UKMO) Unified Model (UM). Radar-derived measurements of mass and displacement speed, along with data from Rothamsted Insect Survey light traps, provided information on the identity of the migrants. We present here three case studies where noctuid and pyralid moths contributed substantially to the observed layers. The major meteorological factors influencing the layer concentrations appeared to be: (a) the altitude of the warmest air, (b) heights corresponding to temperature preferences or thresholds for sustained migration and (c) on nights when air temperatures are relatively high, wind-speed maxima associated with the nocturnal jet. Back-trajectories indicated that layer duration may have been determined by the distance to the coast. Overall, the unique combination of meteorological data from the UM and insect data from entomological radar described here show considerable promise for systematic studies of high-altitude insect layering.
    International Journal of Biometeorology 04/2006; 50(4):193-204. DOI:10.1007/s00484-005-0014-7 · 3.25 Impact Factor
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    • "The radiosondes ascended at a rate of 100 m/min and provided estimates of wind velocity at 50-m intervals and of air temperature approximately every 8 m, the data being automatically recorded into a laptop computer (Feng et al., 2003). Using these wind estimates, the air speed and direction of individual insect targets were calculated from the radar data by vector-subtraction (Riley and Reynolds, 1979). (It was assumed that the speed and direction of the wind did not change within 30 min of an ascent, and air-speed estimates were calculated only for this period.) "
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    ABSTRACT: The high-altitude windborne transport of the second-generation moths of Helicoverpa armigera (Hübner) was observed with radar in July 2001 and 2002 at Langfang, Hebei province, China. Moths of local populations took off at dusk, with “area density” peaking ca. 0.5 h later. The variation of area density through the night suggested that the maximum flight duration was about 8.5 h. The moths generally ascended to altitudes of up to 1500 m above ground level (AGL) and formed layer concentrations at ca. 300, 500, and 1000 m AGL. Multiple layering was very common and layers were located at zones of maxima in wind speed or in wind shear rather than in temperature inversions. Both the displacement direction and the orientation of the insect were at an acute angle to the downwind direction, rather than being distributed at random, and varied as the wind direction changed. The windborne moths were mainly transported into northeastern China where maize at the silking stage would have provided good hosts for the subsequent (third) generation of H. armigera. These population movements may account for the nondevelopment of resistance to Bt cotton in H. armigera in northern China.
    Journal of Insect Behavior 05/2005; 18(3):335-349. DOI:10.1007/s10905-005-3694-2 · 1.14 Impact Factor
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