Resolving the heading‐direction ambiguity in vertical‐beam radar observations of migrating insects

Abstract

Each year, massive numbers of insects fly across the continents at heights of hundreds of meters, carried by the wind, bringing both environmental benefits and serious economic and social costs. To investigate the insects' flight behavior and their response to winds, entomological radar has proved to be a particularly valuable tool; however, its observations of insect orientation are ambiguous with regard to the head/tail direction, and this greatly hinders interpretation of the migrants' flight behavior. We have developed two related methods of using wind data to resolve the head/tail ambiguity, and we have compared their outputs with those from simply assigning the heading direction to be that which is closer to the track direction. We applied all three methods to observations of Australian plague locust migrations made with an insect monitoring radar. For the study dataset, some of the headings selected by the simpler method are shown to be clearly incorrect. The two new methods generally agree and reveal a significantly different, and presumably more accurate, relationship of heading direction to track direction. However, use of these methods leads to quite a large proportion of the sample being lost because the wind values, which derive from a regional‐scale numerical model, are shown to be incompatible with the radar observations. This exploratory study has moreover demonstrated that locusts are frequently oriented at a large angle to their track and that quite often their movement is at least slightly tailfirst. Both new methods appear to be a significant improvement on the simpler method. As well as providing an accurate representation of migratory flight behavior, they allow occasions when the model wind values are unreliable to be eliminated from the data sample.

Insect Monitoring Radar: Maximizing Performance and Utility

“…(ZLC radar observations determine the alignment of an insect's body axis, i.e., an angle in the range 0-180º, but do not resolve whether the insect is heading towards or away from this direction. Simultaneous measurements, or estimates, of the wind at the height of each target are needed to overcome this limitation [10], which also exists for alternative, e.g., scanning, radar configurations [12] (ch. 7).)…”

“…Digitization occurred at the filter outputs at 320 Hz, i.e., after every fourth pulse [17]. This system of data acquisition in 'slow time' (in the parlance of radar signal processing) produced a lot of useful data (e.g., [9,10]), but the fixed broad gates introduced some contamination (see below). The availability from the mid-2000s of affordable and fast A-to-D converters provided an opportunity to both eliminate a demanding in-house hardware build and to improve the radar's data quality and coverage.…”

“…In the case of the IMRs operating in Australia, opportunities provided by new data-acquisition technologies have led to a more general review of the systems' design and operating protocols, as well as recognition that an IMR's effectiveness as a facility for observing insect migration could be considerably increased. Experience gained through analysing IMR data from over a decade (e.g., [6,10,11]) also informed this review by indicating circumstances where larger datasets, or more continuous observation, would likely have enabled additional or stronger scientific inferences. An upgraded unit, designated IMRU ('IMR Upgraded'), was subsequently developed and deployed to Hay, New South Wales, Australia (34.5458 • S, 144.8663 • E), where it commenced observations in August 2017.…”

“…Observations from small, purpose-built radars such as these complement the data on insect movement that are now becoming available from operational weather-surveillance radars (WSRs, e.g., [7]). The narrow beam, high range resolution, and short observing range of the IMRs and VLRs allow them to resolve individual insects, enabling the discrimination of different target types [8,9] and the examination of within-population variances 2 of 20 of migration parameters (e.g., [10]). The relatively low capital and running costs of these units make them more compatible with the modest budgets typical of entomological and aeroecological research programs.…”

“…Remote Sens. 2020, 12, 596 2 of 21 population variances of migration parameters (e.g., [10]). The relatively low capital and running costs of these units make them more compatible with the modest budgets typical of entomological and aeroecological research programs.…”