Passive kHz lidar for the quantification of insect activity and dispersal

Abstract

Background: In recent years, our group has developed electro-optical remote sensing methods for the monitoring and classification of aerofauna. These methods include active lidar methods and passive, so-called dark-field methods that measure scattered sunlight. In comparison with satellite-and airborne remote sensing, our methods offer a spatiotemporal resolution several orders of magnitude higher, and unlike radar, they can be employed close to ground. Whereas passive methods are desirable due to lower power consumption and ease of use, they have until now lacked ranging capabilities. Results:In this work, we demonstrate how passive ranging of sparse insects transiting the probe volume can be achieved with quadrant sensors. Insects are simulated in a raytracing model of the probe volume, and a ranging equation is devised based on the simulations. The ranging equation is implemented and validated with field data, and system parameters that vary with range are investigated. A model for estimating insect flight headings with modulation spectroscopy is implemented and tested with inconclusive results. Insect fluxes are retrieved through time-lag correlation of quadrant detector segments, showing that insects flew more with than against the wind during the study period. Conclusions:The presented method demonstrates how ranging can be achieved with quadrant sensors, and how it can be implemented with or without active illumination. A number of insect flight parameters can be extracted from the data produced by the sensor and correlated with complementary information about weather and topography. The approach has the potential to become a widespread and simple tool for monitoring abundances and fluxes of pests and disease vectors in the atmosphere.

Photonic Monitoring of Atmospheric and Aquatic Fauna

“…Methods such as lunar obscuration, digital in‐line holography and various entomological modulation spectroscopy devices operate in transmission mode close to 0° in the angular domain. Passive dark‐field methods based on sunlight, where the field‐of‐view is terminated in a black cavity, operate at angles between 180° and 90°. In most situations regarding sparse organism detection, organisms would only account for a fraction of the probe volume in efficient detection schemes capable of detecting large numbers.…”

“…Zenith or nadir observations greatly simplify this matter . This is because lateral transport dominates over vertical transport and the anatomical axes are predominantly horizontal such that optical cross sections remain invariant with heading angle. Also certain band ratios such as depolarization or melanization have been shown to be largely angular insensitive .…”

“…In Figure , a spectrogram is presented of a small prey, presumably a fruit fly or similar, which is followed by a large low‐frequency predator, presumable a dragonfly, at the campus of ICIPE, Nairobi . If a quadrant photodiode is employed, range and flight directionality can be assessed . This information can be visualized in color‐coded spectrograms, despite that insects may seem to intersect the probe volume randomly, statistics of the thousands of insects observed over an hour may shed light on the large‐scale movement and dispersal of insects, for example, in relation to migration, pest invasion, or wind conditions.…”

“…Different insects and stages of insects use wind differently, for example, tailwind for energy saving, headwind for olfactory tracking, and side winds for maximizing encountering of odor plumes. Naturally it can be shown that lateral movement is more frequent than vertical movement . Beam splitters or sandwiched photodiodes (e.g., Si/InGaAs) may be employed to retrieve various broad bands of the scattered sunlight at kHz rates .…”

“…Passive ranging (or passive lidar in analogy with the more widespread passive radar) on aerofauna was initially attempted through the O 2 absorption band at 760 nm, but the results were ambiguous . However, it is possible to calculate the insect range in dark‐field methods through employment of a quadrant detector and a correlation method . Such minimal insect monitors may find widespread usage in, for example, agriculture; however, the method has still not been developed for nocturnal species.…”