The movement of small insects in the convective boundary layer: linking patterns to processes

Abstract

In fine warm weather, the daytime convective atmosphere over land areas is full of small migrant insects, among them serious pests (e.g. some species of aphid), but also many beneficial species (e.g. natural enemies of pests). For many years intensive aerial trapping studies were the only way of determining the density profiles of these small insects, and for taxon-specific studies trapping is still necessary. However, if we wish to determine generic behavioural responses to air movements shown by small day-migrating insects as a whole, the combination of millimetre-wavelength ‘cloud radars’ and Doppler lidar now provides virtually ideal instrumentation. Here we examine the net vertical velocities of > 1 million insect targets, relative to the vertical motion of the air in which they are flying, as a succession of fair-weather convective cells pass over the recording site in Oklahoma, USA. The resulting velocity measurements are interpreted in terms of the flight behaviours of small insects. These behaviours are accounted for by a newly-developed Lagrangian stochastic model of weakly-flying insect movements in the convective boundary layer; a model which is consistent with classic characterisations of small insect aerial density profiles. We thereby link patterns to processes.

Linking Small-Scale Flight Manoeuvers and Density Profiles to the Vertical Movement of Insects in the Nocturnal Stable Boundary Layer

“…3) across the full two-month period investigated. This validates previous assumptions that small insects emigrating at dusk actively climb to altitude with minimal atmospheric assistance 13 , in stark contrast to small insect migration in the well-mixed daytime convective boundary layer which relies on assistance from thermals 10 . Figure 2 also shows a second period of insect ascent at dawn, although this is less strong than the clear ascent signal seen at dusk.…”

“…When we go beyond ref. 10 and make the additional assumption that insects tend to be concentrated in the upper half of the layer when updrafts (or lower half when downdrafts) are present, model predictions (Fig. 5b) are broadly consistent with our observations.…”

“…Our analysis and that of Wainwright et al 10 suggests that airborne dispersal of weak fliers across widely-varying atmospheric conditions can be predicted reliably on the basis of high-resolution aerial density profiles. Such data should become increasingly available from combinations of special-purpose entomological radars and operational weather surveillance radars.…”

“…(2020) 10:1019 | https://doi.org/10.1038/s41598-020-57779-0 www.nature.com/scientificreports www.nature.com/scientificreports/ Insect layers formed in the stratified early-morning atmosphere (arising from dawn emigration or even from all-night flight 15 ) sometimes persist for several hours but are usually disrupted by the upward progression of convective turbulence [45][46][47][48] . This strong layering effect in the SBL has previously been suggested to correspond to insect layers forming at heights of localized temperature or wind speed maxima 10 . Discerning whether temperature or wind is the primary driver of insect layer formation has been the subject of previous studies, with mixed results 25,34,49 , and is complicated by the fact that wind and temperature maxima are often collocated so disentangling the role of each variable is not always possible.…”

Linking Small-Scale Flight Manoeuvers and Density Profiles to the Vertical Movement of Insects in the Nocturnal Stable Boundary Layer

“…3) across the full two-month period investigated. This validates previous assumptions that small insects emigrating at dusk actively climb to altitude with minimal atmospheric assistance 13 , in stark contrast to small insect migration in the well-mixed daytime convective boundary layer which relies on assistance from thermals 10 . Figure 2 also shows a second period of insect ascent at dawn, although this is less strong than the clear ascent signal seen at dusk.…”

“…When we go beyond ref. 10 and make the additional assumption that insects tend to be concentrated in the upper half of the layer when updrafts (or lower half when downdrafts) are present, model predictions (Fig. 5b) are broadly consistent with our observations.…”

“…Our analysis and that of Wainwright et al 10 suggests that airborne dispersal of weak fliers across widely-varying atmospheric conditions can be predicted reliably on the basis of high-resolution aerial density profiles. Such data should become increasingly available from combinations of special-purpose entomological radars and operational weather surveillance radars.…”

“…(2020) 10:1019 | https://doi.org/10.1038/s41598-020-57779-0 www.nature.com/scientificreports www.nature.com/scientificreports/ Insect layers formed in the stratified early-morning atmosphere (arising from dawn emigration or even from all-night flight 15 ) sometimes persist for several hours but are usually disrupted by the upward progression of convective turbulence [45][46][47][48] . This strong layering effect in the SBL has previously been suggested to correspond to insect layers forming at heights of localized temperature or wind speed maxima 10 . Discerning whether temperature or wind is the primary driver of insect layer formation has been the subject of previous studies, with mixed results 25,34,49 , and is complicated by the fact that wind and temperature maxima are often collocated so disentangling the role of each variable is not always possible.…”

Evaluation of convective boundary layer height estimates using radars operating at different frequency bands

“…They found that some insects often rise above the CBL top. Wainwright et al (2017) studied the behaviour of small insects with the Ka-band radar and the Halo Doppler lidar and found, as similarly reported by Geerts and Miao (2005a, b), that these small daytime insects travel upwards with a slower pace than the airflow in the rising air plumes.…”

Linking Small-Scale Flight Manoeuvers and Density Profiles to the Vertical Movement of Insects in the Nocturnal Stable Boundary Layer

“…3) across the full two-month period investigated. This validates previous assumptions that small insects emigrating at dusk actively climb to altitude with minimal atmospheric assistance 13 , in stark contrast to small insect migration in the well-mixed daytime convective boundary layer which relies on assistance from thermals 10 . Figure 2 also shows a second period of insect ascent at dawn, although this is less strong than the clear ascent signal seen at dusk.…”

“…When we go beyond ref. 10 and make the additional assumption that insects tend to be concentrated in the upper half of the layer when updrafts (or lower half when downdrafts) are present, model predictions (Fig. 5b) are broadly consistent with our observations.…”

“…Our analysis and that of Wainwright et al 10 suggests that airborne dispersal of weak fliers across widely-varying atmospheric conditions can be predicted reliably on the basis of high-resolution aerial density profiles. Such data should become increasingly available from combinations of special-purpose entomological radars and operational weather surveillance radars.…”

“…(2020) 10:1019 | https://doi.org/10.1038/s41598-020-57779-0 www.nature.com/scientificreports www.nature.com/scientificreports/ Insect layers formed in the stratified early-morning atmosphere (arising from dawn emigration or even from all-night flight 15 ) sometimes persist for several hours but are usually disrupted by the upward progression of convective turbulence [45][46][47][48] . This strong layering effect in the SBL has previously been suggested to correspond to insect layers forming at heights of localized temperature or wind speed maxima 10 . Discerning whether temperature or wind is the primary driver of insect layer formation has been the subject of previous studies, with mixed results 25,34,49 , and is complicated by the fact that wind and temperature maxima are often collocated so disentangling the role of each variable is not always possible.…”

Evaluation of convective boundary layer height estimates using radars operating at different frequency bands

“…They found that some insects often rise above the CBL top. Wainwright et al (2017) studied the behaviour of small insects with the Ka-band radar and the Halo Doppler lidar and found, as similarly reported by Geerts and Miao (2005a, b), that these small daytime insects travel upwards with a slower pace than the airflow in the rising air plumes.…”

Linking Small-Scale Flight Manoeuvers and Density Profiles to the Vertical Movement of Insects in the Nocturnal Stable Boundary Layer

“…3) across the full two-month period investigated. This validates previous assumptions that small insects emigrating at dusk actively climb to altitude with minimal atmospheric assistance 13 , in stark contrast to small insect migration in the well-mixed daytime convective boundary layer which relies on assistance from thermals 10 . Figure 2 also shows a second period of insect ascent at dawn, although this is less strong than the clear ascent signal seen at dusk.…”

“…When we go beyond ref. 10 and make the additional assumption that insects tend to be concentrated in the upper half of the layer when updrafts (or lower half when downdrafts) are present, model predictions (Fig. 5b) are broadly consistent with our observations.…”

“…Our analysis and that of Wainwright et al 10 suggests that airborne dispersal of weak fliers across widely-varying atmospheric conditions can be predicted reliably on the basis of high-resolution aerial density profiles. Such data should become increasingly available from combinations of special-purpose entomological radars and operational weather surveillance radars.…”

“…(2020) 10:1019 | https://doi.org/10.1038/s41598-020-57779-0 www.nature.com/scientificreports www.nature.com/scientificreports/ Insect layers formed in the stratified early-morning atmosphere (arising from dawn emigration or even from all-night flight 15 ) sometimes persist for several hours but are usually disrupted by the upward progression of convective turbulence [45][46][47][48] . This strong layering effect in the SBL has previously been suggested to correspond to insect layers forming at heights of localized temperature or wind speed maxima 10 . Discerning whether temperature or wind is the primary driver of insect layer formation has been the subject of previous studies, with mixed results 25,34,49 , and is complicated by the fact that wind and temperature maxima are often collocated so disentangling the role of each variable is not always possible.…”

Evaluation of convective boundary layer height estimates using radars operating at different frequency bands

“…They found that some insects often rise above the CBL top. Wainwright et al (2017) studied the behaviour of small insects with the Ka-band radar and the Halo Doppler lidar and found, as similarly reported by Geerts and Miao (2005a, b), that these small daytime insects travel upwards with a slower pace than the airflow in the rising air plumes.…”