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

Charlotte E Wainwright,Don R Reynolds,Andy M Reynolds

doi:10.1038/s41598-020-57779-0

Abstract

Huge numbers of insects migrate over considerable distances in the stably-stratified night-time atmosphere with great consequences for ecological processes, biodiversity, ecosystem services and pest management. We used a combination of meteorological radar and lidar instrumentation at a site in Oklahoma, USA, to take a new look at the general assistance migrants receive from both vertical and horizontal airstreams during their long-distance flights. Movement in the nocturnal boundary layer (NBL) presents very different challenges for migrants compared to those prevailing in the daytime convective boundary layer, but we found that Lagrangian stochastic modelling is effective at predicting flight manoeuvers in both cases. A key feature for insect transport in the NBL is the frequent formation of a thin layer of fast-moving air – the low-level jet. Modelling suggests that insects can react rapidly to counteract vertical air movements and this mechanism explains how migrants are retained in the jet for long periods (e.g. overnight, and perhaps for several hours early in the morning). This results in movements over much longer distances than are likely in convective conditions, and is particularly significant for the reintroduction of pests to northern regions where they are seasonally absent due to low winter temperatures.

Highlights

Migration is a key life-history component in many insects with important ecological and evolutionary consequences for the species, as well as significant economic, environmental and cultural impacts on humankind
Other vertical motions in the nocturnal stable boundary layer (NBL) may result from the combination of the shutdown of turbulent mixing at sunset occurring over a laterally-varying buoyancy field, which can produce weak but persistent ascent of magnitude 3–10 cm s−1 alongside a strong nocturnal Blackadar-type low-level jet horizontal wind speed profile[19]
The amplitude of vertical air motion in the NBL is significantly reduced as compared to the daytime convective boundary layer, there exists a need to determine the effect of these motions on nocturnal insect migration and, more generally, to compare the behavioral responses of small insects to vertical air movements throughout the diel cycle of the atmospheric boundary layer (ABL)

Summary

Introduction

Migration is a key life-history component in many insects with important ecological and evolutionary consequences for the species, as well as significant economic, environmental and cultural impacts on humankind (e.g. refs. 1–6). Migrants will usually enter the convective boundary layer (CBL), the portion of the ABL where the vertical air motion is dominated by thermally-driven updrafts and downdrafts, and the (quite subtle) behavioral responses of small insects to vertical air movements under these conditions was the subject of a previous paper The amplitude of vertical air motion in the NBL is significantly reduced as compared to the daytime convective boundary layer, there exists a need to determine the effect of these motions on nocturnal insect migration and, more generally, to compare the behavioral responses of small insects to vertical air movements throughout the diel cycle of the ABL. Knowledge of how insects react to different vertical air movements is a necessary step in understanding their altitudinal selection and improving insect movement forecasting models.To realize this objective, we used a combination of zenith-pointing Doppler lidar and Ka-band dual-polarized profiling cloud radar which together provide precise measurements of the vertical component of air velocity concurrently with a quantification of the movements of insects aloft at various times of diurnal cycle[10]

Methods

Findings

Conclusion