The influence of the nocturnal boundary layer on layers of insects.

Monitoring insect migration is important as it can inform us about insect ecology and population dynamics. Such work has application to agricultural practices as many insect species are important pests, while others are significant natural enemies of pests. Most migrations occur at altitudes of several hundred metres where they are difficult to observe using ground-based methods. Therefore, vertical-looking entomological radar is well suited to retrieving vertical insect profiles. We have used 5 years' worth of data from 2 UK-based radars in this study (one located continuously at Rothamsted, the other located at Malvern then Chilbolton). Vertical insect profiles exhibit different forms, largely dependent on the time of day. The most notable form of profile of relevance to migration is a layered one. Layering is where a significant number of insects in the atmosphere fly in a narrow range of altitudes (e.g. 50-100 m in depth); UK insect layers are typically centred 200-600 m above the ground. The most temporally and spatially consistent layers have been observed nocturnally, when migratory species such as noctuid moths are active.

The key question which this research addresses is whether meteorological conditions have an important influence on insect layers. In stable boundary layers, vertical wind speeds are typically low and so insects have excellent control over vertical flight-speed. It has been suggested previously (and is our hypothesis) that insects can hence accumulate at altitudes with favourable conditions; most particularly in warm temperatures (i.e. nocturnal temperature inversions). Meteorological variables retrieved from radiosondes are useful for field campaign studies, but are too sparse in both space and time for use in a systematic study. Therefore, at the radar sites, vertical profiles of meteorological variables have been retrieved from a mesoscale numerical weather prediction model: the UK Met Office's Mesoscale ‘Unified Model' (UM).

The focus of this research is nocturnal insect layers. Often the nocturnal boundary layer is stable: case studies revealed layers located near the altitude of warmest air (i.e. temperature inversion top) and/or windiest air (i.e. nocturnal jet). Statistical analysis of the insect data-set from the radar and meteorological profiles from the UM over 5 years revealed that layers were strongly correlated with temperature and not correlated with wind speed, but weakly correlated with the presence of a nocturnal jet. Additionally, it was found that neither wind speed- nor directional-shear were associated with layers, suggesting that insect source distribution is homogeneous over distances of 10s of km. These results will be used to form the parameterisations in a stochastic Lagrangian insect-trajectory model (SLIM) and preliminary results will be presented.