Does a ‘turbophoretic’ effect account for layer concentrations of insects migrating in the stable night-time atmosphere?

Large migrating insects, such as noctuid moths and acridoid grasshoppers, flying within the stable nocturnal boundary layer commonly become concentrated into horizontal layers. These layers frequently occur near the top of the surface temperature inversion where warm fast-moving airflows provide good conditions for downwind migration. On some occasions, a layer may coincide with a higher altitude temperature maximum such as a subsidence inversion, while on others, it may seem unrelated to any obvious feature in the vertical profile of meteorological variables. Insects within the layers are frequently orientated, either downwind or at an angle to the wind, but the mechanisms involved in both layer formation and common orientation have remained elusive. Here, we show through the results of numerical simulations that if insects are treated as neutrally buoyant particles, they tend to be advected by vertical gusts (through the ‘turbophoretic’ mechanism) into layers in the atmosphere where the turbulent kinetic energy has local minima. These locations typically coincide with local maxima in the wind speed and/or air temperature, and they may also provide cues for orientation. However, the degree of layering predicted by this model is very much weaker than that observed in the field. We have therefore hypothesized that insects behave in a way that amplifies the turbophoretic effect by initiating climbs or descents in response to vertical gusts. New simulations incorporating this behaviour demonstrated the formation of layers that closely mimic field observations, both in the degree of concentration in layers and the rate at which they form.