features and with the vertical structure of the atmosphere. Our aim in this review is to identify the various' features of the atmospheric environment that affect insect migration, and to indicate how migrating insects respond to these features and how their migratory flights are influenced by them. An analysis along these lines was first made by Wellington (146), although, writing over 40 years ago, he had few relevant entomological observations to draw on. Rainey (104) described some additional atmospheric features that affect migrating insects rn a 1976 paper, which was based mainly on his own pioneering research on migration meteorology (e.g. lOl a, 102). Our coverage is more comprehensive than Rainey's, although we treat in­ dividual atmospheric features and their associated entomological phenomena in much less detail. For each of the atmospheric features that have been shown to affect insects during migratory flights, we first provide a brief description

The Influence of Atmospheric Structure and Motions on Insect Migration

The role of horizontal wind convergence in concentrating flying insects is examined. Because of the inverse relation between intensity and persistence of convergence zones, a tenfold increase in volume density is unlikely to be exceeded in a single atmospheric disturbance, and then only if the insects are not taken through the convergence zone. Radar has shown that the commonest disturbances associated with insect concentrations are windshift lines: modifications of the broad-scale wind flow due to buoyancy and blocking effects. Even in the absence of concentration, a tenfold increase in area density can be brought about by vertical circulations at windshift lines. Insect concentrations are likely to be most frequent and persistent at night, and over and near mountains, where searches are most difficult.

Concentration of Flying Insects by the Wind

Airborne migration is one of the most common adaptations for surviving and exploiting habitat variability. One weather feature that airborne migrants sometimes encounter, convergence, has the potential to concentrate populations in localities where climatic conditions have made a habitat temporarily favourable. Studies of migratory insects have now begun to establish which of several forms of atmospheric convergence are most likely to affect population processes.

The 'aerial plankton' and atmospheric convergence.

Publisher Summary This chapter discusses the effects of local wind systems on insect migration at night. Many insect species undertake migratory flights at night. Such flights are usually dominated by the wind because the insects climb to heights where the wind speed exceeds their own flying speed through the air. Consequently, the distance moved is essentially the product of wind speed and duration of flight. The results throw light on the occurrence of clouds of flying adults and on the clumped distribution of populations of migrants on the ground. Much of the evidence comes from field studies of a few pest species, but it is likely that many other species are similarly affected. Several types of wind system are considered: low-level jet streams, coastal winds, rainstorm outflows, down-slope and down-valley winds, and a variety of local winds caused by barrier effects of mountains. Furthermore, field studies need to put emphasis on atmospheric behavior through supplementary observations of winds and temperatures in the lowest kilometer of the atmosphere. This chapter discusses coastal winds, bores and solitary waves, rainstorm outflows, and mountain winds: drainage winds and barrier winds in reference to the nocturnal insects.

Nocturnal Insect Migration: Effects of Local Winds

Migratory flight close to the Earth’s surface (within the so-called flight boundary layer) occurs in some insects, but the vast majority of migrants ascend above this layer and harness the power of the wind for transport. The resulting displacements range from dispersive movements over a few tens of metres to seasonal migrations covering thousands of kilometres. In this chapter, we summarize knowledge of the use of the aerosphere by insects, focusing particularly on longer migrations, in relation to: the height and duration of flight, direction and speed of movement, seasonal and diel patterns, and responses to atmospheric conditions and phenomena. The seasonal mass movements have major ecological consequences in the invaded areas, and these are discussed briefly. We also highlight recent comparisons of insect movement strategies with those of flying vertebrates and mention interactions between these groups in the atmosphere. We conclude with some suggestions for the future development of these topics.

Riders on the Wind: The Aeroecology of Insect Migrants

On occasion Greenbank et al. (1980) in their 1973 to 1976 study on spruce budworm moth dispersal in New Brunswick, Canada, detected intense line concentrations of airborne moths crossing special radar observing sites located about 100 km inland from both the Bay of Fundy and the Northumberland Straits. Line concentrations of insects reflect the presence of atmospheric convergence lines. Data from a surface mesonetwork and wind and temperature soundings up to 2 km collected in New Brunswick from 1976 to 1978 around the period of spruce budworm moth activity has revealed sea breeze fronts to be the meteorological origin for the line concentrations of moths. Analysis has shown sea breeze fronts penetrating 80 to 100 km inland in New Brunswick can be expected once or twice each year during the moth dispersal period. An equation for maximum inland penetration was developed which gave a correlation coefficient of 0.78 between predicted and observed inland penetrations with a standard error of 17.5 km. Predicted sea-breeze frontal penetrations of 100 km or more were considered likely to produce a line concentration of moths observable at the radar sited. Using this criteria the prediction scheme delineated four out of the five nights when line concentrations of moths were observed and only over-predicted on one occasion. Thus meteorological data can be used to predict the appearance of line concentrations of spruce budworm moths and so provide input into population redistribution studies and into the development of control strategies directed at the adult budworm.

E. I. Mukammal

Papers

Incidence of mesoscale convergence lines as input to spruce budworm control strategies