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Title: Optimal strategies for insects migrating in the flight boundary layer: Mechanisms and consequences

item Srygley, Robert

Submitted to: Integrative & Comparative Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/19/2008
Publication Date: 7/1/2008
Citation: Srygley, R.B., Dudley, R. 2008. Optimal strategies for insects migrating in the flight boundary layer: Mechanisms and consequences. Integrative & Comparative Biology. 48:119-133.

Interpretive Summary: The majority of insects fly near to the Earth’s surface in the flight boundary layer, defined as the range above the ground in which insect flight speed exceeds ambient wind speed. Insects in the flight boundary layer are capable of self-propulsion and self-direction, two features which have been used to define migration. For 17 years, we have studied migrating dragonflies, moths and butterflies in the Neotropics. Using a boat mounted with conventional sailboat navigation equipment, we pace individual insects as they migrate across the Caribbean Sea and Panama Canal, and record ambient wind speed and direction simultaneously with the boat’s speed and direction. This method allows us to evaluate the insects’ abilities to maintain their route in varying crosswinds and to use tailwinds to maximize the distance they can fly. From the amount of fat that the insects carry, we also determine their ability to adjust their flight speed in accordance with their energetic reserves to maximize their migratory distance. We find that dragonflies and butterflies are capable of adjusting for crosswinds to maintain their route but dayflying moths are not. Dragonflies and butterflies also use tailwinds to maximize their migratory distance whereas dayflying moths are not. Butterflies maximize adjust their flight speed in accordance with their energetic reserves, whereas the dayflying moths do not. Thus dragonflies and butterflies both show sophisticated adaptations to migrating long distances, whereas the dayflying moths do not. With this knowledge, we can better predict the distance that beneficial insects, such as predatory dragonflies and pollinating moths and butterflies, travel without refueling. The ability of the insects to remain on course between suitable stopover sites is an important consideration when devising policies to conserve spectacular insect migrations, such as that of the monarch butterfly.

Technical Abstract: Directed aerial displacement requires that a volant organism’s airspeed exceeds ambient wind speed. For biologically relevant altitudes, wind speed increases exponentially with increased height above the ground. Most insects thus disperse according to atmospheric conditions. However, those insects flying close to the Earth's surface displace within the flight boundary layer where insect airspeeds are relatively high. Over the past 17 years, we have studied boundary-layer fliers by following individual insects as they migrate across the Caribbean Sea and the Panama Canal. Although most migrants evade either drought or cold, nymphalid and pierid butterflies migrate across Panama near the onset of the rainy season. Dragonflies of the genus Pantala migrate in October concurrently with frontal weather systems. Migrating the furthest and thereby being the most difficult to study, the diurnal moth Urania fulgens migrates between Central and South America. The migratory butterflies and dragonflies are capable of directed movement towards a preferred compass direction in variable winds, whereas the moths drift with winds over water. The butterflies orient using both global and local cues. In accordance with optimal migration theory, butterflies and dragonflies adjust their flight speeds to maximize migratory distance traveled per unit fuel, whereas the moths do not. Moreover, only butterflies adjust their flight speed in accordance with endogenous fuel reserves. It is likely that these insects use optic flow to gauge their speed and drift, and thus must migrate where sufficient detail in the Earth’s surface is visible to them. The abilities of butterflies and dragonflies to adjust airspeed over water indicate sophisticated control and guidance systems pertaining to migration.