The significance of midsummer movements of Autographa gamma: Implications for a mechanistic understanding of orientation behavior in a migrant moth

Abstract

Abstract The silver Y moth Autographa gamma undertakes windborne spring and fall migrations between winter breeding regions around the Mediterranean and summer breeding regions in northern Europe. Flight behaviors facilitating these migrations include: (i) selection of seasonally-favorable tailwinds; (ii) flying at the altitude of the fastest winds; (iii) adopting flight headings that partially counteract crosswind drift; and (iv) seasonal reversal of preferred directions between spring and fall. In the UK, radar measurements indicate that migratory activity is pronounced during the spring and fall, but is usually very low during midsummer (July). However, an atypically intense period of high-altitude flight was recorded during July 2006, and in this study we compare the flight behavior of A. gamma during these midsummer movements with the more typical spring and fall migrations. During July 2006, activity was most intense at significantly lower altitudes than occurred in spring or fall, and was not associated with the height of the fastest winds; consequently displacement speeds were significantly slower. The most striking difference was an absence of tailwind selectivity in July with windborne movements occurring on almost every night of the month and on tailwinds from all directions. Finally, orientation behavior was quantitatively different during July, with significantly greater dispersion of flight headings and displacements than observed in spring and fall. We discuss mechamsms which could have caused these differences, and conclude that a lack of appropriate photoperiod cues during development of the summer generation resulted in randomly-oriented ‘dispersive’ movements that were strikingly different from typical seasonal migrations.