Relative flight responses of Rhagoletis indifferens as influenced by crowding, sex, and resources

Although flight is believed to be the primary mechanism for dispersal in the Western cherry fruit fly, Rhagoletis indifferens Curran (Diptera: Tephritidae), an orchard pest of both sweet (Prunus avium L.) and sour (Prunus cerasus L.) (Rosaceae) cherry crops, the movement of these flies between host patches is difficult to quantify in the field. A tethered flight mill system was used in the laboratory to examine the flight behaviour of sexually mature flies exposed to different levels of conspecific contact and resource availability. A complete 2 × 2 × 3 factorial design compared the relative influence of the factors ‘context’ (crowded, isolated), ‘sex’ (female, male), and ‘resources’ (low = food only; medium = food + leaf; high = food + leaf + cherries) on flight performance measures including distance flown, net trial time, and stopping patterns. Rather than using a minimum time or distance to determine trial length, flight observations were continued for each fly until a behavioural protocol based on stopping time was met. In this protocol each successful trial was composed of three consecutive flight intervals and included a minimum of three stops lasting a combined total of 5 min. Of the 160 flies tested, 86.9% flew <500 m on the flight mill. Individuals from both sexes were capable of maximum flights in the same order of magnitude, ca. 3 km on the flight mill. Distance flown was significantly influenced by ‘context’ such that crowded individuals flew >1.5-fold farther than isolated individuals. Sex influenced the frequency and duration of stops made, with females stopping more often and longer than males. Although females and males in high resource treatments had the shortest net trial times, the factor ‘resources’ did not produce any highly significant main effects, but did generate significant interaction terms with the factors ‘context’ and ‘sex’, suggesting that past experience with ‘resources’ modifies individual flight behaviour. We have shown for the first time using a tethered flight mill system that R. indifferens flight behaviour is context dependent and sensitive to adult crowding. The implications of this study for improved field experiments on dispersal are discussed.