HOST SEARCH BEHAVIOR BY RHAGOLETIS-POMONELLA FLIES - INTER-TREE MOVEMENT PATTERNS IN RESPONSE TO WIND-BORNE FRUIT VOLATILES UNDER FIELD CONDITIONS

Responses of Rhagoletis pomonella (Walsh) (Diptera: Tephritidae) to synthetic host fruit volatiles were studied in the field. Individually marked females were released in the centre of a 25 m2 patch containing twenty-five host trees (Crataegus mollis var. toba) and followed as they moved within and between trees. Fly response to three experimental conditions was studied: (1) 'clean' air; (2) synthetic host fruit volatiles (apple) permeating the patch; and (3) a single point source of odour placed c. 1 m away from the edge of the patch. Flies in a patch permeated with host odour moved faster, exhibited more straightened-out moves, and reached the edges of the patch more quickly than those exposed to clean air. Flies exposed to a point source of odour exhibited clear orientation responses, arriving consistently at the tree harbouring the source of odour. Odour exposure was intermittent and usually brief (c. 20 s-1 exposure) with intervals between exposure periods averaging 103 s. Wind speed and direction were highly variable. Flies moved during wind ranging in speed from 0.5 to 4.4 m s-1, with their activity being greatest at winds below 2 m s-1. Strong winds (> 3.5 m s-1) either arrested movement or enhanced downwind displacement. Our observations tend to support the 'series-of-steps' hypothesis reviewed by Gibson & Brady (1985) as a mechanism of close-range host location (1-5 m from odour source). Our findings are discussed with respect to theoretical and practical implications of insect orientation mechanisms to odours, dispersal, and control strategies.