HOST ODOR AND VISUAL STIMULUS INTERACTION DURING INTRATREE HOST FINDING BEHAVIOR OF RHAGOLETIS-POMONELLA FLIES

Responses of Rhagoletis pomonella (Walsh) (Diptera: Tephritidae) flies to host fruit visual stimuli (apples or models of apples) and chemica l stimuli (synthetic apple volatile blend) were studied in semidwarf f ield-caged apple trees. Three different fruit or model densities (1, 4 , or 16 fruit or models/tree) and two odor release rates [ca. 0.7 mug/ hr (close to the natural release rate of a ripe apple) and ca. 500 mug /hr (amount of odor released by commercially sold apple maggot traps)] were tested. Individually released flies were followed as they moved within a tree for a maximum of 20 min. We recorded three-dimensional s earch paths followed by foraging flies and computed such variables as total relative distance traveled before alighting on a fruit or model, track length between individual alightment sites, and directness of f light to fruits or models. Effect of odor on propensity to alight on f ruit or models and host-searching behavior prior to alighting on fruit or on models varied according to fruit or model color and density. If the fruit visual stimulus was strong (e.g., red color), odor did not increase the probability of finding fruit or fruit models. As the visu al stimulus became progressively weaker (red to green to clear), odor (irrespective of concentration) appeared to aid flies during the fruit -finding process. As density of fruit or models increased, the probabi lity of flies finding a fruit or model also increased (e.g., 50% of fl ies found a red fruit model at 1 model/tree while 90% found a red mode l at 16 models/tree; 4% of flies found a clear model with odor at 1 mo del/tree while 35% found a clear model with odor at 16 models/tree). F indings reported elsewhere indicate that R. pomonella flies am able to discover a point source of odor (an odor-bearing tree in a patch of t rees) by flying upwind (in the tree patch) in response to intermittent exposure to odor. Findings here indicate that after arrival on a host tree (point source), flies discover individual apparent and abundant host fruit on the basis of vision. If fruit are less apparent or scarc e, odor appears to interact with vision during the fruit-finding proce ss.