Although fruit fly host status determination/designation lies at the heart of strategic decisions on national and international trade of fruit and vegetables, all attempts thus far to define host plant status have been contentious and as a result long-standing disputes between commercial partners throughout the world have lingered over decades. Part of the problem is that too little effort has been devoted to understanding the underlying mechanisms involved in host plant use by fruit flies and that instead economic and political interests usually prevail. Here we review the most important evolutionary, biological, ecological, physiological, and behavioral aspects that drive host use by fruit flies, and then construct a flow diagram rooted in these fundamentals that outlines a series of steps and definitions to determine if a particular fruit or vegetable (and cultivars thereof) is a natural host, or a conditional (potential, artificial) host, or a nonhost. Along the way, we incorporate risk analysis considerations and propose that the underlying complexity determining host plant utilization by fruit flies requires a flexible systems approach capable of realistically dealing with fly/host/environment/geographic variability on a case-by-case basis.

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Asian citrus psyllid (Diaphorina citri) transmits the causal agent of Huanglongbing, a devastating disease of citrus trees. In this study we measured behavioral responses of D. citri to combinations of visual, olfactory, and gustatory stimuli in test arenas. Stimuli were presented to the psyllids in droplets or lines of an emulsified wax formulation in two different arena types in no-choice tests. First, when placed on a colored ring situated halfway between the center and perimeter of a petri dish, D. citri spent more time on yellow versus gray rings; however, this response disappeared when either gray or yellow wax droplets were applied. When the psyllids were presented with droplets scented with terpenes, the response to both scent and color was increased. The addition of a dilute (≍0.1 M) sucrose solution to the wax droplets increased the magnitude of D. citri responses. Next, groups of D. citri were placed on plastic laboratory film covering a sucrose solution, to mimic a leaf surface. Test stimuli were presented via two 'midribs' made from lines of emulsified wax formulation. Probing levels were measured as a function of color saturation and scent composition, and concentration. The test scents were based on qualitatively major volatiles emitted by Murraya paniculata (L.) Jack, Citrus aurantifolia (Christm.) Swingle, and C. sinensis (L.) Osbeck. The highest probing response was observed on the middle concentration (20-μl scent/10 ml wax formulation) of the C. aurantifolia-scented wax lines. Results indicate that there are interactive effects between the different sensory modalities in directing host-plant assessment behavior.

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