Antagonist effects of non‐host fruit volatiles on discrimination of host fruit by Rhagoletis flies infesting apple ( Malus pumila ), hawthorn ( Crataegus spp.), and flowering dogwood ( Cornus florida )

In previous flight‐tunnel tests Rhagoletis pomonella (Walsh) (Diptera: Tephritidae) flies originating from domestic apple ( Malus pumila ), hawthorn ( Crataegus spp.), and flowering dogwood ( Cornus florida ), displayed greater numbers of upwind flights to blends of volatiles identified from their natal fruit compared to non‐natal fruit. Here, we show that when certain non‐host volatiles were added to the host blend, significantly fewer apple, hawthorn, and dogwood flies exhibited sustained upwind flight to the source. Specifically, the upwind flight of apple flies to the apple blend was significantly antagonized by the addition of the hawthorn or dogwood blends, the addition of 3‐methylbutan‐1‐ol alone (a key volatile for hawthorn and dogwood flies), or the combination of 3‐methylbutan‐1‐ol and another key dogwood volatile, 1‐octen‐3‐ol. Similarly, the upwind flight of dogwood and hawthorn flies to their respective natal blends was antagonized by the addition of the apple blend or the key apple volatile butyl hexanoate. Experiments were also conducted to determine whether non‐natal fruit volatiles could disrupt the close‐range flight response of flies to the visual stimulus of fruit alone, represented by an odorless red sphere. Tests with apple‐origin flies showed that when the hawthorn blend, the dogwood blend, or the key antagonist volatiles from each (3‐methylbutan‐1‐ol and 1‐octen‐3‐ol) were added to a red sphere fruit mimic, significantly lower proportions of flies were captured, compared with captures when no odor was present. Our results support the hypothesis that agonist and antagonist properties of fruit volatiles can play an important role in host recognition/discrimination by Rhagoletis flies.