Host fruit chemical stimuli eliciting distinct ovipositional responses from sibling species of Rhagoletis fruit flies

Laboratory experiments tested whether two economically-important sibling species of tephritid fruit flies have evolved distinct egg-laying responses to chemical stimuli on the fruits of their respective hostplants. The egg-laying preferences displayed by apple maggot flies, R. pomonella, and blueberry maggot flies, R. mendax, on artificial fruits treated with apple and blueberry extract paralleled their egg-laying responses to whole apples and blueberries. R. pomonella flies laid more eggs than R. mendax flies in artificial fruits treated with extract from ripe McIntosh apples, and vice versa for artificial fruits treated with extract from ripe Bluehaven blueberries. Furthermore, both species laid more eggs in artificial fruits treated with extract from their respective host fruits than control artificial fruits which were not treated with fruit extract. Prior electroantennogram recordings from R. mendax and R. pomonella flies exposed to volatiles from pentane extracts of apples and blueberries indicate that the antennal sensitivity of both species is selectively tuned to their respective host fruit odors. This differentiation in their olfactory responses to fruit odors could be important in mediating their distinct ovipositional responses to blueberry and apple fruits. Extract from unripe McIntosh apples also elicited egg laying by R. pomonella flies, however, artificial fruits treated with unripe apple extract received 1.9 times fewer eggs than those treated with ripe apple extract. Moreover, the numbers of R. pomonella ovipositor punctures and eggs placed in wax artificial fruits were increased when the artificial fruits were treated with a blend of 7 identified apple esters. Black coloration on these artificial fruits and the presence of apple esters had a synergistic effect on the egg-laying behavior of R. pomonella flies, which caused them to lay substantially more eggs per black fruit than white fruit treated with the same concentration of apple esters. In summary, our results indicate that the egg-laying responses of R. pomonella flies are mediated by the integration of information from fruit chemical and visual cues, and that R. mendax and R. pomonella flies have evolved divergent egg-laying responses to chemical stimuli on the fruits of their respective hostplants. These findings are discussed in the context of other studies on plant compounds which influence the ovipositional behavior of phytophagous Diptera. Resume Stimuli chimiques des pommes et des myrtilles induisant la ponte des especes jumelles, Rhagoletis pomonella et R. mendax Des fruits artificiels en cire traites avec des extraits de fruits ont provoque chez les especes jumelles de R. mendax (Curran) et R. pomonella (Walsh) des reactions de ponte differentes suivant les stimulations chimiques par les fruits. Le comportement de ponte sur des fruits artificiels traites avec des extraits au pentane des myrtilles mures (Vaccinium corymbosum L.) et de pommes mures (Malus pumila Miller = Pyrus malus L.), est le meme que sur des fruits naturels, ce qui montre que la reponse aux stimulations chimiques provenant du fruit constitue un aspect important de la reconnaissance de l'hote. R. pomonella pond plus d'oeufs que R. mendax sur les fruits artificiels traites a l'extrait de pommes mures; c'est l'inverse pour les fruits traites aux extraits de myrtille. Les fruits artificiels traites avec des pommes ou des myrtilles provoquent la ponte de R. pomonella, tandis que les myrtilles mures seules provoquent la ponte de R. mendax. Les extraits de pommes vertes stimulent la ponte de R. pomonella mais elle est alors 2 fois plus faible qu'avec des extraits de pommes mures. Un melange de 7 esters identifies dans l'extrait de pomme induit aussi la ponte de R. pomonella. Le nombre de piqures de tarieresfli dans les fruits artificiels en cire et le nombre d'œufs par fruit ont ete augmentes par addition d'esters de pommes a des fruits blancs ou noirs. La couleur des fruits artificiels influence aussi la reaction de ponte de R. pomonella; la frequence des piqures de tariere contenant un œuf et le nombre d'œufs par fruit etaient significativement plus eleves sur les fruits noirs que sur les fruits blancs traites avec la meme concentration d'esters de pomme. Les fruits artificiels noirs traites avec la concentration la plus stimulante d'esters de pommes ont recu 2, 3 fois plus d'œufs que les fruits blancs avec les memes concentrations en esters. Ces resultats montrent que les esters de pomme et la couleur noire stimulent synergiquement la ponte de R. pomonella sur des fruits artificiels.