The roles of adult and larval specialisations in limiting the occurrence of five species of Dacus (Diptera: tephritidae) in cultivated fruits

The relative importance of adult preferences or specialisations of larval physiology in restricting the host range of five species of Dacine fruit flies (Diptera: Tephritidae) was examined, with particular emphasis on their utilization of cultivated fruits. The species; D. tryoni, D. jarvisi, D. cucumis, D. musae and D. cacuminatus differ widely in host range with D. tryoni being highly polyphagous while D. cacuminatus is virtually monophagous. Laboratory experiments showed that larvae of all species survived and developed in many cultivated fruits in which the specialists never occur in the field. By contrast the oviposition preferences and specificity of adult females differed widely between species. Female D. tryoni oviposited in most species of fruit. The specialised species; D. cucumis, D. musae and D. cacuminatus strongly preferred their usual hosts and would not oviposit in novel fruits even in the absence of the preferred host. In contrast, D. jarvisi consistently preferred its main native host but when this was not offered readily accepted cultivated fruits. These differences in preference are consistent with the pattern of infestation displayed by each species in the field. The study indicate that, in general, the occurrence of these species of Dacus in cultivated fruits is constrained more by the behavioural preferences of adult females than by larval specialisations. A genetic change in some aspect of host recognition or acceptance would be necessary for the specialised species to regularly infest cultivated fruits though no change in larval characteristics may be needed. As the types and concentrations of defensive secondary compounds may differ between native and cultivated fruits this conclusion cannot be extended to host shifts among native fruits.