Why so many polyphagous fruit flies (Diptera: Tephritidae)? A further contribution to the ‘generalism’ debate

Abstract

It has been suggested that generalist herbivorous insects are an evolutionary ‘improbability’. However, many species of tephritid fruit flies are generalists utilising larval hosts across two or more plant families. Focusing on the genus Bactrocera, it is argued that this high frequency of generalism is due to four evolutionary drivers which both decrease the need for specialism and promote generalism. Decreasing the need for specialism, there is firstly no co-evolutionary arms race between host plant and herbivore as Bactrocera do not negatively impact on plant fitness; this removes the need for specialism to counter plant defences. Secondly, the endemic hosts of most Bactrocera are plants with vertebrate-dispersed fleshy fruits. Fruit morphology and their display are highly constrained to attract those seed dispersers; Bactrocera thus do not need to specialize to increase host location capacity as many hosts produce a common signal. Thirdly, in native rainforest ecosystems Bactrocera are rare and so specialization to avoid competition seems unlikely. Promoting generalism are braconid parasitoids, which utilise plant cues to find their hosts. If a fly can use a novel host plant then it moves into enemy-free space, so promoting generalism. From this multi-part hypothesis, a group of testable predictions emerge. If shown to be correct then for the generalism/specialism debate frugivorous tephritids would provide an example where generalism may be a ‘probable’ evolutionary outcome.