Host Range Expansion of an Endemic Insect Herbivore is Associated With High Nitrogen and Low Fibre Content in Exotic Pasture Plants

Endemic moth species of the genus Wiseana spp. (Hepialidae) have become serious pests of introduced pasture plants in New Zealand. The original native host plants of these moths have not been confirmed. This study investigated the performance (survival, development time, weight gain) of three Wiseana species on seven putative host plants: five native and two exotic species. The aim was to identify native hosts for the three Wiseana species and to compare their performance on native plants and exotic pasture plants. The chemical composition of the seven putative host plants was investigated to compare native and exotic plant chemistries, and to test for associations between plant characteristics and performance of selected Wiseana species. Carbon, nitrogen, silica and fibre contents were measured for each plant species; primary metabolite composition was determined by gas chromatography-mass spectrometry. For the three moth species, increased survival and weight gain were significantly associated with high nitrogen and low fibre contents in one exotic host plant, white clover ( Trifolium repens ), although one species, W. umbraculata , did not complete development to adult on any of the plants tested, including clover. Two exotic plants ( T . repens, Lolium perenne × Lolium multiflorum ), and two native plants ( Aciphylla squarrosa and Festuca actae ) supported W. copularis development to the adult stage, but only one exotic ( T . repens ) and one native ( F. actae) species supported complete development of W. cervinata . Exotic and native plant species had distinct metabolite profiles, but there was no significant association between metabolite composition and Wiseana performance. We conclude that W. copularis and W. cervinata , but not W . umbraculata , have expanded their host range, because of their ability to use both native and new hosts. No evidence was found for a host shift, i.e., a loss of performance on the ancestral host compared with the new host.