Polyphagy in an uncertain environment: 'Helicoverpa punctigera' in inland Australia

Abstract

We investigated potential primary hosts for 'Helicoverpa punctigera', a polyphagous lepidopteran pest which affects many crops, but also has many native hosts in non-cropping regions of inland Australia. The most common hosts are Asteraceae and Fabaceae, but there is little information on the relative importance of different species in these families for the population dynamics of 'H. punctigera' in the inland. We tested three common hosts, one legume ('Cullen cinereum') and two daisies, 'Rhodanthe floribunda' and 'Polycalymma stuartii', for their suitability for survival and growth of early instar larvae. Survival was high on the flowers of all three species, and on the leaves of 'C. cinereum' and 'R. floribunda', but low on leaves of P. stuartii. There were significant differences in growth rates, measured by head widths, in the order of 'C. cinereum'>'R. floribunda'>'P. stuartii', and flowers>leaves. They were strongly correlated with nitrogen content, measured by fresh weight. Field surveys of larval populations in westernQueensland indicated that two legume species, 'C. cinereum' and 'Cullen pallidum', were widely utilized and supported many larvae. Daisies were generally less widely used and/or had lower numbers of larvae. When 'C. cinereum' or 'C. pallidum' was found near daisies, the legume had more larvae if both the daisy and the legume were flowering, but this was reversed if only the daisy was flowering. The seasonality and habitats of the daisies and legumes are different. 'C. cinereum' and 'C. pallidum' (a new host record for 'H. punctigera') may be primary hosts of 'H. punctigera' in inland Australia and key hosts for maintaining populations in flood plains. However the areas occupied by daisies can be more extensive depending on rainfall, and some species may be critical in generating outbreaks that affect cropping areas in southeast Australia.