Abstract
Four Eurasian Hieracium spp. (Asteraceae) are noxious weeds in New Zealand and targets for biological control. Cheilosia urbana (Meigen) and Cheilosia psilophthalma (Becker) (Diptera, Syrphidae) are univoltine, sympatric hoverflies from Central Europe which have been screened to determine their safety as biological control agents. Females of both species oviposit on Hieracium pilosella L. rosettes in April and May but their larvae occupy different feeding niches. C. urbana larvae move into the soil where they feed externally on the roots, making small holes, whereas C. psilophthalma larvae remain on and feed on the aboveground plant parts. During no-choice larval-transfer tests, neonate C. urbana larvae fed and completed development to the adult stage on eight of nine Hieracium spp. tested, whereas C. psilophthalma larvae completed development to the adult stage on all nine Hieracium spp. tested. Neither C. urbana nor C. psilophthalma developed on species from genera other than Hieracium. A successful method was developed to conduct single-choice oviposition tests with both species. Two critical test plant species outside the genus Hieracium were each offered to gravid C. urbana females in the presence of the target weed H. pilosella but none of the nontarget test plants were accepted for oviposition. The screening results indicated that both species of hoverflies are sufficiently host specific for release in New Zealand where no native Hieracium species exist. Invasive hawkweeds of Eurasian origin are also considered serious weeds in North and South Americas, where in contrast to the situation in New Zealand, native Hieracium spp. are also present. Thus, additional host specificity tests with native Hieracium species and other American Asteraceae will be necessary to determine the experimental host range of both hoverflies within the genus Hieracium and to examine whether other plant species would be at risk.
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