Host-range investigations of potential biological control agents of alien invasive hawkweeds (Hieracium spp.) in the USA and Canada: an overview.

Several European Hieracium species, e.g. Hieracium caespitosum Dumort. and Hieracium aurantiacum L., are noxious weeds in North America. A project for the biological control of alien invasive hawkweeds has therefore been initiated in 2000. Five European insect species investigated before their release in New Zealand and two additional gall wasps have been tested on North American test plants. The stolon-tip galling cynipid, Aulacidea subterminalis Niblett (Hym., Cynipidae) proved to be the most specific candidate attacking four Hieracium spp. in the subgenus Pilosella. Aulacidea hieracii (L.), a gall wasp reared from Hieracium procerum Fries (subgenus Pilosella) and Hieracium robustum Fries (subgenus Hieracium), which severely galls the flower stalks, did not attack any of the target weeds. Another gall wasp, Aulacidea pilosellae (Kieffer), galling the midrib of leaves, stolons and flower stalks, attacked two native North American hawkweed species under no-choice conditions but none of the natives exposed in open-field tests. As a negative effect on the target weeds has not yet been shown, host-range investigations are postponed. Preliminary tests with Oxyptilus pilosellae Zeller (Lep., Pterophoridae) were stopped due to attack of non-target species. Macrolabis pilosellae (Binnie) (Dipt., Cecidomyiidae), a multivoltine gall midge galling the rosette centre, flower heads and stolon tips, can develop on most native North American Hieracium spp. As attack occurred also in field cages and in the field, this agent was removed from the list of potential agents. The root-feeding hoverfly, Cheilosia urbana (Meigen) (Dipt., Syrphidae), and the rosette-feeding hoverfly, Cheilosia psilophthalma (Becker), develop on seven and at least two native hawkweed species, respectively, in no-choice larval transfer tests. However, under open-field conditions, attack rates of C. urbana on native Hieracium spp. are much lower than on H. caespitosum. Further experiments are planned to explore the level of C. urbana attack in the field. Neither test nor control plants were attacked by C. psilophthalma in open-field tests in 2005 and 2006. Therefore, host-range tests with C. psilophthalma are progressing slowly.