Phenologies of Canada thistle (Cirsium arvense) andCirsiumspecies native to the upper Midwest: implications for the ecological host range of the biocontrol agentHadroplontus litura

Abstract

AbstractNativeCirsiumspecies play an important role in landscapes across North America.Hadroplontus litura(F.) (formerlyCeutorhynchus litura), the stem-mining weevil and biological control agent of Canada thistle [Cirsium arvense(L.) Scop.] can complete its life cycle on fiveCirsiumspecies native to the upper Midwest. Although these fiveCirsiumspecies are within the fundamental host range ofH. litura, as determined by host-range tests, we wanted to explore whether phenological differences amongCirsiumspecies help define the field ecological host range ofH. litura. The objective of this study was to determine the phenology ofCirsiumspecies native to the upper Midwest in relation toC. arvenseandH. litura.Our goal was to explore whether shoots of nativeCirsiumspecies could escapeH. liturashoot oviposition in spring due to delayed shoot emergence relative toC. arvense. Soil cumulative growing degree days (GDD) were a superior predictor of shoot emergence for perennialCirsiumspecies or initiation of leaves in biennialCirsiumspecies, with a 2.4 times larger effect on time to emergence relative to air GDD. All nativeCirsiumspecies initiated new leaves or shoots beforeC. arvenseshoot emergence, even when nativeCirsiumspecies growth was delayed in the spring. In turn,C. arvenseshoots emerged approximately 1 to 3 wk before femaleH. liturabegan to lay eggs. As such, all nativeCirsiumplants had shoots available forH. lituraoviposition. There was no phenological separation between nativeCirsiumandC. arvenseshoot emergence or initiation that would render nativeCirsiumspecies safe fromH. lituraattack. Based on the phenology of shoot emergence or initiation in the spring, all testedCirsiumspecies native to the upper Midwest would be within the ecological host range ofH. litura.