Laboratory Feeding Preferences of Three Larinus Weevil Species on a Threatened Thistle and a Co- occurring Invasive Knapweed: Implications for Host Choice and Conservation

Nonnative invasive species are a threat to biodiversity worldwide. One common method for invasive species management is biological control (biocontrol). However, biocontrol can have unanticipated nontarget effects on native species. Cirsium pitcheri (Pitcher's thistle) is a federally threatened plant endemic to the Great Lakes dunes. Unfortunately, a formerly-recommended biocontrol weevil (Larinus carlinae, used in an effort to control Cirsium arvense and previously referred to as Larinus planus within the biocontrol literature) impacts Ci. pitcheri seed production in large portions of its range and is contributing to population declines. Two congeneric species of weevil (L. minutus and L. obtusus), introduced to control spotted knapweed (Centaurea stoebe subsp. micranthos), may pose a similar risk of nontarget interactions with Ci. pitcheri. All three weevil species have been known to use both Cirsium and Centaurea species in their European range. To understand weevil preferences, we performed a set of choice and nonchoice trials under laboratory conditions in order to test whether the three weevil species are more flexible in their host preferences than previously known, using adult weevils and substituting Ci. arvense for Ci. pitcheri in the case of L. carlinae, due to limited availability of the latter, endangered thistle. All three weevil species spent more time feeding on their intended/known hosts but also fed on the nontarget species, and in some cases the differences were not significant. All three weevil species always oviposited on their known hosts, with no ovipositions on the other test species. Based on our results, neither L. minutus nor L. obtusus is likely to be a major threat to Ci. pitcheri, and L. carlinae is unlikely to use Ce. stoebe as a host.