Degree-day model for establishment of the garlic mustard biological control agent, Ceutorhynchus scrobicollis (Coleoptera: Curculionidae) in North America

Garlic mustard (Alliaria petiolata), an invasive plant of the North American forest understory, is the target of importation biological control programs in both the United States and Canada. After host range testing was completed in the US, a garlic mustard rosette crown-mining weevil from Europe, Ceutorhynchus scrobicollis, was released in Canada in 2018 and has recently been approved for release in the United States. We defined temperature-dependent thresholds for C. scrobicollis oviposition behaviour and cumulative thermal units required for F1 development within garlic mustard rosettes. These thermal requirements of C. scrobicollis were then used to inform a degree-day model to predict the potential geographic distribution of the imported biological control agent in the Great Lakes region of North America. We found that Cumulative Degree Days accumulated during the rosette seasons in 2020 through 2024 were sufficient for C. scrobicollis parasitism of garlic mustard rosettes to take place in Southern Ontario, as well as the central and upper Midwest and Northeast regions of the United States.