Development of a growing degree-day model to estimateLinaria vulgarisshoot emergence and prospects for improving biological control efforts

Abstract

AbstractYellow toadflax (Linaria vulgarisMill.; Scrophulariaceae) is an invasive herbaceous perennial weed of agricultural and natural habitats throughout North America. In pastures or native rangelands, use of biological control is an attractive option, particularly if the agent can be established quickly.Rhinusa pilosa(Gyllenhal) (Coleoptera: Curculionidae), a stem-galling weevil, was first released in Canada in 2014 to evaluate its potential to controlL. vulgaris.Rhinusa pilosarequires young, vigorously growing shoots to establish. Ability to estimate when adequate shoots will be available could inform release timing, thus improving establishment success. There is currently no growing degree-day (GDD) model forL. vulgaris. Our main objective was to develop a GDD model for the emergence ofL. vulgarisshoots and discuss the utility of such a model in relation to the establishment ofR. pilosain Nova Scotia. Four sites containing five randomly placed 1-m2quadrats were monitored for the emergence ofL. vulgarisshoots twice weekly in spring to summer 2017 and 2018 by recording number of shoots and shoots with flower buds. A GDD (Tbase2 C) model for shoot emergence ofL. vulgariswas developed and validated using independent shoot emergence data. Shoots emerged in the spring between 124 and 244 GDD with 90% of all shoots emerged between 681 and 1,117 GDD. Model estimation for the initiation of shoot emergence was 74 GDD, with 10%, 50%, and 90% shoot emergence estimated to occur at 179, 409, and 811 GDD, respectively.Rhinusa pilosaadults were released in 2016 (three sites) and 2017 (one site), and number of shoots with galls was recorded. Galls were observed in all three sites in 2016 and in three of the four sites in 2017, with none found in 2018. Timing of release and soil moisture are discussed as factors affecting establishment ofR. pilosain Nova Scotia.