Research Scientist Agriculture and Agri-Food Canada Lethbridge, Alberta, Canada
European stem-galling weevil Rhinusa pilosa (Coleoptera:Curculionidae) was recently introduced to Canada as a biocontrol for the invasive, perennial yellow toadflax (Linaria vulgaris). Rhinusa pilosa requires young host tissues for gall formation, with adults present over several weeks in spring to oviposit into newly emerging shoots that change in size and vigor during development. To advise on timing of spring release of R. pilosa, greenhouse experiments were conducted to explore effects of shoot phenology at the time of oviposition on both insect (gall production, population size) and plant (agent's impact on growth). These were set up in a randomized block design using artificially overwintered toadflax brought to three stages of shoot development (3, 16, 30 days growth after removal from cold storage) before exposure to R. pilosa. When examining the effects of phenology on host impact, plants at the same stages, but without R. pilosa exposure (control), were included. Insect measurements per plant were; number of galls and F1 weevils produced, gall volume. Plant measurements were length of galled and ungalled shoots, number of dead shoots. Unexpectedly, older stages of shoot growth produced more and larger galls and more weevils. However, there was greater shoot death and a greater impact on shoot length for plants galled at an earlier stage of shoot development. A key recommendation is to release later in spring to achieve strong establishment of R. pilosa.
