Abstract
Host specificity and efficacy are essential prerequisites for the release of a prospective weed biological control agent (BCA). Conventional pre-release testing forms the foundation of environmental safety assessments for weed BCAs; however, it often misses early behavioral barriers before the utilization of plant, such as pre-alightment host-finding behavior, which provides mechanistic insights into how and why specialist herbivores select their hosts under natural field conditions. Similarly, despite broad agreement on the importance of post-release evaluation, quantitative studies remain limited, and the efficacy of seed feeders continues to be widely discussed.This thesis investigated the host specificity and efficacy of Mogulones borraginis Fabricius (Coleoptera: Curculionidae), a prospective seed-feeding biocontrol agent targeting Cynoglossum officinale L. (Boraginaceae), an invasive plant species in North America.
Chapter 2 examined pre-alightment host selection by testing female weevil responses to olfactory, visual, and combined cues from North American threatened and endangered (T&E) species and Eurasian confamilials. M. borraginis preferred C. officinale over all non-targets, with strong discrimination emerging when combined cues were presented. The weevils were repelled by floral volatiles of the T&E species Oreocarya crassipes and did not discriminate between the host and some non-targets based on individual cues likely due to shared bioactive compounds and spectral similarities.
Chapter 3 explored the role of foliar cues in the host-selection mechanisms of M. borraginis, which targets reproductive parts of its host. Results showed that floral visual cues were more attractive than foliar visual cues. However, foliar olfactory and combined cues were as important as floral cues. The relative importance of olfactory and visual cues was plant-part specific.
Chapter 4 quantified the impact of M. borraginis, showing up to a 46.1% and 53.0% reduction in seed output from adult and larval feeding alone, respectively, and approximately 66.9% reduction from combined feeding, compared to plants without weevils.
Chapter 5 presented a global meta-analysis of recent post-release studies. Biocontrol agents substantially reduced target weed mass, flower production, density, and coverage, with moderate effects on plant size and comparatively smaller effects on seed production. Agents in the order Coleoptera, family Curculionidae and the combined use of multiple agents were associated with greater reductions in weed reproductive fitness. Long-term studies revealed stronger impact in weed density and native species recovery.
Collectively, these findings demonstrate that M. borraginis exhibits a narrow host range and highlight the value of incorporating pre-alightment behavioral assays using multimodal cues to better interpret conventional host-range data. This study indicates that M. borraginis could be an effective agent for the management of C. officinale and also contributes to our understanding of recent biocontrol programs outcomes by identifying key factors driving their success.