Interactions Between Mogulones Crucifer and Target and Non-Target Boraginaceae Species in Montana and Washington

Abstract

Invasive plant species can significantly harm the environment and economy. Classical biological control (biocontrol), the deliberate release of a specialized natural enemy from the invasive plant’s native range to reduce its abundance or spread in its introduced range, is a tool commonly used to manage invasive plants. However, concerns persist regarding biocontrol agents affecting non-target species. Therefore, my study focused on the root weevil (Mogulones crucifer Pallas), approved in Canada as a biocontrol agent for the invasive plant houndstongue(Cynoglossum officinale L.), to assess its effect on its target and closely related non-target plants. Over three years at 14 sites in Montana and Washington, where M. crucifer has self-dispersed, I monitored M. crucifer’s oviposition and herbivory on C. officinale and five non-target species (Hackelia micrantha (Eastw.) J.L. Gentry, Hackelia floribunda (Lehm.) I.M. Johnson, Lithospermum ruderale Dougl. Ex Lehm., Mertensia ciliata (James ex Torr.) G. Don, Mertensia oblongifolia (Nutt.) G. Don., and an unidentified annual Boraginaceae), tracking the observed plants' growth and demography. Our findings suggest that M. crucifer prefers C. officinale but can oviposit and feed on the non-target plants. However, these non-target interactions seem unlikely to lead to substantial population declines of these plants. Notably, C. officinalepopulations consistently declined across sites, supporting the effectiveness of M. crucifer in suppressing the target species. The limited oviposition and juvenile development on non-target plants align with spillover effects observed in previous studies, where M. crucifer’s impact did not extend to non-target population-level changes. However, there were observed declines of some non-target plant populations, yet our models indicated that these declines were not related to M. crucifer herbivory or oviposition. Regardless, I recommend further long-term studies to clarify the nature and extent of these population declines where M. crucifer interacts with the non-target plant of interest.