Optimizing classical biological control through the deployment of environmentally resilient agents may provide a sustainable, cost-effective and selective management option for large scale infestations of fire adapted weeds. Ongoing research is exploring the efficacy of a candidate agent, the stem mining weevil Mecinus heydenii, for biocontrol for invasive toadflax. The weevil shows differential levels of host acceptance and suitability for a range of parental (yellow [Linaria vulgaris] and Dalmatian [L. dalmatica] toadflax) and hybrid toadflax genotypes. Test using hybrid host plants with yellow toadflax as the maternal parent generated nearly as many live adult offspring as the typical host (yellow toadflax), with nearly no larval mortality.

Background

Wildfire and other landscape level disturbances on western rangelands and forests increase their vulnerability to weed invasions. Weeds such as Dalmatian (Linaria dalmatica) and yellow (L. vulgaris) toadflax evolved in native range ecosystems characterized by frequent fire events. Both species are therefore well-adapted to rapidly re-establish or successfully colonize post-disturbance landscapes. As a result, minor pre-fire infestations of toadflax can ‘explode’ post-fire, producing dense and persistent patches that quickly dominate entire drainages or watersheds.

The scale of toadflax infestations warrants investigation of alternatives to costly herbicide treatments. The sustainable and highly selective nature of biological control is well-matched to management challenges presented by large toadflax infestations, particularly those that co-occur with culturally or ecologically sensitive species. 

Weed biocontrol agents become available to U.S. stakeholders through a costly and complicated process.  ‘Foreign exploration’ is conducted by sponsored overseas collaborators to identify a pool of potential weed biocontrol agents through extensive literature reviews and native range surveys of herbivores associated with the target weed(s). Host specificity, determined through rigorous, systematic tests of each herbivore species’ fidelity to the target weed species, is guided by a phylogenetically-based test plant list, which narrows the initial large pool of potential agents to a restricted pool of candidate agents. Combined results of host specificity tests, climate matching assessments and biological and ecological characterizations are used to further narrow the pool of candidate agents.

Recent advances in molecular diagnostics have revealed that many invasive weeds targeted for classical biocontrol programs are not single species but rather hybrids of two species, or even species complexes (hybrids of multiple species). Classical biological control of weeds ‘works’ primarily because it intentionally reunites target weeds and biocontrol agents that have evolved together in their common native (overseas) range. Shared history allows the agent to circumvent the target weed’s chemical defenses (phytoprotectants) so that host plants can be safely consumed. Hybridization between exotic weed species in their adopted, invaded North American range, which skips this long term co-evolution process, typically results in management targets that have no suitable biocontrol agents.

Biological control of hybrid weeds poses a significant challenge, especially when, as is the case with hybrids of yellow and Dalmatian toadflax, the parental weed species do not have sympatric distributions in the native range. Insect species collected from native range toadflax as potential toadflax biocontrol agents have not co-evolved with hybrids of Dalmatian and yellow toadflax. Two toadflax agents already established in North America, the yellow toadflax stem mining weevil Mecinus janthinis and the closely related Dalmatian toadflax stem miner M. janthiniformis, both highly host specific and seldomly successful in exploiting their non-typical host.