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.
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.
Initial phases of this research began in spring-summer 2014 with a candidate agent for yellow toadflax, another stem mining weevil Mecinus heydenii. Laboratory and field tests were conducted.
The weevil Mecinus heydenii showed differential levels of host acceptance and suitability for a range of parental (yellow and Dalmatian 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. Hybrid host plants with a Dalmatian toadflax maternal parent were much less suitable, producing few live adult progeny.
Reproductive success of the weevil was highly variable on clones of plants that were field collected in Montana and Idaho and verified to be hybrids of yellow and Dalmatian toadflax. Plants from one site produced many adult progeny with low larval mortality; plants from a second site produced very few live adults with very high larval mortality; and plants from the third site appeared to be unsuitable, with no live or dead adult or larval progeny produced.
Mecinus heydenii’s acceptance of and reproductive success on hybrid toadflax were an unexpected outcome. Adventive or approved toadflax feeding insect species already established in North America have proven to be either ineffective or, as in the case of the established yellow toadflax agent M. janthinus, susceptible to significant overwintering mortality. Current year progeny (F1s) of both of the established and candidate yellow toadflax stem miners reach adulthood at approximately the same time, early August.
The established agent, M. janthinus, remains inside the same host stem from egg deposition in spring of one year until adult emergence in spring of the following year, overwintering as a pre-reproductive adult within its pupal chamber. Infested toadflax stems weakened by internal stem mining of developing M. janthinus are easily damaged by high winds or passing animals. Mecinus janthinus is also vulnerable to wide fluctuations in temperature and relative humidity for the entire time it resides within the natal stem, and particularly while overwintering if snow depth fails to provide adequate insulation.
In contrast, the candidate agent M. heydenii exits the natal stem once it reaches adulthood in early August, then overwinters in the soil near host plants.
The difference in overwintering site--natal stem (M. janthinus) vs. soil (M. heydenii)--may make a significant difference in survival of pre-reproductive adult weevils, translating into more rapid buildup of agent populations and hopefully, declines in hybrid and yellow toadflax. Options to deploy environmentally resilient weed biocontrol agents will likely become even more important under predicted climate change.