The invasion of non-native plant species has resulted in significant environmental damage and economic costs worldwide. Invasive plants have been shown to alter ecosystem processes, ecological interactions, and habitat quality in their novel environments. Although plant chemical defenses are thought to play an important role in the impacts, invasion success, and management of invasive plants, often little is known about natural variation in the allelochemicals of these plants. Such is the case for Dalmatian toadflax, Linaria dalmatica (Plantaginaceae), which produces iridoid glycosides, a group of terpenoid secondary metabolites that have been shown to act as defense compounds against herbivores and pathogens. To investigate the chemical ecology of L. dalmatica, I conducted field and greenhouse studies, which examined: (1) quantitative variation in iridoid glycoside concentrations, (2) factors that contribute to this variation, and (3) plant interactions with two toadflax biological control agents, Calophasia lunula (Noctuidae) and Mecinus janthinus (Curculionidae).
The results of these studies showed that, overall, L. dalmatica plants can contain high levels of iridoid glycosides (up to 15-20% dry weight). Moreover, research indicated that C. lunula sequesters one iridoid glycoside, antirrhinoside, from L. dalmatica, at levels ranging from 2.7 to 7.5% dry weight. There was no evidence of sequestration by Mecinus janthinus. However, research demonstrated that this biocontrol agent had a negative effect on the defensive chemistry and reproduction of L. dalmatica. Field studies showed that plant iridoid glycoside concentrations are highly variable within and among populations and also depend on a number of factors. For example, iridoid glycoside concentrations decreased with increased plant age, increased soil nitrogen availability, and plant injury by M. janthinus larvae. Linaria dalmatica also showed within-plant patterns of iridoid variation, with the highest iridoid glycoside concentrations in leaves and flowers and lowest concentrations in stems.
Greenhouse experiments investigating the effects of soil nitrogen (N) enrichment on L. dalmatica and C. lunula revealed that the consequences of increased N availability can be complex and context dependent. For L. dalmatica, N enrichment increased plant growth and reproduction, but the effects on iridoid glycosides were more complicated. Increased N availability decreased shoot (leaves + stems) iridoid glycoside concentrations for pre-flowering plants by approximately 30%. However, for plants that were in the flowering stage of development, increased N availability decreased flower iridoids, dramatically increased root iridoids, but did not affect shoot iridoid glycoside concentrations. For C. lunula, N enrichment increased larval growth, decreased development time to pupation, and decreased larval iridoid glycoside concentrations.
Human alteration of the nitrogen cycle represents a major global change and anthropogenic N inputs continue to increase at a rapid rate. My dissertation research documents direct effects of N enrichment on an ecologically and economically important invasive plant species and indirect effects on one of its biocontrol agents. Moreover, this research represents the first quantitative investigation of the defense compounds and chemical ecology of L. dalmatica and its specialist herbivores, which may help provide important insights into the impacts, invasion success, and management of this invasive plant.