Numerous invasive plants have large floral displays that are highly attractive to flower visitors. The large floral displays may be due, in part, to the loss of enemies in the invaded range. Plants that are not attacked can place more resources into growing large displays, and this may indirectly benefit plants if larger displays attract proportionally more flower visits. Multiple studies in native systems have demonstrated that herbivory can have direct effects on plant fitness as well as indirect effects through a decrease in floral visits. However, relatively few studies have addressed this issue in invasive plants. We hypothesized that the biological control agent Mecinus janthinus (Coleoptera: Curculionidae) would reduce female reproductive output of the invasive plant Linaria dalmatica (Scrophulariaceae) directly through a decrease in flower, fruit, and seed set and indirectly through a reduction in proportion of flowers visited and number of seeds per fruit. Linaria dalmatica is a common, prolific invasive plant found throughout the western United States. Mecinus janthinus was released as biological control in 1995. The adults feed on the leaf and stem tissue. In late spring, females deposit eggs in the stem where the larvae develop, pupate, and emerge the following spring. We randomly assigned potted L. dalmatica plants to either M. janthinus attack or no attack. After females had oviposited in the stems, we placed the L. dalmatica in the field using a completely randomized design and then measured flower visitation rates and the number of flowers, fruits, and seeds.
Results/Conclusions
We found evidence for direct effects of M. janthinus attack: there were fewer total flowers, fruits, seeds and the number of flowers visited were reduced in plants exposed to the biological control agent. However, we did not find a decrease in the proportion of flowers visited per plant or the number of seeds per fruit in response to M. janthinus attack. This suggests that there was not an indirect effect of the herbivore on the L. dalmatica mediated through flower visitors.
