Olfactory preference and reproductive isolation of two Mecinus species (Coleoptera: curculionidae): implications for biological control of dalmatian, yellow, and hybrid populations of Toadflax, Linaria species

Abstract

Classical biological control of the exotic, invasive toadflaxes Linaria vulgaris (L.) Mill. and Linaria dalmatica Mill. has had both successes and failures. One of the new challenges land managers face is the apparent increase in vigor shown by naturally occurring hybrid populations of the two toadflax species. This has presented practical problems because managers now are unable to decide which weevil species to use on these hybrids: Mecinus janthinus, which is found on L. vulgaris, or M. janthiniformis which has preference for L. dalmatica. This key question was addressed using olfactometer experiments to determine if the volatile profile for each plant establishes host fidelity for the naturally-occurring associated Mecinus species. Adults of both insect species were paired in cages on clones of naturally occurring and synthetic reciprocal cross hybrids and the parent toadflax species to quantify mating events and to determine the number of offspring produced on clones of each plant type of plant in incomplete randomized blocks. We did this using both intraspecific and interspecific pairs of Mecinus species to determine how many offspring are produced by intraspecific adults and also to explore the possibility of establishing a hybrid weevil population using clones of each plant type. Host plant preference for both Mecinus species is influenced by olfactory responses, but this was only evident for adult females. Our first series of no-choice experiments with intraspecific mating pairs indicated that M. janthiniformis is more successful in terms of offspring produced for all types of hybrid toadflax tested. The results also showed that M. janthinus had a higher percentage of survival on all types of hybrids. As expected each Mecinus species performed best on its natural host plant. The results of our second series of no-choice experiments suggest that these two weevil species can produce viable interspecific offspring on clones of most of the plant types evaluated. This suggests that the newly described M. janthiniformis is very similar to M. janthinus and the separation between the two species is controlled at least in part, by olfactory cues from the favored host. In the future, land managers can better decide which Mecinus species to use based on whether the hybrid weed species is influenced more by L. vulgaris, or L. dalmatica.