Biological control of Dalmatian toadflax breaks down in an urban ecosystem

Background/Question/Methods

Urbanization can alter the structure of communities and affect ecosystem services. Both urban and non-urban sites in Spokane County, Washington, have infestations of Dalmatian toadflax (Linaria dalmatica). A stem-boring weevil (Mecinus janthiniformis) has been introduced as a potential biological control agent of Dalmatian toadflax. We hypothesized that urbanization would affect the interaction of Dalmatian toadflax and weevils. During the summer of 2010, we measured characteristics of Dalmatian toadflax such as stem height and stems per ramet at sites that differed in their level of urbanization. During 2011, we measured weevil densities in addition to Dalmatian toadflax characteristics. We used the proportion of maintained area within a 200-meter radius around each site as our index of urbanization.

Results/Conclusions

Dalmatian toadflax height increased at sites that were more urban during 2010 (linear regression, r² = 0.656, p = 0.003), but not during 2011 (linear regression, r² = 0.117, p = 0.136). During 2011, fewer weevil eggs and larvae were found per Dalmatian toadflax stem at more urban sites (linear regression, r² = 0.500, p = 0.003). Finally, during 2011, we found a positive correlation (0.689, p = 0.009) between weevil abundance per ramet and the distance between the ramet and its nearest neighboring ramet. Our data suggest that Dalmatian toadflax thrives at urban sites, while its biological control is not successful at urban sites. The contrasting response of Dalmatian toadflax and the stem boring weevil causes a dilemma for urban land owners and managers who have Dalmatian toadflax at their sites. Multiple hypothetical mechanisms might be responsible for changing the interaction between Dalmatian toadflax and the stem boring weevil in urban areas.