Print PageDalmatian toadflax (Linaria dalmatica) is a perennial figwort native to Asia and Europe. It was introduced to North America in the 1800’s and has become an invasive weed in the northwest United States and Canada. Dalmatian toadflax is a concern due to the detrimental effect it has on livestock, and its ability to outcompete native plants. Our study was conducted during the summer of 2010 in Spokane County, Washington. We compared the growth of Dalmatian toadflax at ten sites, half of which we initially classified as urban, and the other half as non-urban. To produce an index of urbanization, we measured the building density at each site, and its distance from the center of downtown Spokane. In addition, at each of the sites we measured air temperature, the size of Dalmatian toadflax, and the density of an introduced biological control agent, a stem-boring weevil (Mecinus janthinus) that feeds on Dalmatian toadflax.
Results/Conclusions
As the urbanization index of sites increased, mean air temperature increased (ranged major axis regression, r2=0.42, p=0.05), and Dalmatian toadflax grew larger (ranged major axis regression, r2=0.57, p=0.01). The relationship between stem-boring weevil abundance and urbanization was more complex. At all of the sites, weevils were more abundant on larger toadflax plants (2-way ANOVA, effect of plant size on weevil abundance, F1,167 = 7.84, p = 0.006). However, the increase in weevil density on larger plants at the five non-urban sites was significantly greater than the increase in weevils at the five urban sites (2-way ANOVA, interaction between plant size and site type, F1,167 = 31.7, p < 0.001). Our results suggest that stem-boring weevils might be more effective at helping to control Dalmatian toadflax at less urban sites. However, other site characteristics such as soil moisture and alternative weed control practices are likely to influence the interaction between Dalmatian toadflax and herbivorous insects.
