Resource dilution increases the effectiveness of a biological control weevil

Background/Question/Methods

Dalmatian toadflax (Linaria dalmatica) is an introduced plant that infests several sites in Spokane County, Washington. A stem-boring weevil (Mecinus janthiniformis) has been introduced as a biological control agent of Dalmatian toadflax. Previous work at our sites found that at urban sites, weevil abundances were low, while Dalmatian toadflax densities were high. Our observations suggested that reducing densities of Dalmatian toadflax might increase the effectiveness of the weevil. During 2012, we conducted a field experiment in which we reduced the above-ground abundance of Dalmatian toadflax by clipping some ramets at ground level at some sites, while unmanipulated sites were controls. We began the experiment with 7 treatment and 7 control sites, but 2 treatment sites were lost during the summer. We sampled plant characteristics and weevil abundances on focal Dalmatian toadflax ramets at three times during the summer, and dissected stems at the end of the summer to count juvenile weevils.

Results/Conclusions

The clipping treatment increased the distance of focal ramets to the nearest neighboring ramet (1-sided t-test: t = -3.020, df = 5, p = 0.015). In turn, increasing distances to the nearest neighboring ramet increased the number of adult weevils observed on focal ramets (linear regression: r² = 0.598, F_{(1, 10)} = 17.36, p = 0.002). Increasing distances between ramets also produced a weaker, but still statistically significant, increase in the number of juvenile weevils found within the stems at the end of the summer (linear regression: r² = 0.324, F_{(1, 9)} = 5.78, p = 0.040). Finally, a negative relationship was found between the local density of Dalmatian toadflax ramets and the number of weevils in that area (linear regression: r² = 0.420, F_{(1, 10)} = 8.96, p = 0.014). Our results suggest that the weevils might be more effective biological control agents in areas of lower Dalmatian toadflax density. Resource concentration might be a mechanism that alters the interaction between Dalmatian toadflax and the stem-boring weevil in urban areas within Spokane County.