Plant plasticity in response to insect herbivory differs between native and introduced populations of Verbascum thapsus

Biological plant invasions are ideal for understanding how insects drive the evolution of plant traits, as plants often experience known differences in herbivory between their native and introduced ranges. Therefore, a common garden approach with both herbivory and reduced herbivory treatments is the best way to gain insight into these interactions. I followed this approach using Verbascum thapsus, a common plant invader within Colorado, to test the following questions: (1) are leaf, floral, and fruit traits plastic and, if so, (2) does trait plasticity in response to insect herbivory differ between native and introduced plant populations? To answer these questions, I measured a suite of morphological plant characteristics (flower, fruit, and leaf size and shape) of plants from 16 native European and 22 introduced North American populations of V. thapsus grown in a common garden. Our measured traits are plastic in response to herbivory. However, the direction of plasticity differs between European and North American populations of V. thapsus.  While our data suggests that North American populations experience a cost of herbivory (i.e., smaller leaves, flowers, fruits), European populations instead appear to overcompensate (i.e., larger leaves, flowers, fruits).  This follows what we know about differences in herbivory between the European and North American range, as European plants often experience much higher levels of herbivory than they experienced in our North American garden. Differences in floral traits did influence Apis mellifera and Rhinusa tetra preference, but the main driver of this preference was not measured.  Future directions include measuring plant defense, which we expect will give greater clarity to these differences in insect attraction. However, that the direction of floral plasticity differs between ranges is an important novel finding. Although plasticity is known to contribute to the success of invader establishment, our results suggest we need to adopt a more long-term perspective that investigates how plasticity evolves after introduced populations that have had the time to evolve and adapt to their new insect communities.