Comparing insect herbivory between native and invasive Senecio species in NE Spain: a test for the Host Switching hypothesis

Plant invasion success can be determined by changes in insect-plant interactions. After invading a new habitat, local herbivores consuming native plants can establish new interactions with introduced plants (Host Switching Hypothesis; Keane & Crawley, 2002). New interactions can affect decreasing fitness of exotic species and thus invasion capacity. The process of native insects switching to exotic hosts can be facilitated by a) the presence at the area of invasion of native plant species chemically related to the exotic plant, either from the same genus or family, which facilitates host recognition by herbivores (Janzen, 1968), and b) the time since introduction, with higher probabilities of herbivores switching to new hosts with a longer invasion period (Colautti, 2004). We tested the predictions of the Host Switching Hypothesis on the genus Senecio. Senecio is represented in our study area by two native species, S. lividus and S. vulgaris, and two exotics with different colonization times, S. inaequidens (introduced to NW Europe more than a century ago) and S. pterophorus (introduced to NE Spain 30- 40 years ago) (Chamorro, 2006). We compared herbivory on these four species by sampling the flower heads from 15-30 individual plants at 6 populations in Montseny Natural Park (NE Spain). Sampling was conducted from May to November covering the flowering stage for all species. Flower heads were dissected in the lab and presence of phytophagous insects was recorded. S. lividus and S. inaequidens had the highest herbivore consumption, with 25% and 10% of damaged flower heads, respectively. S. vulgaris and S. pterophorus showed low values of predation (less than 5%). Damage was mainly caused by the Senecio specialist Sphenella marginata (Diptera: Tephritidae) and the Asteraceae specialist Phycitodes albatella (Lepidoptera: Pyralidae). Other unknown Lepidoptera were also found consuming flower heads. Sphenella marginata was associated to the native species S. lividus and S. vulgaris, but also to S. inaequidens, illustrating a case of Host Switching only for the exotic plant with longer introduction time. Sphenella marginata was not found on S. pterophorus even though plant and insect phenology were coincident. Phycitodes albatella was present only on the exotic species, affecting predominantly S.inaequidens (29% of total predation). The presence of Phycitodes albatella on S. pterophorus was marginal (less than 1%) because insect- plant interactions were constrained by phenology. Our results show that the presence of chemically related hosts and a longer time since invasion may have facilitated host switching for the diptera Sphenella marginata from native Senecio to the exotics S.inaequidens but not for S.pterophorus. Newly established interactions between native herbivores and exotic plants may decrease plant invasive capacity in the long run. References : Chamorro, L., Caballero, B. et al. (2006) Anales del Jardín Botánico de Madrid 63: 55-62. Colautti RI., Ricciardi A., et al. (2004). Is invasion success explained by the enemy release hypothesis? Ecology Letters 7 (8): 721-733. Janzen, DH. (1968) Host Plants as islands in evolutionary and contemporary time. The American Naturalist 102: 592-595. Kean R.M & Crawley M.J. (2002) Exotic plant invasions and the enemy release hypothesis. Trends in Ecology & Evolution 17 (4). 164-17