The role of induced defences in the success of an exotic pine: the importance ofrecognizing your enemies

Abstract: The Enemy Release Hypothesis (ERH) is one of the mostly cited theories to explain howexotic species become invasive out of its natural range. This hypothesis posits that plants speciesbecome invasive because of reduced regulation by herbivores in their introduced range. Thishypothesis has been widely tested in different plant-animal systems and results are controversial.We investigate whether differences in inducibility between a native (Pinus pinaster) and an exoticpine (P. radiata) may explain the differences in the attack patterns of a local generalist insectherbivore, Hylobius abietis (Coleoptera, Curculionidae). We evaluated the damage rate of thisinsect in i) in vitro cafeteria experiments, ii) in vivo bioassays, and iii) in two naturally infectedgenetic trials of both species, jointly planted on a coniferous clear-felled area. Contrary to the ERHpredictions, debarked area caused by the pine weevil was significantly greater in the exotic pine inboth field trials. However, in vitro bioassays with the same material showed the opposite, and thepine weevil clearly preferred the species with which it has coevolved. No significant differenceswere observed in the in vivo bioassays after a 72h feeding period. The higher resistance ofP. pinaster in field conditions could derive from induced resistance mechanisms preferentiallyelicited in the native species following the insect damage. Indeed, the induction of resin in the stems(the main resistant trait in conifers) after a 72h feeding period was twice in the native than in theexotic pine. These results suggest that the native pine, although constitutively more susceptible, isable to recognize the potential enemy, and elicit the appropriate defence mechanisms, resulting insignificantly better defended seedlings. Considering the capability to elicit induced resistance traitsagainst alien and local insects appeared to be essential to correctly interpret the predictions of theERH.