O
The role of induced defenses in the success of an exotic pine: the importance of
recognizing your enemies
Zas R.1, Sampedro L.2 and Moreira X.2
1
Misión Biológica de Galicia (CSIC). Apdo. 28. E-36080 Pontevedra, Spain.
Centro de Investigación e Información Ambiental de Lourizán. Apdo. 127. E-36080
Pontevedra, Spain.
2
The Enemy Release Hypothesis (ERH) is one of the mostly cited theoretical
frameworks to explain how exotic species become invasive out of its natural range. This
hypothesis predicts a reduction of the impact of biotic enemies on populations
established in new environments, resulting in a selective advantage regarding similar
indigenous species, with which they now co-occur. This hypothesis has been widely
tested in different plant-animal systems and results are controversial. Some studies
found exotics to be more damaged and/or more impacted in terms of fitness by
herbivores than native species, whereas others found the opposite. We investigate here
whether differences in inducibility between a native (Pinus pinaster) and an exotic pine
(P. radiata) may explain the differences in the attack patterns of a local insect
herbivore, Hylobius abietis (Coleoptera, Curculionidae). This insect is an important
forest pest in Europe that strongly hampers the regeneration of coniferous forests.
Adults of this species fed on the bark and phloem of young seedlings of different
coniferous species, causing stem girdling and high seedling mortality. We evaluated the
effects of this insect in i) in vitro cafeteria experiments, ii) in vivo bioassays, and iii) in
two naturally infected genetic trials of P. pinaster and P. radiata, jointly planted on a
coniferous clear-felled area. Each trial includes 90 replicates of 31 open pollinated
families of each pine species and one control of the opposed species. Contrary to the
ERH predictions, one year after planting, debarked area caused by the pine weevil was
significantly greater in the exotic pine in both trials. However, in vitro bioassays with
the same material cultivated in the greenhouse showed the opposite, and the pine weevil
clearly preferred the species with which it has coevolved. No significant differences
were observed in the in vivo bioassays after 48 h exposing greenhouse seedlings of both
species to the insect. The higher resistance of P. pinaster in field conditions could
derive from induced resistance mechanisms preferently elicited in the native species
following the insect damage. These mechanisms are unable to be expressed in cut twigs,
whereas the short time of the in vivo bioassay may have impede the switch of the insect
preferences observed at field conditions after an attack that lasted for several months.
According to this hypothesis, the induction of resin in the stems (the main resistant trait
in conifers) after a 48 feeding period was twice in the native than in the exotic pine.
These results suggest that the native pine, although constitutively more susceptible, is
able to recognize the potential enemy, and elicit the appropriate defense mechanisms,
resulting in significantly better defended seedlings. Considering the capability to elicit
induced resistance traits against alien and local insects appeared to be essential to
correctly interpret the predictions of the ERH.