Seedling herbivory by slugs in a willow hybrid system: developmental changes in damage, chemical defense, and plant performance

We evaluated feeding preference and damage by the slug, Arion subfuscus, on seedlings of two willow species, Salix sericea and S. eriocephala, and the ir F-1 interspecific hybrids. Trays of seedlings were placed in the field a nd excised leaves were presented to slugs in choice tests. Slugs preferred feeding on and caused the most damage to S. eriocephala seedlings. S. seric ea seedlings were least preferred and least damaged. F-1 hybrid seedlings w ere intermediate in preference and damage. Slug preference of and damage to these seedlings decreased over time, suggesting developmental changes in r esistance. Seedlings were sampled for phenolic glycoside and tannin chemist ry weekly to coincide with the field and laboratory experiments. Concentrat ions of phenolic glycosides and tannins increased linearly with seedling ag e, coincident with changes in slug preference and damage, indicating a deve lopmental change in defense. Slug deterrence was not detected at low concen trations of salicortin when painted on leaves or discs, but both salicortin and condensed tannins deterred slug feeding at concentrations between 50 a nd 100 mg/g, levels found in adult willows. Seedling performance was relate d to damage inflicted by slugs. Due to lower levels of damage when exposed to slugs in the field, S. sericea plants had significantly greater biomass than S. eriocephala plants. Biomass of F-1 hybrids was equal to S. sericea when damaged. However, undamaged S. eriocephala and F-1 hybrid plants had t he greatest biomass. Because F-1 hybrid seedlings performed as well as the most fit parent in all cases, slugs could be an important selective factor favoring introgression of defensive traits between these willow species.