Does the differential seedling mortality caused by slugs alter the foliar traits and subsequent susceptibility of hybrid willows to a generalist herbivore?

Abstract

Abstract1. Many Salicaceae species naturally form hybrid swarms with parental and hybrid taxa that differ in secondary chemical profile and in resistance to herbivores. Theoretically, the differential mortality in the seedling stage can lead to changes in trait expression and alter subsequent interactions between plants and herbivores. This study examines whether herbivory by the generalist slugArion subfuscus, which causes extensive mortality in young willow seedlings, causes shifts in (a) the foliar chemistry of F2 willow hybrids (Salix sericeaandSalix eriocephala), and (b) the subsequent susceptibility to Japanese Beetles,Popillia japonica.2. In 2001, two populations of F2 seedlings were generated: those that survived slug herbivory (80–90% of seedlings placed in the field were killed by the slugs) were designated as S‐plants, whereas C‐plants (controls) experienced no mortality.3. Common garden experiments with cuttings from these populations, in 2001 and 2002, revealed extensive variation in the phenolic chemistry of F2 hybrids, but revealed no significant difference between S‐ and C‐plants, although the levels of foliar nutrients, proteins and nitrogen tended to be higher in S‐plants.4. Concentrations of salicortin and 2′‐cinnamoylsalicortin explained 55 and 38% of the the variation in leaf damage caused by Japanese beetles, and secondary chemistry was highly correlated within replicate clones (salicortinR2= 0.85, 2‐cinnamoylsalicortinR2= 0.77, condensed tanninsR2= 0.68).5. Interestingly, Japanese beetle damage and condensed tannins were positively correlated within the S‐plants, but not in the C‐plants, suggesting that slugs had selected for plants with a positive relationship between tannins andP. japonicadamage. This is unlikely to be a consequence of a preference for tannins, but is suggested to be related to the elevated nutrient levels in the S‐plants, perhaps in combination with the complex‐binding properties of tannins.6. The damage was highly correlated within replicate clones and a model choice analysis suggested that Japanese beetle damage may be explained by four factors: concentrations of salicortin, condensed tannins, and nitrogen, as well as the specific leaf area (thick leaves were damaged less).