Trichome production and variation in young plant resistance to the specialist insect herbivore Plutella xylostella among natural populations of Arabidopsis lyrata

The strength of plant-herbivore interactions varies spatially and through plant ontogeny, which may result in variable selection on plant defense, both among populations and life-history stages. To test whether populations have diverged in herbivore resistance at an early plant stage, we quantified oviposition preference and larval feeding by Plutella xylostella (L.) (Lepidoptera: Plutellidae) on young (5-6weeks old) Arabidopsis lyrata (L.) O'Kane & Al-Shehbaz (Brassicaceae) plants, originating from 12 natural populations, six from Sweden and six from Norway. Arabidopsis lyrata can be trichome-producing or glabrous, with glabrous plants usually receiving more damage from insect herbivores in natural populations. We used the six populations polymorphic for trichome production to test whether resistance against P.xylostella differs between the glabrous and the trichome-producing morph among young plants. There was considerable variation among populations in the number of eggs received and the proportion of leaf area consumed by P.xylostella, but not between regions (Sweden vs. Norway) or trichome morphs. Rosette size explained a significant portion of the variation in oviposition and larval feeding. The results demonstrate that among-population variation in resistance to insect herbivory can be detected among very young individuals of the perennial herb A.lyrata. They further suggest that trichome densities are too low at this plant developmental stage to contribute to resistance, and that the observed among-population variation in resistance is related to differences in other plant traits.