Results/Conclusions Dietary specialist caterpillars suffered less bird predation than did generalists. Specifically, the magnitude of the difference in caterpillar density between bagged and unbagged experimental units (density difference) was greater for generalist than for specialist caterpillars across all 8 tree species. Moreover, specialists gained an enemy-free space advantage over generalists even when caterpillar density was taken into account. In conclusion, the enemy-free space advantage of specialist caterpillars is at least partly trait-mediated. These results support the general hypothesis that tri-trophic interactions shape herbivore adaptation to host plants, including dietary specialization.