NITROGEN-CONTENT OF FOOD PLANTS AND VULNERABILITY OF PIERIS-RAPAE TO NATURAL ENEMIES

We tested the commonly made assumption that reduction in leaf quality reduces the survivorship of herbivores indirectly by increasing their exposure to natural enemies. Caterpillars of the butterfly Pieris rapae were placed on collard plants that had been experimentally manipulated to have leaves either rich or poor in nitrogen. On low-nitrogen plants caterpillars both developed more slowly and devoted more of their time to feeding than on high-nitrogen plants. When exposed to large (> 2 mm) predators and parasites in a natural setting, the survivorship of caterpillars on low-nitrogen collards was reduced greatly compared to caterpillars protected from predators and parasites. Survivorship of caterpillars on high-nitrogen plants was reduced significantly less dramatically when exposed to predators and parasites. Small (< 1-2 mm) pupal parasites that were able to reach the caterpillars through the mesh of the predator exclusion cages emerged more frequently from individuals reared on high-nitrogen plants.