Diamondback moth oviposition: effects of host plant and herbivory

The oviposition behaviour of Plutella xylostella L. (Lepidoptera: Plutellidae) on Chinese cabbage (Brassica rapa L. Pekinensis, cv. Wombok), canola (Brassica napus L. cv. Thunder TT), and cabbage (Brassica oleracea L. Capitata, cv. sugarloaf) (Brassicaceae) was studied in the laboratory. In no-choice experiments moths laid most eggs on the stems and lower three leaves of cabbage plants, the lower three leaves of canola plants, but on the upper three leaves of Chinese cabbage plants. The effects of conspecific herbivore damage to foliage could be replicated by mechanical damage. When foliage was damaged, injured cabbage and canola plants were preferred for oviposition over intact conspecifics, whereas injured Chinese cabbage plants were less preferred than intact conspecifics. However, when root tissue was damaged, intact cabbage and canola plants were preferred over injured conspecifics, whereas moths did not discriminate between root-damaged and intact Chinese cabbage plants. Injury to upper leaves significantly affected the intra-plant distribution of eggs. In cabbage and canola plants, injury to leaf 6 significantly increased the number of eggs laid on this leaf, resulting in a significant decrease in the number of eggs laid on the lower foliage/stem of plants, whereas in Chinese cabbage plants it significantly decreased the number of eggs laid on leaf 6. Following oviposition on intact plants, neonate larvae established the vast majority of feeding sites on leaves 58 in all three host plants, indicating that larvae moved a considerable distance from preferred oviposition sites in cabbage and canola plants. The growth rate of neonates fed on leaf-6 tissue was significantly greater than that of those fed on leaf-1 tissue; >90% of larvae completed development when fed exclusively on leaf-6 tissue but no larvae completed development when fed exclusively on leaf-1 tissue. The study demonstrates the complex and unpredictable interactions between P.similar to xylostella and its host plants and provides a basis from which we can begin to understand observed distributions of the pest in Brassica crops.