Development and Harmfulness of the Cabbage Moth Plutella xylostella (L.) (Lepidoptera, Plutellidae) on Spring Rape Crops in Leningrad Province

Results of long-term research have shown that the cabbage moth Plutella xylostella poses a serious threat to spring rape crops in Leningrad Province. Several outbreaks of the pest resulting in heavy injury to the rape leaves and buds were recorded during the last decade. Mass reproduction of the moth was observed under the conditions of forward and warm spring, when the adults infested the crops earlier and simultaneously, and the larvae fed on rape plants during the initial stages of their development. The yield was strongly reduced in such cases, especially in the crops damaged previously by Phyllotreta flea beetles, as well as in hot and dry weather, when the moth larvae fed more actively. Due to damage caused by the larvae, the yield of spring rape decreased by 4.7% (0.65 dt/ha) on average over the entire period of research, by 84% in the outbreak years, and by 2.5% in the years of low population density of the pest. The harmfulness of cabbage moth larvae increased by 2.3 and 4.5 times when they damaged 26–50% and more than 50% of the leaf surface area, respectively, as compared with damage not exceeding 25% of the leaf area. Application of full dosage of mineral fertilizers increased the ability of spring rape plants to recover from damage and reduced injury to the leaves and buds.