Integration of endemic natural enemies and Bacillus thuringiensis to manage insect pests of Brassica crops in North Korea

Brassica crops, principally varieties of Brassica oleracea and Brassica campestris, account for over half the total vegetable production in the Democratic Peopleʹs Republic of Korea (DPRK). The crops are attacked by a complex of insects and the two major pest species, Plutella xylostella L. (Lepidoptera: Plutellidae) and Pieris rapae L. (Lepidoptera: Pieridae) represent the principal constraints to Brassica crop production. On co-operative farms pests are managed by scheduled application of broad spectrum insecticides and there is no attempt to conserve or utilize endemic natural enemies. The natural enemy complexes attacking P. xylostella and P. rapae in summer white cabbage (Brassica oleracea var. 6–21) on three co-operative farms in the south west of DPRK were studied. The hymenopteran parasitoids Cotesia vestalis Haliday (=plutellae Kurdjumov) (Braconidae), Oomyzus sokolowskii Kurdjumov (Eulophidae) and Diadromus collaris Gravenhorst (Ichneumonidae) attacked P. xylostella while Cotesia glomeratus L. (Braconidae), Cotesia rubecula Marshall (Braconidae) and Pteromalus puparum L. (Pteromalidae) and an unidentified species of Exorista (Diptera: Tachinidae) attacked P. rapae. The generalist epigeal predatory fauna was dominated by Lycosidae (most abundant species: Lycosa t-insignata) and Carabidae (most abundant species: Chlaenius naeviger Morawitz, C. pallipes Gebler, Calathus helensis Schaller). Larval and pupal parasitism rates of both P. xylostella and P. rapae increased during the early summer months but natural enemy exclusion experiments showed that generalist predators were the most important natural enemy guild attacking P. xylostella populations in the agro-ecosystem. In a step towards the development of conservation biological control strategies, field experiments compared current pest management practice with pest threshold based interventions using commercial formulations of Bacillus thuringiensis var. aizawai (Bt). The Bt-natural enemy strategy significantly increased crop yields and the impact of both parasitoid and predator natural enemies on pest populations. However, even under the threshold based Bt regimes, natural enemy impact remained relatively low. The ecology of the major pests and their natural enemies is discussed and recommendations for improved biological control and more effective management of key Brassica pests are made within the constraints posed by current agricultural circumstances in DPRK.