The pest status of diamondback moth and the role of Cotesia plutellae in suppressing pest populations in South Africa

摘要

A study conducted over three years ( February 2003-January 2006) on unsprayed cabbage plots near Brits, North West Province of South Africa, demonstrated that parasitoids play an important role in suppressing diamondback moth, Plutella xylostella ( L.) ( Lepidoptera: Plutellidae), populations. Parasitism of P. xylostella larvae was high, reaching 100% on several occasions during late spring, summer and autumn (November-May) each year. Infestations were low during these seasons, often fluctuating between zero and two larvae per plant. The only period that infestations were high was during spring ( September-October) . Eight indigenous parasitic Hymenoptera were reared from P. xylostella: the larval parasitoids Cotesia plutellae (Kurdjumov) and Apanteles halfordi ( Ullyett) ( Braconidae); the larval-pupal parasitoids Oomyzus sokolowskii ( Kurdjumov) ( Eulophidae) and Diadegma mollipla ( Holmgren) (Ichneumonidae); the pupal parasitoid Diadromus collaris ( Gravenhorst) ( Ichneumonidae); and the hyperparasitoids Eurytoma sp. ( Eurytomidae), Mesochorus sp. (Ichneumonidae), and Pteromalus sp. ( Pteromalidae) . Cotesia plutellae was the most abundant parasitoid followed by O. sokolowskii, which is a facultative hyperparasitoid of C. plutellae. The population densities of P. xylostella during the current study were much lower compared to previous studies in the same region. Although parasitism levels of P. xylostella were as high as in previous studies, the parasitoid species diversity has been reduced from 21 to 8 parasitoids. The reduction in parasitoid diversity in this study is attributed to parasitoid competition resulting from a decline in P. xylostella population density. However, as in previous studies, C. plutellae remained the dominant parasitoid associated with P. xylostella in South Africa.