Effect of parasitoid elimination on populations of diamondback moth in cabbage

An insecticidal exclusion method was used to assess the effect of parasitoids on level of infestation by the diamondback moth (DBM), Plutella xylostella (L.) (Lepidoptera: Plutellidae), in cabbage. In field trials conducted at Rietondale, Pretoria, Gauteng Province and Brits, North-West Province, South Africa, a selective organophosphate pesticide, with systemic and contact action, dimethoate, was applied twice weekly to three plots in cabbage fields that had been divided into six plots. The three remaining plots served as controls. At weekly intervals, ten plants were randomly selected from each plot and thoroughly scouted for DBM infestation. Infestation levels were recorded and larvae, pupae and parasitoid cocoons were collected and taken to the laboratory. To determine parasitism levels all collected larvae were kept individually in Petri dishes with fresh cabbage leaves, and pupae and parasitoid cocoons were kept individually in glass vials until either parasitoids or moths emerged. Incidences of parasitism were high in the control plots, peaking above 90% on several occasions. The fauna of DBM parasitoids was rich; during the study period 23 species of parasitoids and hyperparasitoids were identified. The most abundant parasitoids were the larval parasitoid Cotesia plutellae (Kurdjumov), the larval-pupal parasitoid Oomyzus sokolowskii (Kurdjumov), the pupal parasitoid Diadromus collaris Gravenhorst, and the hyperparasitoids Mesochorus sp. and Pteromalus sp. Egg parasitoids were not recorded. At both sites, infestation levels in the sprayed plots were significantly higher than those in the control plots. On the other hand, parasitism levels of DBM in the control plots were significantly higher than in the treated plots. It was concluded that the higher infestation level of cabbage by DBM in the sprayed plots was because of partial elimination of parasitoids by the pesticide.