Characteristics of parasitism of diamondback moth by Oomyzus sokolowksii (Hymenoptera: Eulophidae)

Laboratory, greenhouse and field studies were conducted on Oomyzus sokolowskii Kurdjumov, a parasite of diamondback moth, Plutella xylostella (L.), to judge its suitability for introduction in the field to control the plutellid. Oomyzus sokolowskii preferred the third and fourth instar diamondback moth larvae over fresh pupae for parasitization. It is thus a larval parasite. Within the range of 10 °C to 35 °C, the higher the temperature the higher was the parasitism rate. High parasitism at temperatures of 30 °C and 35 °C indicates that this insect is suitable for introduction in the tropical lowlands. In a no-choice test where only fresh pupae of Cotesia plutellae Kurdjumov (another potentially competing larval parasite of diamondback moth) were offered, O. sokolowskii failed to parasitize the pupae. In a choice test where the fourth instar diamondback moth larvae and fresh C. plutellae pupae were offered, O. sokolowskii parasitized only diamondback moth larvae. This parasite, therefore, is not a hyperparasite of diamondback moth. When C. plutellae-oviposited diamondback moth larvae were offered at intervals for parasitism by O. sokolowskii, it parasitized only freshly oviposited host larvae. The longer the period that elapsed after C. plutellae oviposition of diamondback moth larvae, the lesser was the parasitism of these larvae by O. sokolowskii. In a field cage study, as the diamondback moth population increased, the parasitism of the pest by the eulophid increased, parasitism by C. plutellae, however, decreased. Host-plant (cabbage) age did not affect the parasitism of diamondback moth larvae by O. sokolowskii; in both seedlings and mature plants the level of parasitism of the plutellid larvae was comparable. Most organic insecticides tested were toxic to both pupae and adults of O. sokolowskii but Bacillus thuringiensis was not toxic. Introduction of O. sokolowskii in a large field cage erected over a cabbage field reduced the infestation of cabbage by diamondback moth and doubled the yield of cabbage over the control plot where no parasite was used.