Diamondback moth, Plutella xylostella (L.), on peas in Kenya: impact of the host shift on the pest and its parasitoid

A population of the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae) (DBM), was detected in 1999 on sugar snap peas (Pisum sativum) in the export vegetable growing area south of Lake Naivasha in the Rift Valley Province of Kenya. The performance of this new pea strain (DBM-P) was compared with the normal crucifer strain (DBM-C) in laboratory studies. DBM-P survived equally well on kale and on pea, whereas DBM-C survival on pea was only 2.4%. Larval development of DBM-C took five days longer on pea than on kale and larval growth was severely reduced. Pupal weights of DBM-C on pea (3.8 mg) were significantly lower than those of DBM-P (4.6 mg) and those of both strains on kale (5.7 and 5.3 mg, respectively). Crosses between the strains in both directions produced fertile offspring. The influence of the host shift on Diadegma mollipla (Holmgren), the most abundant indigenous parasitoid of DBM in the Kenyan highlands, was also investigated. The number of larvae parasitised in a no-choice situation by two strains of D. mollipla, one collected on pea and the other from cabbage, was higher on pea than on cabbage. In a choice situation with entire plants, parasitism on pea offered alone was four times higher than on cabbage. When both host plants were offered simultaneously, the level of parasitism dropped to the level of cabbage offered alone. In conclusion, it is suggested that D. mollipla is a non-specialist parasitoid in the crucifer/DBM system and the significance of this observation for biological control of DBM is discussed.