Einfluss des Wirtswechsels der Kohlmotte, Plutella xylostella L. auf Erbsen auf ihre natürlichen Feinde in Kenia

Abstract

A population of the diamondback moth (DBM), Plutella xylostella L., known as an oligophagous pest on crucifers was recently found to infest sugar snap- and snowpeas (Diadegma semiclausum L.) in the Rift Valley in Kenya, causing heavy damage. The impact of this host shift on the interaction with associated parasitoids was investigated. In laboratory experiments parasitation levels, development and survival, and the role of host plants for host location of two important parasitoids were studied: The indigenous Diadegma mollipla (Holmgren), regarded as a relative generalist, and the highly specific Diadegma semiclausum (Hellen), introduced to Kenya for biocontrol of DBM on cruciferous vegetables. Tested individually D. mollipla surprisingly proved to be more effective on the new host plant than on cabbage. Diadegma semiclausum parasitized more efficiently on DBM on cabbage. Bioassay tests, conducted with a Y-tube olfactometer, showed that the specialisation of Diadegma semiclausum is mediated by host plant signals, associated with crucifers, which are not encountered in DBM feeding on peas. Whereas for D. mollipla crucifer volatiles seem not to be used as primary cues for host location. Differential performance and host location abilities of the two parasitoids suggested a difference in competitiveness on peas as compared to cabbage. On cabbage, Diadegma semiclausum clearly out-competes its congenus. On peas, confined to small containers D. mollipla parasitized equally as good or even better than Diadegma semiclausum. However, under more natural conditions in the greenhouse, Diadegma semiclausum parasitized significantly more larvae than its competitor on both host plants. Chemical interference between the two species could be a possible explanation. Olfactory learning is known to enable parasitoids to modify their behavioural responsiveness to host and plant- derived odours. In view of increasing the potential of Diadegma semiclausum for biocontrol of DBM in peas, the parasitoid was reared for three subsequent generations on the pea strain of DBM. Adaption to the new host plant through pre-emergence learning seemed to be possible. Response to DBM-infested pea and levels of parasitation increased in the pea generations. However, fitness trade-offs, especially an extreme shift in sex ratio to males reduced reproductive success of Diadegma semiclausum on peas. In order to investigate the effect of the unusual host plant on the entire local parasitoid community associated with DBM, field studies were conducted close to the farm where DBM was first discovered on peas. Peas provided an enemy-free space for DBM. Local parasitoids occurred only sporadically and in very low numbers on DBM on peas. The release of Diadegma semiclausum in the study site led to a reduction of DBM in kales. It did not establish in peas. The number of local parasitoids gradually decreased after the introduction. With a strong competition on kales especially D. mollipla was assumed to find a niche in peas. However, it did not occur in larger numbers in peas. The species might have moved to other host instead. We demonstrated, that the host plant had a strong influence on parasitation, fitness and host loaction. The inclusion of a new host plant into its feeding repertoire enables the herbivore host to avoid enemy attack. But due to the ability to adapt to a new host plant, parasitoids could follow their host with time.