Guild structure of aphid parasitoids in broccoli: influence of host and neighbouring crops

Naturally occurring parasitoids may provide sufficient control of aphid pests in fields undisturbed by insecticides. Yet the efficacy of these parasitoids may be strongly influenced by characteristics of their hosts and by the availability of alternative hosts in neighbouring crops. In the present study, parasitoid guild structure was compared in two aphid species, Brevicoryne brassicae and Lipaphis erysimi, in broccoli and in adjacent wheat and cotton fields. Diaeretiella rapae constituted 81 and 75% of all parasitoids (including hyperparasitoids) recovered from B. brassicae and L. erysimi, respectively. Aphidius species were also represented at low levels. Diaeretiella rapae does not appear to attack other aphids in neighbouring noncruciferous crops; Aphids in wheat and cotton are attacked primarily by Aphidius and Lysiphlebus species, respectively. Another factor that hampered parasitoid population build-up in broccoli was the action of hyperparasitoids, which were active throughout the broccoli-growing season (November-March). To enhance biological control, our data suggest that it is important to keep specific non-crop vegetation as a parasitoid source near broccoli fields. This vegetation must not harbour aphid species that may move onto broccoli but instead be infested with other aphids that are readily attacked by D. rapae.