Hedges. VI. Habitat diversity and crop pests: a study of Brevicoryne brassicae and its Syrphid predators.
The following is based largely on the author's summary. A study was made in eastern England in 1969-70 on aphidophagous Syrphids and Syrphid predation on Brevicoryne brassicae (L.) on brussels sprouts in two contrasting areas of farmland 5 km apart. One area was rich in hedges, woodland, pasture and other habitats outside the crop, and the other was mainly arable. The work consisted of studies on the phenology and habitat preferences of aphidophagous Syrphids, studies on oviposition by Syrphids on artificially infested sprouts in a variety of habitats, and studies on the development of aphid populations on sprouts in the two areas. The phenological work showed that the main predator in the study, Syrphus balteatus[Episyrphus balteatus] (Deg.) [cf. RAE A 58 1635], has only one generation each year, and not several, as suggested in the literature. Some Syrphid species were found to be restricted to woodland, while others occurred in the woodland and also in an adjoining hedge. However, these woodland-based species did not lay many eggs on brussels sprouts, except on plants in pots placed in the wood and on plants in the immediate shelter of the hedge. Even 30 m from the hedge, no more eggs were laid than in a site in the middle of an arable area. Neither was there any evidence that Syrphid species that were more generally distributed and those that were mainly found in open habitats laid more abundantly in the area of diverse habitats than in the mainly arable area. The experimental study of aphid populations showed that Syrphids reduced aphid numbers significantly only in the mainly arable area. This supports the view that the presence of rich habitats without crops does not increase Syrphid oviposition, at least on the scale of this experiment.