Provision of Plant Resources for Beneficial Arthropods in Arable Ecosystems.

Abstract

Plant resources were provided for epigeal beneficial polyphagous predators of cereal aphids, as a raised grassy bank 290m long, sown with grass species of contrasting structure, within a 7ha field. Densities of polyphagous predators overwintering in the grasses were estimated by sampling from grass plots during winters (1987/88 to 1993/94). Data are presented which illustrate how densities of taxa of polyphagous predators have fluctuated during the seven winters. Samples measuring densities of polyphagous predators over the last three winters of the study showed that the within-field grassy bank supported greater densities of beneficial predators than an adjacent permanent field boundary .
Floral resources in the form of the crucifer Labularia maritima, provided along an arable field margin for hoverflies (Diptera: Syrphidae), in the summer, were selectively foraged upon and preferred to native arable weeds by a variety of hoverfly species with aphidophagous larvae. A white coloured variety, compared with a purple coloured variety of L.maritima was consistently visited and foraged upon more frequently, by Episyrphus balteatus, in laboratory and field experiments.
Observational studies showed that potential alternative crops were also foraged upon by beneficial Syrphidae. A field trial, using the potential alternative crop coriander, did not show any significant redistribution of Syrphidae between fields with and without border strips of coriander. Further observational studies on a hedgerow, with its floral diversity and available resources increased by sowing a mixture of alternative crops in a strip, between the hedge and crop, showed that E.balteatus remained at the enhanced strip for significantly longer periods than at the control strip. A simple model shows that the difference in numbers of E.balteatus caught in water-traps at either site cannot be fully explained simply by E.balteatus remaining at the florally diverse site for longer. To explain the differences in trap catches, E.balteatus must have immigrated to the florally diverse strip at a higher rate than to the control strip. Higher syrphid immigration rate to the florally diverse strip may have been due to the visual attractiveness of the diverse site.
Although Syrphidae are highly mobile, linear features on arable land influenced their distribution. A road and tall hedge were shown to be significant obstacles preventing the even distribution of at least one syrphid species. The potential and limitations of habitat enhancement techniques for biological control agents on farmland are discussed.