The foraging ecology of hoverflies (Diptera:Syrphidae) and the potential for manipulating their distribution on farmland

Abstract

Observations of the foraging behaviour of commonly occurring syrphid species were recorded in two floristically different field margin types. Individual syrphid species did not visit flowers according to their abundance in the habitat. The study has identified the arable weeds and hedgerow plants which are used selectivity by syrphids. The growth of several selected plant species is encouraged via the implementation of the Game Conservancy Trust's 'conservation headlands' technique. Comparison of the numbers of syrphid adults in fully sprayed and conservation headland plots showed that when there were significant differences in floral area (the number of flowers) between the two treatments there were also significant differences in the proportion of the total number of flies in the two treatments; the provision of selected adult food sources resulted in a local redistribution of flics with higher numbers of several syrphid species being recorded in the conservation headland plots. Flies were retained in these plots as a result of their foraging behaviour. Diurnal activity patterns for individual species are discussed in relation to microclimatic factors. Syrphid eggs collected from the two treatments were identified to genus using the characteristic surface patterning of the eggs. Pipiiella, Episyr-phus and Metasyrphus were numerically the most important genera. Comparison of the number of eggs per aphid in fully sprayed and conservation headland plots showed that on several dates higher numbers of eggs per aphid were recorded from the conservation headland plots. This was in part due to the fact that the non-crop plants within the crop acted as ovipo-sition sites for syrphids belonging to the genus Melanostoina. Females belonging to this group did not respond to aphid density when ovipositing; however, there was a significant positive relationship between the number of Episyrphus and Metasyrphus eggs and aphid density. Analysis of the distribution of unidentified syrphid eggs within a single field showed that the flies displayed a positive reproductive numerical response to aphid density.

The study has shown the importance of field margin composition in determining the distribution and species composition of syrphids; beneficial manipulation of field margin composition has the potential to enhance bio-control potential of this important group of aphid predators.

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