Use of Phacelia tanacetifolia strips to enhance biological control of aphids by hoverfly larvae in cereal fields

Hoverflies (Diptera: Syrphidae) are potentially important in arable and horticultural crops as biological control agents. Many species lay their eggs near aphid colonies, and their larvae are aphidophagous. Adult hoverflies need nectar for energy and the protein from pollen for sexual maturation and egg development. Phacelia tanacetifolia Bentham (Hydrophyllaceae), a North American annual species that is a good source of pollen for syrphids, was drilled in the margins of 3 winter-wheat fields on a farm in North Hampshire, southern United Kingdom in 1992 and in different fields in 1993. Numbers of hoverflies in yellow water traps, oviposition rates, and aphid densities in these fields were compared with those in control fields. In 1992 in the fields bordered with P. tanacetifolia, significantly more hoverflies were captured in the traps but differences in oviposition or aphid numbers were not significantly different between treatments. This may have been because the wheat matured early so that it was less suitable for syrphid oviposition when gravid females were in the field. In 1993, differences between numbers of adults caught in experimental and control fields were not significantly different. However, significantly more eggs were found in fields with P. tanacetifolia than in control fields and significantly fewer aphids were present in these fields than in controls during the 4th wk of the experiment when many 3rd-instar syrphid larvae were present in the crop. Our results provide support for the hypothesis that a management strategy of providing flower borders for fields can result in more efficient biological control of aphids by syrphid larvae, thus reducing the reliance on insecticidal control.