The usefulness of Phacelia tanacetifolia strips as a resource for aphidophagous hoverflies (Diptera:syrphidae) on arable land

This project examines the effect of providing boundary strips of Phacelia tanacetifolia (Hydrophyllaceae) beside cereal fields to attract adult hoverflies, on biological control of aphids in the adjacent crops in the adjacent crop by hoverfly larvae. Phacelia, a North American annual species that is a good source of pollen for adult hoverflies, was drilled in the margins of three winter wheat fields in 1992 and in different fields in 1993. Numbers of hoverflies caught 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 Phacelia more hoverflies were captured in the traps but neither the amount of oviposition, nor aphid numbers, differed significantly 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, numbers of adults caught in experimental and control fields were not significantly different. However, significantly more eggs were found in fields with Phacelia borders than in control fields and fewer aphids were present in these fields than in controls during the 4th week of the experiment when many third instar syrphid larvae were found in the crop. The results of a supplementary experiment comprising yellow water trap catches of hoverflies at crop height and at ground level indicated that the crop high traps were selectively attracting newly emerged and hungry individuals.An investigation into the feeding preferences of the most common aphidophagous syrphid species in New Zealand revealed that anemophilous pollen was an important component of its diet, but that Phacelia pollen was also eaten by this species.The extent to which this work has clarified the relationship between flowers and oviposition by hoverflies is discussed along with ideas for continued work in this field.