USE OF PHACELIA-TANACETIFOLIA STRIPS TO ENHANCE BIOLOGICAL-CONTROL OFAPHIDS BY HOVERFLY LARVAE IN CEREAL FIELDS

Hoverflies (Diptera: Syrphidae) are potentially important in arable an d horticultural crops as biological control agents. Many species lay t heir eggs near aphid colonies, and their larvae are aphidophagous. Adu lt hoverflies need nectar for energy and the protein from pollen for s exual maturation and egg development. Phacelia tanacetifolia Bentham ( Hydrophyllaceae), a North American annual species that is a good sourc e 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 co mpared with those in control fields. In 1992 in the fields bordered wi th P. tanacetifolia, significantly more hoverflies were captured in th e traps but differences in oviposition or aphid numbers were not signi ficantly different between treatments. This may have been because the wheat matured early so that it was less suitable for syrphid ovipositi on 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 i n fields with P. tanacetifolia than in control fields and significantl y fewer aphids were present in these fields than in controls during th e 4th wk of the experiment when many 3rd-instar syrphid larvae were pr esent in the crop. Our results provide support for the hypothesis that a management strategy of providing flower borders for fields can resu lt in more efficient biological control of aphids by syrphid larvae, t hus reducing the reliance on insecticidal control.