Purple tansy is a native North American plant that is a good pollen source for syrphids. Field long strips of tansy (20" wide) were seeded on two borders of wheat fields in 1992 and 1993. Each year strips were added to three wheat fields and compared to three nearby wheat fields without tansy borders. In 1992, more syrphid flies were found in fields with tansy borders during June and July, but significantly more flies were found only when the tansy was flowering. Elevated levels of syrphid flies were found as far as 200 yards from the tansy borders. After the tansy had died down (late July), the differences in syrphid numbers disappeared. Very low numbers of syrphid eggs were found and no significant differences occurred. In 1993 the number of syrphid flies found was not significantly influenced by the tansy border on any date; however, signficantly more syrphid eggs (2-5 times) were found in fields with tansy borders.

In 1992, no significant differences were found in aphid numbers, but in 1993 significantly fewer aphids were found in early July in fields bordering tansy. One factor influencing the year to year variation was the relationship between time of tansy flowering and wheat maturity. In 1992 the wheat senesced two weeks earlier than normal, and the condition of wheat may have inhibited syrphid egg laying.

Source:

J. M. Hickman & S. D. Wratten. 1996. Use of Phacelia tanacetifolia strips to enhance biological control of aphids by hover fly larvae in cereal fields. J. Econ. Entomol. 89:832-840.