The tachinid A. xylota is a common parasitoid of larvae of Trirhabda virgata and T. borealis (Coleoptera: Chrysomelidae) in central New York [USA]. Flies aggregated on herbaceous vegetation along the borders between old fields and more shaded habitats (shrubby fields or woodlots). Aggregations comprised from less than 100 to a few thousand individuals, almost all of which were male. Males continuously perched and moved about in sun flecks on the vegetation and frequently grappled with each other. In the old field adjacent to the primary aggregation site, the tachinid sex ratio was strongly skewed toward females. Aggregations of A. xylota are probably involved in mating.