Field boundaries play an important role as refuges, food sources and corridors for invertebrates and vertebrates, and increasing farmland fragmentation impacts on these functions. However, hedgerows and other structures can also impede dispersal by flying insects. The current work uses the pollen of Phacelia tanacetifolia in hover fly guts as a marker to assess hover fly movement in farm landscapes. In the United Kingdom and New Zealand, Phacelia pollen was found in the guts of Ephisyrphus balteatus and Metasyrphus corollae (United Kingdom) and Melanostoma fasciatum (New Zealand) at distances up to 200 m from the source, when there were no barriers between the flowers and the traps used to catch the flies. The rate of decline over distance in the proportion of flies containing pollen was similar for the two countries. The extent to which four replicated field boundary types impeded hover fly movement was tested using post-and-wire fences, lines of poplars (Populus spp.) with gaps, dense poplars and controls (no potential barriers). Phacelia was planted on one side of each boundary, and along the centre of the control plots. The relative presence of the pollen in flies on both sides of the barriers showed that both types of poplar boundary restricted the movement of the flies, but the fence had no effect. In a separate experiment, gravid females of M. fasciatum were captured at a greater height on a shade-cloth fence than were non-gravid females and males. The implications of this work include the functioning and persistence of metapopulations and the influence of field boundaries on population recovery of beneficial invertebrates following pesticide-induced mortality. If field boundaries contribute to a temporal asynchrony between pest and natural enemy populations, this needs to be considered along with the well-established roles of boundaries as refugia for, and sources of, beneficial arthropods.

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The phenology and sex ratios of three hover fly species on Canterbury farmland, New Zealand were recorded weekly from five yellow watertraps from October 15, 1993 to January 31, 1997. Gut fullness, egg number and pollen content were recorded weekly from September 8, 1994 to July 25, 1996. Catches of Melanostoma fasciatum (Macquart) and Melangyna novaezelandiae (Macquart) were higher during the summer months, with maximum male M. fasciatum catches at 0.7 per trap/day in January, 1997. Catches of female Eristalis tenax Z97035 Received 15 October 1997, accepted 8 September 1998 L. peaked at 0.24 in August, 1994. Gut fullness and egg number in M. novaezelandiae and M. fasciatum were low from June to September, 1995, a pattern opposite to that of E. tenax. For all species combined, gut fullness was significantly higher in gravid females compared with non-gravid females, and males. Thirty-nine different pollen types were recorded from all hover fly classes. Male E. tenax (the largest of the three species) consumed the widest range of pollen in summer and autumn whereas females consumed the widest range in winter and spring, a pattern opposite to that of M. novaezelandiae. The value of the work in possible population enhancement programmes for hover flies in crops is discussed.