Spider distribution in agroecosystems in Canterbury, New Zealand

The spider assemblage from four shelterbelts and their adjacent grazed pastures in South Island, New Zealand was suction sampled from August 1994 to July 1995 and from March 1996 to March 1997. Spider density decreased rapidly with distance from the field margin (mean 241/ m²) to 72.5/m² at 2.5 m and 10.3/ m² at 5 m into the pasture. Fenced shelterbelts had 6.7 times more spiders and 3-10 more species than did the adjacent pasture. The most common pasture species was the European linyphiid Lepthyphantes tenuis (Blackwall) (37% of individuals) but, in shelterbelts, L. tenuis was equally common at 26% with an unidentified endemic theridiid species at 25%. Eleven shelterbelts were suction sampled in November 1995 and had spider densities ranging from 62/ m² to 369/ m², and species richness ranging from 6 to 12 from a 1 m² sample area at each site. Twenty-three species were recorded; three were introduced European linyphiids, the rest were native or endemic to New Zealand. Spider density was compared in three pasture types (cocksfoot and clovers, lucerne, and rye grass and clovers) both in open and agroforestry pasture (between rows of Pinus radiata trees). There were no differences between either open and agroforestry pasture, or the three pasture types. Spider density was not correlated with vegetation height or cover, although L. tenuis density was positively correlated with vegetation height characteristics in open pasture. Mowing ungrazed pasture plots reduced both spider density and species richness. Unmown plots had a fauna and density similar to those of shelterbelts, but mown plots had a density similar to those in grazed pasture. Field cage experiments in a lucerne (Medicago sativa L.) crop and in broad bean (Vicia faba L.) plots to investigate the effects of spider density on pea aphid (Acyrthosiphon pisum (Harris)) abundance, showed that a large number of replicates (46 or more) were needed to detect differences in aphid abundance between spider density treatments. Aphid densities reached unnaturally high levels due to the sheltered cage conditions. The results are discussed in the context of the effects of pastoral agricultural practices in New Zealand on spider density, distribution, species composition, and potential in pest management.