Effect of IPM practices and conventional spraying on spider population dynamics in an apple orchard

Integrated pest management (IPM) is now practiced in commercial orchards in the Czech Republic. The effect of two IPM practices and conventional spraying on arboreal and understorey spider population dynamics was studied over four years. Conventional spraying included frequent applications of non-selective pesticides; IPM plots were treated with selective pesticides less frequently. IPM plots were undersown with various plantings or grass. The plot under conventional spraying, was grown by a naturally established weed stand. Conventional practice negatively influenced species diversity, particularly hunting spiders (Thomisidae and Philodromidae). A spraying programme applied on IPM plots reduced abundance of spiders less than the non-selective chemicals applied to the plot under conventional practice. Plantings on IPM plots increased overall species diversity, whereas undersowing of grass supported abundance of arboreal spiders. In general, IPM practice maintained the seasonal abundance of spiders at a balanced level, whereas conventional spraying caused violent fluctuations of late-summer spider populations. Late-summer abundances were determined by reproduction rather than aerial dispersal. Spring abundance of Theridion impressum was presumably influenced by winter predation by Philodromus cespitum.