Comparison of Predator and Pest Communities in Washington Potato Fields Treated with Broad-Spectrum, Selective, or Organic Insecticides

Abstract

In the northwestern United States, insect pests of potatoes (Solanumtuberosum L.) have typically been controlled using broad-spectrum insecticides. However, the loss or impending loss of many broad-spectrum chemicals is increasing the use of selective insecticides, and organic potato production is growing in the region. In the 2001 and 2002 growing seasons, we intensively sampled the arthropods in 31 (14 in 2001 and 17 in 2002) production potato Þelds under three pest management regimens: conventional Þelds treated with broad-spectrum insecticides (Hard), conventional Þelds treated with selective insecticides (Soft), and organic Þelds treated with insecticides certiÞed for organic production (Organic). All Þelds were within the Columbia Basin of Washington, grown under center-pivot irrigation. We sampled arthropods using three techniques: D-vac suction sampling, pitfall trapping, and visual searching. Geocoris spp. and Nabis spp. bugs (Hemiptera) and spiders (Araneae) were the most abundant predators in plant foliage, together making up 90% of the foliar predator community in both years. Total predator densities and densities of Geocoris, Nabis, and spiders not in the family Linyphiidae were highest in Organic and Soft, and lowest in Hard Þelds. Linyphiid spider densities were marginally elevated in Soft compared with Hard Þelds. On the ground, carabid (Coleoptera: Carabidae) and staphylinid (Coleoptera: Staphylinidae) beetles and a diverse group of spiders dominated the community, together making up 90% of the ground-active predator com- munity in both years. Total predator densities and densities of carabid beetles, staphylinid beetles, and linyphiid spiders were highest in Organic and Soft and lowest in Hard Þelds, whereas densities of these arthropods did not differ between Organic and Soft. Densities of nonlinyphiid spiders were highest in Organic, intermediate in Soft, and lowest in Hard Þelds. While predator densities were generally high in organic Þelds, these Þelds also had the highest densities of the two most injurious insect pests, the green peach aphid (Myzus persicae Sulzer) and the Colorado potato beetle (Leptinotarsa decem- lineata Say). Selective insecticides allowed conventional growers to achieve predator densities similar to those seen in organic Þelds while retaining low pest densities typical of Þelds treated with broad-spectrum insecticides.