Influence of below-ground feeding by garden symphylans (Cephalostigmata: Scutigerellidae) on plant health

The influence of garden symphylan (Scutigerella immaculata Newport) root feeding on crop health was measured in the laboratory and in the field. In the laboratory, the relative susceptibility of sweet corn (Zea mays L.), potato (Solanum tuberosum L.), spinach (Spinacia oleracea L.), and tomato (Lycopersicon esculentum L.) to S. immaculata feeding was investigated by subjecting each crop to three levels of pest pressure (0, 15, and 45 S. immaculata). As S. immaculata densities increased from 0 to 45, root length and dry weight of above-ground plant parts of 1-wk-old spinach and tomato seedlings were reduced from 85 to 98%, whereas corn root length was reduced by 34%. Potato and corn dry weight and potato root length were not reduced. The pest-host association was investigated in the field by examining the relationships between estimated S. immaculata densities and two vegetational parameters: (1) crop health measured by the normalized difference vegetation index (NDVI) and (2) post-tillage spring weed densities. S. immaculata densities were estimated using a baiting method. NDVI in corn and potato was only slightly reduced with increasing within-field S. immaculata densities after adjusting for spatial autocorrelation. Squash and broccoli showed sharp decreases in NDVI at densities from 1 to 10 S. immaculata. The nonlinear asymptotic form of the Bleasdale-Nelder curve was selected as the best curve to describe the damage/pest relationship for all crops using Akaike information criteria. The relative susceptibilities of direct seeded crop varieties to S. immaculata in western Oregon reported in 1937 by Morrison were reanalyzed to make comparisons with our data. In further analysis, seed size seemed to be linearly related with susceptibility of direct-seeded crops to S. immaculata. Stand count was predicted to increase by 0.24% for each 1-mg increase in seed size. These findings will be used to help develop action thresholds and sample size requirements and to help reduce S. immaculata damage by avoiding the planting of highly susceptible crops at sites with high S. immaculata populations.