Geospatial patterns of soil properties and the biological control potential of entomopathogenic nematodes in Florida citrus groves

Entomopathogenic nematodes (EPNs) are widely distributed in natural and managed ecosystems worldwide. Due to the cryptic nature of soil food webs, EPN ecology and their role in modulating insect population dynamics remain largely a matter of speculation. A weevil pest of citrus, Diaprepes abbreviatus, is less abundant in orchards on the central ridge (hilly topography, deep, coarse sand soils) than in the flatwoods (flat topography, fine sand soils with a high water table). We speculate that native EPNs are a key factor regulating these weevils and thus hypothesized that EPNs are most abundant and/or species diverse in central ridge orchards. In this study, we measured and analyzed the natural distributions of EPNs in these two regions concomitantly with those of selected abiotic and biotic soil components. Our objective was to identify physical properties that can potentially be manipulated to conserve native EPNs that serve to control D. abbreviatus. We used species-specific qPCR probes for 1) 13 EPN species, ii) two species of Paenibacillus that are ectoparasitically associated with EPNs, iii) free-living bacteriophagous nematodes (Acrobeloides-group) that might compete with EPNs, and iv) oomycete pathogens of citrus roots, Phytophthora nicotianae and Phytophthora palmivora. Citrus orchards were surveyed in eco-regions categorized as central ridge (23 localities) and flatwoods (30 localities). EPNs and Acrobeloides-group were detected in all sites and the abundances of the two guilds were positively related. Heterorhabditids comprising two species occurred in more localities at higher numbers than did five steinernematid species. Heterorhabditis indica dominated flatwoods communities, whereas communities with abundant Steinernema diaprepesi, Heterorhabditis zealandica and H. indica occurred on the central ridge. Spatial patterns of S. diaprepesi were more aggregated than those of H. indica and other dominant species. The central ridge supported greater EPN evenness, diversity and species richness. For the first time, quantitative natural positive associations between EPNs and two species of Paenibacillus bacteria were assessed. The oomycete pathogen P. palmivora was only detected in the flatwoods, whereas P. nicotianae was widespread and equally abundant in both regions. Four variables that affect soil water potential (groundwater depth, available water capacity, clay and organic matter content) significantly contributed to explain the variability in a redundancy analysis of the selected soil communities. Management of soil water potential may aid in establishing and conserving diverse EPN communities that provide more effective control of Diaprepes root weevils. (C) 2013 Elsevier Ltd. All rights reserved.