Entomopathogenic nematodes, root weevil larvae, and dynamic interactions among soil texture, plant growth, herbivory, and predation

Abstract

Greenhouse experiments were conducted to assess the influence of soil texture on the persistence, efficacy and plant protection ability of entomopathogenic nematodes (EPNs) applied to control larvae of the Diaprepes root weevil (DRW), Diaprepes abbreviatus, infesting potted citrus seedlings. Seedlings were grown in pots containing either coarse sand, fine sand, or sandy loam. Three DRW larvae were added to each of 80 pots of each soil type. 24 h later, 20 pots of each soil type that had received weevil larvae were inoculated with EPN infective juveniles (IJs) of one of the following species: Steinernema diaprepesi, Steinernema riobrave and Heterorhabditis indica. Pots of each soil without EPNs were established as controls with DRW and controls without DRWs. Subsequently, pots with larvae received three additional larvae monthly, and the experiment continued for 9 months. Plant root and top weights at the end of the experiment were affected by both soil (P≤0.0001) and nematodes (P≤0.0001), and nematode species protected plants differently in different soils (interaction P≤0.0001). Soil porosity was inversely related to plant damage by DRW, whether or not EPNs were present; and porosity was directly related to the level of plant protection by EPNs. Mortality of caged sentinel weevil larvae placed in pots near the end of the experiment was highest in pots treated with S. diaprepesi. In a second, similar experiment that included an additional undescribed steinernematid of the Steinernema glaseri-group, soil type affected root damage by DRW and root protection by EPNs in the same manner as in the first experiment. Final numbers of S. diaprepesi and Steinernema sp. as measured by real-time PCR were much greater than those of S. riobrave or H. indica in all soils. Across all treatments, the number of weevil larvae in soil at the end the experiment was inversely related to soil porosity. In all soils, fewer weevil larvae survived in soil treated with S. diaprepesi or Steinernema sp. than in controls with DRW or treatments with S. riobrave or H. indica. The results of these experiments support the hypothesis that EPNs provide greater protection of seedlings against DRW larvae in coarse textured soil than in finer textured soil. However, less vigorous growth of the control without DRW seedlings in the two finer textured soils suggests that unidentified factors that stressed seedlings in those soils also impaired the ability of seedlings to tolerate weevil herbivory.

Analyzing spatial patterns linked to the ecology of herbivores and their natural enemies in the soil

“…Across the two regions of the geospatial survey, three of the four most frequently encountered EPN species were significantly aggregated as measured by the SADIE Ia (Campos-Herrera et al, 2013). The spatial patterns of Steinernema diaprepesi and Heterorhabditis zealandica were significantly associated with that of the central ridge eco-region, supporting the possibility that EPNs influence the D. abbreviatus spatial pattern, because S. diaprepesi is reported to be more virulent to this weevil than other native EPN species (El-Borai et al, 2012). Variables that might affect EPN patterns were identified from redundancy analyses.…”

Entomopathogenic Nematodes in the Soil Environment: Distributions, Interactions and the Influence of Biotic and Abiotic Factors

“…In Florida citrus groves, soil type was not correlated with infection of root weevils by S. carpocapsae (Beavers, McCoy, & Kaplan, 1983) but suppression of root weevils by S. riobrave was greater in coarse, sandy soils than in fine textured soils (Duncan, Genta, Zellers, Fares, & Stansly, 2001;Shapiro et al, 2000). Indeed, across a broad range of Florida citrus groves, soil texture (and associated variables that influence soil water potential; see below) appears to strongly influence the distribution of EPN species and the natural regulation of root weevils (Campos-Herrera, Pathak, ElBorai, Schumann, Abd-Elgawad, et al, 2013; Campos-Herrera, Pathak, El-Borai, Duncan, Stuart, El-Borai, Campos-Herrera, Pathak, et al, 2013;El-Borai, Stuart, Campos-Herrera, Pathak, & Duncan, 2012;Stuart et al, 2008).…”

A survey of entomopathogenic nematode species in continental Portugal

“…Soil type and texture are also very important parameters which influence EPN distribution (Campos-Herrera et al, 2011;El Borai et al, 2012). Continental Portugal has a wide diversity of crops, such as fruit trees, cereals and vegetables, and natural habitats, such as conifer forests and grasslands.…”