The first record of entomopathogenic nematodes (Rhabiditiae: Steinernematidae and Heterorhabditidae) in natural ecosystems in Lebanon: A biogeographic approach in the Mediterranean region

Natural occurrence of entomopathogenic nematodes (Steinernema and Heterorhabditis) and Pristionchus nematodes in black truffle soils from Spain

The European truffle beetle Leiodes cinnamomeus is the most important pest in black truffle (Tuber melanosporum) plantations. Current control methods against it are inefficient, so entomopathogenic nematodes (EPNs) could play an important role in their population regulation due to their efficacy against many soil-dwelling insect pests. A survey of EPNs and Pristionchus nematodes was conducted in truffle soils of Spain, considering environmental and physical-chemical soil factors. A total of 164 soil samples were collected from forests, productive plantations and null-low productive plantations, representing three distinct black truffle-growing habitat types. EPNs were isolated from seven soil samples (4.3%); four nematodes were identified as Steinernema feltiae and three as Heterorhabditis bacteriophora. Both species were sampled in three types of soil texture (loam, sandy loam or sandy clay loam), characterized by alkaline pH (7.5 to 8.5) and high organic matter (2.1-11.04%). The presence of these EPNs was influenced by habitat type and organic matter content. Pristionchus nematodes were isolated from truffle soil, around truffle fruit bodies and under the elytra of L. cinnamomeus, with Pristionchus maupasi being the most commonly identified species. No significant associations were found between environmental and soil factors and the occurrence of Pristionchus nematodes. These nematodes were found in alkaline soils (pH 7.75 to 8.7), across all seven sampled soil textures, with variable organic matter content (0.73%-5.92%). The ecological trends and the presence of Pristionchus may affect the occurrence of EPNs and their prospective use as biological control agents against L. cinnamomeus in black truffle plantations.

Isolation, identification of entomopathogenic nematodes with insights into their distribution in the Syrian coast regions and virulence against Tuta absoluta

The occurrence and distribution of entomopathogenic nematodes (EPNs) in the Syrian coast regions remain relatively uncharted. To address this gap in our knowledge, an extensive survey of these ecosystems was essential. This study aims to isolate and identify EPNs from diverse ecosystems within the coastal regions. The distribution of EPNs in cultivated and natural environments was analyzed according to habitat, altitude, and sampling season factors. Between 2017 and 2020, EPNs were recovered from 27 out of 821 soil samples (3.28%) and collected from 24 out of 375 sampling sites (6.4%). Based on morphological, morphometric, and molecular (ITS) characteristics, four EPN species were identified: Heterorhabditis indica (51.85%), representing the first report of its occurrence in the coastal regions, H. bacteriophora (33.33%), H. pakistanense (7.4%), which is also reported for the first time in Syria, and Steinernema affine (7.4%). There were statistical differences in the abundance and recovery frequency of EPNs in each type of habitat. Additionally, there were statistical differences in the altitude and sampling season recovery frequency. Co-inertia analysis revealed correlation between the distribution and occurrence of EPNs in vegetation habitats, altitude, and sampling seasons, as well as some soil characteristics. H. indica and H. bacteriophora were associated with citrus orchards, low-altitude ranges, moderate organic matter, and acidic soil. More specifically, H. indica isolates were correlated with olive orchards, vegetable fields, autumn season, and clay, sandy, and sandy loam soils. Meanwhile, H. bacteriophora isolates were correlated with tobacco fields, grasslands, alkaline pH, spring season, silty loam, and clay loam soils. H. pakistanense was linked to pear orchards, vineyards, moderate pH, and low organic matter. S. affine occurred in walnut orchards, silty soil, higher altitudes, and winter season. The virulence levels of three native EPN isolates (S. affine, H. indica and H. bacteriophora) were evaluated against 3rd and 4th instar larvae (outside and inside mines) and pupae of T. absoluta, a destructive pest in Syria. All three native EPN species exhibited ability to infect and kill the insect, with observed significant differences in their virulence. This study provides an understanding of EPN occurrence, distribution, and their potential for application in sustainable pest control strategies in Syria.

Potential of Oscheius tipulae nematodes as biological control agents against Ceratitis capitata

A survey to collect soil nematodes with potential to control Ceratitis capitata flies was carried out in different locations in Tunisia. Several nematode isolates were recovered, laboratory colonies were established, and their taxonomic identities were determined based on molecular methods. Among all the recovered nematode isolates, two of them, Oscheius tipulae TC2 and OC2, were evaluated for their capacity to control C. capitata flies and for their ability to kill and reproduce on Galleria mellonella larvae. Our results show a great potential of these two isolates as biocontrol agents as they kill C. capitata eggs and pupae and interfere with the metamorphosis of C. capitata larvae. More specifically, TC2 and OC2 nematodes killed 39 and 31% of C. capitata eggs, respectively, impaired the metamorphosis of up to 77% and up to 67% of C. capitata larvae, respectively, and killed up to 66% and up to 58% of C. capitata pupae, respectively. The efficacy of TC2 and OC2 nematodes was particularly high on C. capitata pupae, and significant insect mortalities were observed even at concentrations of 1 and 5 nematodes/pupae, respectively. We also found that TC2 and OC2 nematodes efficiently kill and reproduce in G. mellonella larvae, suggesting that these insects could be used for mass-multiplication of these nematodes. These results reveal the potential of O. tipulae to complement integrated pest management programs against C. capitata flies.

A study on Xenorhabdus and Photorhabdus isolates from Northeastern Thailand: Identification, antibacterial activity, and association with entomopathogenic nematode hosts

Xenorhabdus and Photorhabdus are gram negative bacteria that can produce several secondary metabolites, including antimicrobial compounds. They have a symbiotic association with entomopathogenic nematodes (EPNs). The aim of this study was to isolate and identify Xenorhabdus and Photorhabdus species and their associated nematode symbionts from Northeastern region of Thailand. We also evaluated the antibacterial activity of these symbiotic bacteria. The recovery rate of EPNs was 7.82% (113/1445). A total of 62 Xenorhabdus and 51 Photorhabdus strains were isolated from the EPNs. Based on recA sequencing and phylogeny, Xenorhabdus isolates were identified as X. stockiae (n = 60), X. indica (n = 1) and X. eapokensis (n = 1). Photorhabdus isolates were identified as P. luminescens subsp. akhurstii (n = 29), P. luminescens subsp. hainanensis (n = 18), P. luminescens subsp. laumondii (n = 2), and P. asymbiotica subsp. australis (n = 2). The EPNs based on 28S rDNA and internal transcribed spacer (ITS) analysis were identified as Steinernema surkhetense (n = 35), S. sangi (n = 1), unidentified Steinernema (n = 1), Heterorhabditis indica (n = 39), H. baujardi (n = 1), and Heterorhabditis sp. SGmg3 (n = 3). Antibacterial activity showed that X. stockiae (bMSK7.5_TH) extract inhibited several antibiotic-resistant bacterial strains. To the best of our knowledge, this is the first report on mutualistic association between P. luminescens subsp. laumondii and Heterorhabditis sp. SGmg3. This study could act as a platform for future studies focusing on the discovery of novel antimicrobial compounds from these bacterial isolates.

An Entomopathogenic Nematode Extends Its Niche by Associating with Different Symbionts

Bacterial symbionts are increasingly recognised as mediators of ecologically important traits of their animal hosts, with acquisition of new traits possible by uptake of novel symbionts. The entomopathogenic nematode Heterorhabditis downesi associates with two bacterial symbionts, Photorhabdus temperata subsp. temperata and P. temperata subsp. cinerea. At one intensively studied coastal dune site, P. temperata subsp. cinerea is consistently more frequently isolated than P. temperata subsp. temperata in H. downesi recovered from under the bare sand/Ammophila arrenaria of the front dunes (where harsh conditions, including drought, prevail). This is not the case in the more permissive closed dune grassland further from the sea. No differences were detected in ITS1 (internal transcribed spacer) sequence between nematode lines carrying either of the two symbiont subspecies, nor did they differ in their ability to utilise insects from three orders. The two symbionts could be readily swapped between lines, and both were carried in equal numbers within infective juveniles. In laboratory experiments, we tested whether the symbionts differentially affected nematode survival in insect cadavers that were allowed to dry. We assessed numbers of nematode infective juveniles emerging from insects that had been infected with H. downesi carrying either symbiont subspecies and then allowed to desiccate for up to 62 days. In moist conditions, cadavers produced similar numbers of nematodes, irrespective of the symbiont subspecies present, while under desiccating conditions, P. temperata subsp. cinerea cadavers yielded more nematode progeny than P. temperata subsp. temperata cadavers. Desiccating cadavers with the same nematode isolates, carrying either one or the other symbiont subspecies, confirmed that the symbiont was responsible for differences in nematode survival. Moreover, cadavers harbouring P. temperata subsp. cinerea had a reduced rate of drying relative to cadavers harbouring P. temperata subsp. temperata. Our experiments support the hypothesis that H. downesi can extend its niche into harsher conditions by associating with P. temperata subsp. cinerea.

Biodiversity of entomopathogenic nematodes in Italy

An investigation was carried out on the distribution and biodiversity of steinernematid and heterorhabdtid entomopathogenic nematodes (EPN) in nine regions of Italy in the period 1990–2010. More than 2000 samples were collected from 580 localities and 133 of them yielded EPN specimens. A mapping of EPN distribution in Italy showed 133 indigenous EPN strains belonging to 12 species: 43 isolates of Heterorhabditis bacteriophora, 1 of H. downesi, 1 of H. megidis, 51 of Steinernema feltiae, 12 of S. affine, 4 of S. kraussei, 8 of S. apuliae, 5 of S. ichnusae, 3 of S. carpocapsae, 1 of S. vulcanicum, 3 of Steinernema ‘isolate S.sp.MY7’ of ‘S. intermedium group’ and 1 of S.arenarium. Steinernematids are more widespread than heterorhabditids and S. feltiae and H. bacteriophora are the most commonly encountered species. Sampling sites were grouped into 11 habitats: uncultivated land, orchard, field, sea coast, pinewood, broadleaf wood, grasslands, river and lake borders, caves, salt pan and moist zones; the soil texture of each site was defined and the preferences of habitat and soil texture of each species was assessed. Except for the two dominant species, S. feltiae and H. bacteriophora, EPN occurrence tends to be correlated with a specific vegetation habitat. Steinernema kraussei, H. downesi and H. megidis were collected only in Sicily and three of the species recently described – S. apuliae, S. ichnusae and S. vulcanicum – are known only from Italy and seem to be endemic.