Phylogenetic evidence for the invasion of a commercialized European Phasmarhabditis hermaphrodita lineage into North America and New Zealand

Nematodes associated with terrestrial gastropod molluscs in Belgium and additional characterisation of Pellioditis californica and P. hermaphrodita

A survey for slug- and snail-associated nematodes was conducted in forests, parks, botanical gardens, and nature reserves at 13 localities in Belgium to uncover more diversity of gastropod mollusc-associated nematodes and to characterise Pellioditis populations found in the country. A total of 319 slugs and snails belonging to nine species were examined. Arion vulgaris was the most commonly found mollusc species in this study (eight locations), and 19.4% of the examined mollusc specimens were found infected by nematodes. The highest prevalence of nematodes was observed in Cornu aspersum (60%) followed by A. vulgaris (34.8%), Limax maximus (28.6%), and Cepaea sp. (20%). Eleven nematode species belonging to eight families were isolated and identified from the mollusc hosts including Alloionema appendiculatum, Angiostoma dentiferum, A. gandavense, Angiostrongylus vasorum, Cosmocerca longicauda, Panagrolaimus cf. subelongatus, Pellioditis californica, P. hermaphrodita, Rhabditis sp., Tetrameres cf. fissispina, and Troglostrongylus cf. brevior.Pellioditis was the most commonly found nematode genus (at nine localities) and C. longicauda and P. californica were reported in Belgium for the first time. Co-infections of more than one nematode species were observed in eight (2.5%) molluscs specimens. Most co-infections consisted of two nematode species. In one A. vulgaris specimen, a co-infection of three nematode species (A. vasorum, P. hermaphrodita, and Tetrameres cf. fissispina) was observed. Four ex vivo cultures of P. californica and six ex vivo cultures of P. hermaphrodita were established from single hermaphrodites, and both species were described based on light microscopy, scanning electron microscopy, and morphometric, morphological, and molecular data.

The biocontrol nematode Phasmarhabditis hermaphrodita infects and increases mortality of Monadenia fidelis, a non-target terrestrial gastropod species endemic to the Pacific Northwest of North America, in laboratory conditions

Inundative biological control (biocontrol) efforts in pest management lead to the mass distribution of commercialized biocontrol agents. Many 'biocontrol gone awry' incidents have resulted in disastrous biodiversity impacts, leading to increased scrutiny of biocontrol efforts. The nematode Phasmarhabditis hermaphrodita is sold as a biocontrol agent on three continents and targets pest gastropods such as Deroceras reticulatum, the Grey Field Slug; P. hermaphrodita is not presently approved for use in the United States. Investigations into the potential for P. hermaphrodita to infect non-target gastropod species of conservation relevance, however, are limited. We examined the effects of three strains of P. hermaphrodita on mortality in Monadenia fidelis, the Pacific Sideband, a snail species endemic to the Pacific Northwest of North America, in laboratory conditions. Across a 71-day laboratory infectivity assay, snails exposed to each of the three nematode strains, each analyzed at two doses, experienced a mean 50% mortality by days 20-42. All nematode-treated snails were dead by the end of the study. By contrast, 30/30 water-control snails experienced no mortality. Nematodes killed smaller, juvenile-stage snails significantly faster than those in larger and more developmentally advanced hosts. Our results provide direct evidence that the biocontrol nematode P. hermaphrodita infects and kills M. fidelis, a non-target gastropod species endemic to the Pacific Northwest, in laboratory conditions. This study suggests that introduction of P. hermaphrodita to new ecosystems might negatively impact endemic gastropod biodiversity and advocates for further investigation of non-target effects, including in conditions closer to the natural environments of non-target species.

CHEMOTAXIS RESPONSE OF PHASMARHABDITIS CALIFORNICA (FAMILY: RHABDITIDAE) AND PRISTIONCHUS ENTOMOPHAGUS (FAMILY: NEODIPLOGASTERIDAE) TO THE MUCUS OF FOUR SLUG SPECIES

The chemotaxis responses of soil nematodes have been well studied in bacteriophagic nematodes, plant-parasitic nematodes, entomopathogenic nematodes, and to a lesser extent malacopathogenic nematodes. Free-living stages of parasitic nematodes often use chemotaxis to locate hosts. In this study, we compared the chemotaxis profile of 2 slug-associated nematodes with overlapping host ranges. Phasmarhabditis californica is a facultative parasite that has been shown to express strain-dependent variation in chemoattraction profile. We tested 4 slug species to determine the attraction index of a Canadian strain of Ph. californica and a sympatric necromenic nematode, Pristionchus entomophagus. When tested against a control (distilled water), Ph. californica showed a clear (positive) attraction towards the mucus of slugs Ambigolimax valentianus, Arion rufus, and Arion fasciatus, but not Deroceras reticulatum. However, when given a choice between the mucus of D. reticulatum and Ar. fasciatus in a pairwise test, Ph. californica was strongly attracted to the former. Other pairwise comparisons did not reveal a clear preference for either slug species in the following pairs: D. reticulatum-Ar. rufus, Am. valentianus-Ar. rufus, D. reticulatum-Am. valentianus. The chemotaxis assay for Pr. entomophagus showed an attraction toward D. reticulatum and Ar. fasciatus (tested against controls); the attraction index for Am. valentianus was positive, but this was not statistically significant. In contrast, the attraction index for Ar. rufus was negative, suggesting possible repulsion to the mucus of this slug species. Given that Pr. entomophagus and Ph. californica occupy overlapping habitats, utilize similar hosts, and exhibit similar chemotaxis profiles, there is a potential for direct interaction between these 2 nematodes. Like other members of the genus Pristionchus, Pr. entomophagus may be able to prey upon the co-occurring Ph. californica, such antagonistic interactions could have important implications for the coexistence of these 2 species and Ph. californica in particular as a biocontrol agent against pestiferous slugs.

Synopsis of gastropod-associated nematodes of Ciscaucasia (Russian Federation) with the description of a new species of Pellioditis Dougherty, 1953 (syn. Phasmarhabditis Andrássy, 1976)

Caucasus is known as one of the few biodiversity hotspots in Europe and is characterised by rich gastropod fauna while the nematode fauna in association with gastropods has remained largely understudied. Surveys conducted in 2019 and 2021 in the North Caucasus of the Russian Federation (Stavropol Upland and western and central parts of Krasnodar Krai) has revealed the presence of three new species of Pellioditis, a facultative parasite of land gastropods, and two species of obligate parasites, the intestinal parasite Angiostoma kimmeriense and a new, still undescribed species of a larval ectoparasite Alloionema sp. The new associations of Cruznema sp. and Rhabditophanes sp. with land gastropods were recorded for the first time in the Russian Federation. The new species of Pellioditis Dougherty, 1953 described here is based on the analysis of morphology and molecular studies of two distant and morphologically distinct strains, thermalis and sindicae. Pellioditis thermalis n. sp. was characterised by females possessing a tail of about 95-100 μm long, broadly conical in shape in the thermalis and sindicae strains, with a rounded anterior part and a subulate terminal part as long as the former; prominent phasmids located at the mid-tail, equatorial vulva position, a lateral field of three ridges (four incisions), males with spicules featuring a hole at the distal tip, ensheathed infective juveniles with average length 717 μm in the thermals strain and 771 μm in the sindicae strain, and exsheathed ones 644 μm and 682 μm, respectively. ITS-based phylogenetic analyses revealed that all Pellioditis species found in Ciscaucasia and Transcaucasia probably belong to two separate clades, with independent evolutionary histories of colonisation of this area. The entire Caucasus range area appears to serve as a biodiversity hotspot for the genus Pellioditis, presumably due to its complicated geological history and repeated isolation events for its terrestrial mollusc hosts.

Thirty years of slug control using the parasitic nematode Phasmarhabditis hermaphrodita and beyond

Several slug species are highly pestiferous and threaten global sustainable agriculture. Current control methods rely heavily on metaldehyde pellets, which are often ineffective, harm nontarget organisms and have been banned in some countries. A viable alternative is the parasitic nematode Phasmarhabditis hermaphrodita (and recently P. californica), which has been formulated into a biological control agent (Nemaslug®) to control slugs across northern Europe. Nematodes are mixed with water and applied to soil where they seek out slugs, penetrate behind the mantle and kill them in 4-21 days. Phasmarhabditis hermaphrodita has been on the market since 1994 and since then there has been ample research on its use. Here we review the research carried out on P. hermaphrodita over the last 30 years since its development and release as a commercial product. We provide information on life cycle, worldwide distribution, history of commercialisation, gastropod immunity, host range, ecological and environmental factors that affect its success in the field, bacterial relationships, and summarise results of field trials. Finally, we suggest future directions for P. hermaphrodita research (and other Phasmarhabditis species) to enhance its use as a biological control agent to control slugs for the next 30 years. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Pellioditis pelhamensis n. sp. (Nematoda: Rhabditidae) and Pellioditis pellio (Schneider, 1866), earthworm associates from different subclades within Pellioditis (syn. Phasmarhabditis Andrássy, 1976)

Recently, much attention has been focused on a group of rhabditid nematodes called Phasmarhabditis, a junior synonym of Pellioditis, as a promising source of biocontrol agents for invasive slugs. Pellioditis pelhamensis n. sp. was first isolated from earthworms near Pelham Bay Park in Bronx, New York, USA, in 1990 and has been found to be pathogenic to slugs as well as some earthworms. It has also been used in several comparative developmental studies. Here, we provide a description of this species, as well as a redescription of a similar earthworm-associated nematode, Pellioditis pellio Schneider, 1866, re-isolated from the type locality. Although P. pelhamensis n. sp. and P. pellio are morphologically similar, they are reproductively isolated. Molecular phylogenetic analysis places both species in a clade that includes all species previously described as Phasmarhabditis which are associated with gastropods. Phasmarhabditis Andrássy, 1976 is therefore a junior synonym of Pellioditis Dougherty, 1953. Also, Pellioditis bohemica Nermut', Půža, Mekete & Mráček, 2017, described to be a facultative parasite of slugs, is found to be a junior synonym of Pellioditis pellio (Schneider, 1866), adding to evidence that P. pellio is associated with both slugs and earthworms. The earthworm-associated species P. pelhamensis n. sp. and P. pellio represent different subclades within Pellioditis, suggesting that Pellioditis species in general have a broader host range than just slugs. Because of this, caution is warranted in using these species as biological control agents until more is understood about their ecology.

Avoidance and attraction behaviour of slugs exposed to parasitic nematodes

Avoidance of pathogens and parasites is the first line of defense to survive. Several slug species avoid the parasitic nematode Phasmarhabditis hermaphrodita to reduce infection however, there is nothing known about whether slugs avoid other members of the Phasmarhabditis genus. I exposed two slug species (Deroceras invadens and Limax maculatus) to Phasmarhabditis californica and P. neopapillosa. D. invadens avoided P. californica but was strangely attracted to P. neopapillosa. L. maculatus did not avoid P. californica, but on day 1 and 3 significantly more slugs were found with P. neopapillosa. Reasons for host attraction to P. neopapillosa are discussed.

Howe D, R. Denver D. First Report of the Gastropod-Killing Nematode, Phasmarhabditis californica, in Washington State, U.S.A

Phasmarhabditis californica, a commercially available biological control agent in England, Scotland, and Wales (Nemaslug 2.0 ®), was discovered for the first time in Washington State during 2022. Nematodes were recovered from the invasive gastropods Arion hortensis, Deroceras reticulatum, and Oxychilus sp. in garden centers in both Vancouver and Spokane. The 18S ribosomal RNA gene was used to identify the nematodes. This discovery builds upon previous reports of P. californica in California and Oregon and suggests that the species is widespread throughout the west coast of the U.S. Future research directions with P. californica are proposed.

The Native Microbial Community of Gastropod-Associated Phasmarhabditis Species Across Central and Southern California

Nematodes in the genus Phasmarhabditis can infect and kill slugs and snails, which are important agricultural pests. This useful trait has been commercialized by the corporation BASF after they mass produced a product labeled Nemaslug®. The product contains Phasmarhabditis hermaphrodita, which has been cultured with Moraxella osloensis, a bacterial strain that was originally thought to be responsible for causing mortality in slugs and snails. The exact mechanism leading to death in a Phasmarhabditis infected host is unknown but may involve contributions from nematode-associated bacteria. The naturally occurring microbial community of Phasmarhabditis is unexplored; the previous Phasmarhabditis microbial community studies have focused on laboratory grown or commercially reared nematodes, and in order to obtain a deeper understanding of the parasite and its host interactions, it is crucial to characterize the natural microbial communities associated with this organism in the wild. We sampled Phasmarhabditis californica, Phasmarhabditis hermaphrodita, and Phasmarhabditis papillosa directly from their habitats in Central and Southern California nurseries and garden centers and identified their native microbial community via 16S amplicon sequencing. We found that the Phasmarhabditis microbial community was influenced by species, location, and possibly gastropod host from which the nematode was collected. The predominant bacteria of the Phasmarhabditis isolates collected included Shewanella, Clostridium perfringens, Aeromonadaceae, Pseudomonadaceae, and Acinetobacter. Phasmarhabditis papillosa isolates exhibited an enrichment with species belonging to Acinetobacter or Pseudomonadaceae. However, further research must be performed to determine if this is due to the location of isolate collection or a species specific microbial community pattern. More work on the natural microbial community of Phasmarhabditis is needed to determine the role of bacteria in nematode virulence.

Distribution of Phasmarhabditis (Nematode: Rhabditidae) and Their Gastropod Hosts in California Plant Nurseries and Garden Centers

Three species of Phasmarhabditis were recovered from 75 nurseries and garden centers in 28 counties in California during fall and winter 2012-2021. A total of 18 mollusk species were recovered, most of them invasive. Nematodes were identified by sequencing the D2-D3 expansion segments of the large subunit (LSU or 28S) rRNA. Based on these surveys, P. californica was the most widespread species (37 isolates, 53.6% recovery); followed by P. hermaphrodita (26 isolates; 37.7% recovery); P. papillosa and a closely related P. papillosa isolate (6 isolates; 8.7% recovery). Nematode isolates were mainly collected from four invasive slugs (Deroceras reticulatum, D. laeve, Arion hortensis agg, Ambigolimax valentianus) and snails (Oxychilus spp. and Discus spp.). Results suggest that P. californica and P. hermaphrodita share an ecological niche in Northern, Central, Coastal, and Southern California, north of Los Angeles County.

Microbiome Analysis of Malacopathogenic Nematodes Suggests No Evidence of a Single Bacterial Symbiont Responsible for Gastropod Mortality

Nematodes and bacteria are prevalent in soil ecosystems, and some have evolved symbiotic relationships. In some cases, symbionts carry out highly specialized functions: a prime example being entomopathogenic nematodes (EPNs), which vector bacteria (Xenorhabdus or Photorhabdus) into insect hosts, killing them to provide a food source for the nematodes. It is thought that the commercially available malacopathogenic (kills slugs and snails) biocontrol nematode Phasmarhabditis hermaphrodita vectors a bacterium (Moraxella osloensis) into slugs to kill them. To investigate this further we used a metagenomic approach to profile the bacteria present in the commercial strain of P. hermaphrodita, a wild strain of P. hermaphrodita and two other Phasmarhabditis species (P. californica and P. neopapillosa), after they had killed their slug host (Deroceras invadens). We show that these nematodes do not exclusively associate with one bacterium but a range of species, with members of the phyla Pseudomonadota, Bacillota, Actinobacteriota and Bacteroidota the most prevalent. The commercial strain of P. hermaphrodita had the least diverse bacterial community. Furthermore, we found that the bacterium P. hermaphrodita has been cultured on for 25 years is not the expected species M. osloensis but is Psychrobacter spp. and the only strain of the Phasmarhabditis species to associate with Psychrobacter spp. was the commercial strain of P. hermaphrodita. In summary, we found no evidence to show that P. hermaphrodita rely exclusively on one bacterium to cause host mortality but found variable and diverse bacterial communities associated with these nematodes in their slug hosts.

Terrestrial Slugs in Neotropical Agroecosystems

Slugs can be important agricultural pests in tropical regions. They are also intermediate hosts of parasitic nematodes, such as Angiostrongylus costaricensis and A. cantonensis, which can cause abdominal and cerebral angiostrongyliasis in humans. Management of slugs in conventional agriculture has relied heavily in the use of pellets containing metaldehyde. In this article, we review cases of slug problems and their management in neotropical agroecosystems.

Biological Control of Pest Non-Marine Molluscs: A Pacific Perspective on Risks to Non-Target Organisms

Simple Summary

As malacologists long concerned with conservation of molluscs, we present empirical evidence supporting the proposition that biological control of nonmarine mollusc pests has generally not been demonstrated to be safe and effective, which are the basic measures of success. Yet claims of success often accompany contemporary biological control programs, although without rigorous evaluations. Failed molluscan biocontrol programs include well known classical control efforts that continue to devastate native biodiversity, especially on Pacific islands, as well as more contemporary programs that claim to be safer, with minimal non-target impacts. We do not condemn all biological control programs as ineffective, unsafe, and poorly evaluated, but emphasize the need for programs targeting non-marine molluscs to incorporate the lessons learned from past failures, and to do a better job of defining and measuring success both pre- and post-release of biocontrol agents. Most importantly, we call for the biocontrol community not to rely on entomologists with backgrounds in use of host-specific agents, who yet promote generalist parasites/predators for mollusc control, but to engage more actively with those knowledgeable in molluscan biology, particularly conservation. In doing so, maybe some programs targeting molluscan pests can become safe and effective.

Abstract

Classic biological control of pest non-marine molluscs has a long history of disastrous outcomes, and despite claims to the contrary, few advances have been made to ensure that contemporary biocontrol efforts targeting molluscs are safe and effective. For more than half a century, malacologists have warned of the dangers in applying practices developed in the field of insect biological control, where biocontrol agents are often highly host-specific, to the use of generalist predators and parasites against non-marine mollusc pests. Unfortunately, many of the lessons that should have been learned from these failed biocontrol programs have not been rigorously applied to contemporary efforts. Here, we briefly review the failures of past non-marine mollusc biocontrol efforts in the Pacific islands and their adverse environmental impacts that continue to reverberate across ecosystems. We highlight the fact that none of these past programs has ever been demonstrated to be effective against targeted species, and at least two (the snails Euglandina spp. and the flatworm Platydemus manokwari) are implicated in the extinction of hundreds of snail species endemic to Pacific islands. We also highlight other recent efforts, including the proposed use of sarcophagid flies and nematodes in the genus Phasmarhabditis, that clearly illustrate the false claims that past bad practices are not being repeated. We are not making the claim that biocontrol programs can never be safe and effective. Instead, we hope that in highlighting the need for robust controls, clear and measurable definitions of success, and a broader understanding of ecosystem level interactions within a rigorous scientific framework are all necessary before claims of success can be made by biocontrol advocates. Without such amendments to contemporary biocontrol programs, it will be impossible to avoid repeating the failures of non-marine mollusc biocontrol programs to date.

A Literature Review of Biological and Bio-Rational Control Strategies for Slugs: Current Research and Future Prospects

Terrestrial gastropod molluscs (slugs and snails) (Mollusca: Gastropoda) cause significant crop damage around the world. There is no formal approach for differentiating between slugs and snails; however, an organism is usually considered a slug when there is no external shell, or when the shell is small in comparison to the body, and a snail when there is a large external shell. Although snails are an important pest of many crops, this review focuses on slug pests and their nonchemical control measures. A recent study by the UK Agriculture and Horticulture Development Board concluded that the failure to control slugs could cost the UK agriculture industry over GBP 100 million annually, with similar figures reported around the world. Whilst slugs are mostly controlled using chemical molluscicide products, some actives have come under scrutiny due to their detrimental environmental effects and impact on nontarget organisms. This has resulted in the ban of actives such as methiocarb in the UK and EU, and, more recently, the ban of metaldehyde in the UK. Therefore, there is an urgent need to find alternative and effective nontoxic solutions in the interest of global food security. In this paper, we have integrated extant literature on the three main biological control agents of slugs, namely nematodes, carabid beetles and sciomyzid flies, and various promising bio-rational slug control strategies. The review also highlights current research gaps and indicates some relevant potential future directions towards developing environmentally benign slug control solutions.