Nematodes that associate with terrestrial molluscs as definitive hosts, including Phasmarhabditis hermaphrodita (Rhabditida: Rhabditidae) and its development as a biological molluscicide

The use of Phasmarhabditis nematodes and metabolites of Xenorhabdus bacteria in slug control

Many species of slugs are considered serious pests in agriculture and horticulture around the world. In Europe, slugs of the genera Arion and Deroceras are the most harmful pests in agriculture. Therefore, the main goal of this study was to evaluate the effect of the whole-cell metabolites of 10 strains of five Xenorhabdus and three slug-parasitic nematodes (Phasmarhabditis hermaphrodita, Phasmarhabditis bohemica, and Phasmarhabditis apuliae) on the feeding behaviour and repellent effect on target slugs and evaluate a new possible means of biocontrol of these pests. The repellent and anti-feedant effects of nematode-killed insects, metabolites, slug-parasitic nematodes and a combination of metabolites and nematodes were studied through experimental designs: sand-filled plastic boxes divided into two parts in several modifications: with dead Galleria mellonella killed by nematodes, lettuce treated with bacterial metabolites and lettuce placed on the treated sand. We found that slugs avoid eating G. mellonella killed by nematodes, while they eat freeze-killed G. mellonella. Similarly, they avoid the consumption of lettuce in areas treated with bacterial metabolites (the most effective strains being Xenorhabus bovienii NFUST, Xenorhabdus kozodoii SLOV and JEGOR) with zero feeding in the treated side. All three Phasmarhabditis species also provided a significant anti-feedant/repellent effect. Our study is the first to show the repellent and anti-feedant effects of metabolites of Xenorhabdus bacteria against Arion vulgaris, and the results suggest that these substances have great potential for biocontrol. Our study is also the first to demonstrate the repellent effect of P. apuliae and P. bohemica. KEY POINTS: • Slugs avoid eating G. mellonella killed by entomopathogenic nematodes. • Bacterial metabolites have a strong repellent and antifeedant effect on slugs. • Presence of slug parasitic nematodes increases the repellent effect of metabolites.

Immune-Related Gene Profiles and Differential Expression in the Grey Garden Slug Deroceras reticulatum Infected with the Parasitic Nematode Phasmarhabditis hermaphrodita

The grey garden slug (Deroceras reticulatum), a common terrestrial slug native to Europe with a global distribution including North America, is commonly considered the most severe slug pest in agriculture. The nematode Phasmarhabditis hermaphrodita, which has been used in the U.K. and Europe as a commercial biocontrol agent since 1994, has also recently been collected in Oregon and California and has long been considered a candidate biocontrol agent for slug management in the U.S. In this study, we report differential gene expressions in nematode-infected slugs using RNA-seq to identify slug immune-related genes against nematodes. Comparison of gene expression levels between the whole bodies of a nematode-infected slug (N-S) and an uninfected control slug (C-S) revealed that there were a total of 39,380 regulated unigenes, of which 3084 (3%) were upregulated and 6761 (6%) were downregulated at greater than 2-fold change (FC > 2) in the nematode-infected slug. To further investigate the biological functions of differentially expressed genes (DEGs), gene ontology (GO) and functional enrichment analysis were performed to map the DEGs to terms in the GO, eukaryotic ortholog groups of proteins (KOG) and Kyoto Encyclopedia of Genes and Genome Pathway (KEGG) databases. Among these DEGs, approximately 228 genes associated with immunity or immune-related pathways were upregulated 2-fold or more in the N-S compared to C-S. These genes include toll, Imd, JNK, scavenger receptors (SCRs), C-type lectins (CTLs), immunoglobulin-like domains, and JAK/STAT63 signaling pathways. From the RNA-seq results, we selected 18 genes and confirmed their expression levels by qRT-PCR. Our findings provide insights into the immune response of slugs during nematode infection. These studies provide fundamental information that will be valuable for the development of new methods of pest slug control using pathogenic nematodes in the field.

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.

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.

Invasive Alien Slug Arion vulgaris Moquin-Tandon, 1855 (Gastropoda: Pulmonata: Arionidae) in Moscow Parks and Its Co-introduced Parasite Alloionema appendiculatum Schneider, 1859

PURPOSE

The present study investigates the origin of Arion vulgaris slugs in the parks of Moscow city and their parasites.

METHODS

Snails and slugs inhabiting green areas of Moscow city were collected in the summer season of 2020 and examined on the presence of gastropod-associated nematodes and trematodes using morphological and molecular methods.

RESULTS

The presence of the alien slug species, Arion vulgaris, was recorded in several locations, and the mitochondrial gene-based analysis has shown that slug populations inhabited Moscow parks originated from West and Central Europe. Out of a total of 15 gastropod species examined, A. vulgaris was the only species infected by the nematode Alloionema appendiculatum Schneider, 1859, a larval parasite of molluscs. It is the first record of this nematode from the territory of the Russian Federation. COX1 mtDNA sequences of A. appendiculatum obtained from 3 populations of infected slugs were identical with those from Western and Central Europe similarly to their gastropod hosts thus indicating that the nematodes travelled with their hosts. No parasites dangerous for humans or animals were found.

CONCLUSION

The complex life cycle of A. appendiculatum includes a free-living stage in soil which offers a source of infection for other potentially susceptible gastropod species but the capacity of A. appendiculatum to change hosts in local conditions needs to be further investigated. The particular susceptibility and tolerance of A. vulgaris to nematodes in our study was in concordance with earlier data while in contradiction with the enemy release hypothesis.

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.

Ex vivo development of Phasmarhabditis spp. associated with terrestrial molluscs

The success of Phasmarhabditis hermaphrodita (Schneider) Andrássy (Rhabditida: Rhabditidae) as a biological control agent of molluscs has led to a worldwide interest in phasmarhabditids. However, scant information is available on the lifecycle development of species within the genus. In the current study, the development of P. hermaphrodita, Phasmarhabditis papillosa, Phasmarhabditis bohemica and Phasmarhabditis kenyaensis were studied using ex vivo cultures, in order to improve our understanding of their biology. Infective juveniles (IJs) of each species were added to 1 g of defrosted homogenized slug cadavers of Deroceras invadens and the development monitored after inoculated IJ recovery, over a period of eight-ten days. The results demonstrated that P. bohemica had the shortest development cycle and that it was able to produce first-generation IJs after eight days, while P. hermaphrodita, P. papillosa and P. kenyaensis took ten days to form a new cohort of IJs. However, from the perspective of mass rearing, P. hermaphrodita has an advantage over the other species in that it is capable of forming self-fertilizing hermaphrodites, whereas both males and females are required for the reproduction of P. papillosa, P. bohemica and P. kenyaensis. The results of the study contribute to the knowledge of the biology of the genus and will help to establish the in vitro liquid cultures of different species of the genus.

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.

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

Simple Summary

Terrestrial molluscs (slugs and snails) pose a major threat to agriculture, causing severe yield losses in a wide range of crops worldwide. The limited number of chemical molluscicides on the market, along with their negative impact on nontarget organisms and the environment, make mollusc control a real concern for growers and farmers. Therefore, the exploration of alternative, effective and eco-friendly control measures has become a dire need. This study focuses on slugs, as opposed to snails, and reviews the literature on three natural enemies of slugs, namely nematodes, carabid beetles and marsh flies, along with various natural products with slug control potential (for example, essential oils), and this study contributes to providing a comprehensive understanding of how slugs can be better controlled by using nonchemical measures. In doing so, this study also draws attention to the limitations of current research and discusses some important future research avenues in order to develop effective nonchemical slug control measures.

Abstract

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.

3D morphology of nematode encapsulation in snail shells, revealed by micro-CT imaging

Many parasites and hosts are embroiled in an on-going arms race that affects the evolution of each participant. One such battle is between parasitic nematodes and terrestrial gastropods which have co-evolved for 90–130 MY. Recently, snails have been shown to encase and kill invading nematodes using their shell as a defence mechanism. However, there is remarkably little known about this process in terms of understanding where, when and how nematodes are fixed within the shell. Also there has never been any attempt to observe this process using methods other than light microscopy. Therefore, we used micro CT scanning of a Cepaea nemoralis shell (a common host for nematodes) to 3D visualise encased nematode parasites and quantify morphological parameters. By taking this approach future studies could use micro CT scanning of fossil shells in conchology collections to understand nematode/snail co-evolution.

Nematodes associated with terrestrial slugs in the Edmonton region of Alberta, Canada

A survey of nematodes associated with terrestrial slugs was conducted in residential gardens, nurseries, greenhouses and agricultural sites located in and around Edmonton, Alberta, Canada. A total of 2406 slugs were collected from 82 sites. Slugs were decapitated and cadavers were incubated for two weeks, with emerging nematodes removed and processed for identification. Nematodes were identified using molecular sequence data for the 18S ribosomal DNA. Nematodes were recovered from 20 of the 82 sites surveyed, with 24.4% of the slugs infected with nematodes. A total of seven nematodes were identified to species level, including Caenorhabditis elegans, Panagrolaimus papillosus, Pellioditis typica, Pelodera pseudoteres, Rhabditella axei, Rhabditoides inermiformis and Phasmarhabditis californica. An additional four specimens were identified to genus level, including Oscheius sp. (9), Pristionchus sp., Rhabditis sp. and Rhabditophanes sp. (1). The two most common nematode species were C. elegans and P. pseudoteres. The facultative parasite, P. californica, was recovered from a single Arion rufus specimen, collected from a seasonal nursery. To our knowledge, this study represents the first survey of slug-associated nematodes in Canada.

Pathogenicity of wild and commercial Phasmarhabditis hermaphrodita exposed to the pestiferous slug Deroceras invadens

Many terrestrial gastropods are pestiferous and pose a significant threat to agriculture, horticulture and floriculture. They are usually controlled by metaldehyde based pellets but an alternative control method is the slug parasitic nematode Phasmarhabditis hermaphrodita, which has been formulated into a biological control agent (Nemaslug®) for use by farmers and gardeners to kill certain pestiferous slug species in 4-21 days. The current strain of P. hermaphrodita (called DMG0001) has been used in commercial production since 1994, but there is little information about the pathogenicity of wild strains of P. hermaphrodita towards slugs. Here, we exposed the pestiferous slug Deroceras invadens to nine wild isolated strains of P. hermaphrodita (DMG0002, DMG0003, DMG0005, DMG0006, DMG0007, DMG0008, DMG0009, DMG0010 and DMG0011) and the commercial strain (DMG0001) to three doses (0, 500 and 1000 nematodes per ml). Survival and feeding were recorded over 14 days. All wild P. hermaphrodita strains (other than DMG0010) and P. hermaphrodita (DMG0001), applied at 500 nematodes per ml, caused significant mortality to D. invadens compared to an uninfected control. Similarly, all P. hermaphrodita strains applied at 1000 nematodes per ml, caused significant mortality to D. invadens compared to an uninfected control. Overall, all wild P. hermaphrodita strains (other than DMG0011) caused significantly more mortality than P. hermaphrodita DMG0001 at one or both nematode concentrations. In summary, we have found some wild P. hermaphrodita strains were more virulent than P. hermaphrodita (DMG0001). Ultimately, these strains could potentially be developed as alternative, efficient biological control agents for use against slugs.

Angiostoma norvegicum n. sp. (Nematoda: Angiostomatidae) a parasite of arionid slugs in Norway

Angiostoma norvegicum n. sp. (Angiostomatidae) is described from the oesophagus, crop and the buccal mass of five species of slugs of the family Arionidae, Arion vulgaris (Moquin-Tandon), Arion ater (L.), Arion fasciatus (Nilsson), Arion fuscus (Müller) and Arion rufus/Arion ater hybrid), collected throughout Norway. Angiostoma norvegicum n. sp. was found parasitising arionids at seven of the 30 sample sites examined (23.3%), and 9.9% of all Arion spp. were infected with this nematode. The new species is characterised by its large size (4.0–8.6 mm long) and in having: lateral alae; 6 + 6 papillae at the cephalic end; a large circular mouth aperture; a spacious stoma; a pharyngeal basal bulb without valvular apparatus; an excretory pore near the base of bulb; a distal part of posterior ovary always outstretched; an anterior ovary distally nearly always outstretched; a vulva situated anterior to mid-body; long, nearly straight spicules and a small gubernaculum; three circumcloacal papillae and caudal genital papillae (GP) arranged in a pattern 1+2/3+3 with GP 5 and GP 8 opened on dorsal side of narrow bursa not reaching tail tip; short conical tails in both sexes with tips supplied by 4 short, unequal denticles. Morphologically, A. norvegicum n. sp. is similar to Angiostoma limacis Dujardin, 1845, which diagnostic characteristics are given based on examination of specimens from Norway and the UK. Conversely, the phylogenetic analyses based on D2D3 large subunit (LSU) rRNA gene sequences performed in the present study did not support the morphological affinity of these two species. Phylogenetic analyses demonstrated that although Angiostoma spp. cluster together, A. norvegicum n. sp. forms a tight monophyletic clade with the milacid nematode parasites Angiostoma margaretae Ross, Malan & Ivanova, 2011 and Angiostoma milacis Ivanova & Wilson, 2009.