Survey, host preference and infectivity of Phasmarhabditis californica (Family: Rhabditidae) on pest slugs
Creators: Patuwatha Withanage, Dayani B. M.
Publisher: name: University of Alberta Library
Year: 2023

<p>Certain slug species are considered agricultural and horticultural pests worldwide. Nematodes offer a potential solution as biocontrol agents in controlling slug populations due to their natural associations with terrestrial gastropods. In some cases, they provide higher specificity and more efficient pest management outputs than many chemical or physical practices currently available. One of the most well-known biocontrol agents of slugs is a facultative parasite, Phasmarhabditis hermaphrodita, which has been widely established as a biocontrol agent after it was patented and commercialized as a molluscicide product (Nemaslug ® with the associated bacteria symbiont, Moraxella osloensis) in 1994 in the UK. However, Canada had no previous record of any Phasmarhabditis species until a recent discovery of a Canadian strain of P. californica collected from a local nursery in Edmonton, Alberta. This species was originally isolated from California and subsequently marketed by BASF as a biocontrol agent (Nemaslug® 2.0) against slugs in England, Scotland, and Wales in 2022. However, the immediate use of this species as a biocontrol agent is currently not available in Canada until a proper risk assessment of the biocontrol product Nemaslug 2.0 with the active organisms P. californica (with the bacteria symbiont, Moraxella osloensis) is made and its biology fully understood. First, I conducted an extensive survey to identify the diversity, distribution, and abundance of pest slug species and their associated nematodes in selected agricultural and horticultural sites in Alberta. I further investigated if any Phasmarhabditis species were present in the survey sites. I collected 1331 slugs belonging to nine species, with Deroceras reticulatum being the most common. Forty-five samples (3.38%) were positive for nematodes, the majority were identified to species level: Alloionema appendiculatum, Caenorhabditis briggsae, Caenorhabditis elegans, Panagrolaimus subelongatus, and Mesorhabditis spiculigera. I did not isolate P. californica from any of the slugs collected from these survey sites, which included the original site where P. californica was discovered. However, four D. reticulatum slugs retrieved from a residential garden sample were infected with P. californica, thereby suggesting a possible fragmented distribution for this strain in the province. I then used an agar-based chemotaxis assay to evaluate the host preference of the laboratory-cultured Canadian strain of P. californica against four pest slug species, D. reticulatum, A. rufus, A. fasciatus, and A. valentianus. I showed that P. californica was strongly attracted to mucus of all slug species except for D. reticulatum for which I observed a weak attraction. In addition, I checked the host preference of a co-occurring nematode, Pristionchus entomophagus, a necromenic nematode on the same host species to check if they would have a similar host preference as P. californica. P. entomophagus showed a significant attraction to the mucus of D. reticulatum while being strongly repulsive to A. rufus. Given that these two nematode species have potential similarities in chemoattraction profiles towards D. reticulatum, I then investigated the efficacy of the infectivity of P. californica as a biocontrol agent in the presence of P. entomophagus. The ability to cause mortality in slugs infected by P. californica was the same in single and mixed infections, i.e., mortality rates remained the same despite its co-occurrence with P. entomophagus. Both in single and mixed infection treatments, the number of P. californica that entered the slug host also remained comparable and statistically non-significant. However, the number of progeny (F1) in mixed treatments was lower than that of the single treatments for P. californica. Interestingly, P. entomophagus was not affected by concomitant infection with P. californica. These discoveries on the local strains of Phasmarhabditis support the possibility of using P. californica as a biological control agent within Canada. Still, further investigation is needed on the persistence and efficacy of P. californica in the presence of other nematode species in the soil community.</p>