The Role of Terrestrial Mollusks in Phoresis and Vectoring of Plant-Parasites

The Brown Garden Snail, Helix aspersa, is a terrestrial mollusk that was introduced to California in the 1850s, and many other parts of the world as food (escargot), through the movement of plants, and by hobbyists who collect snails. Today, H. aspersa is a pest that can be found almost anywhere within California. This research determined the nematodes, fungi, and bacteria associated with H. aspersa collected in three locations in San Diego, California, and assessed where on or in the snail body the microbes were carried. All the microbes recovered from individual snails were identified as operational taxonomic units (OTUs) through comparison of their DNA sequence homology with available data. Twenty-three nematode operational taxonomic units (OTUs) were recovered from the snails. Bacterivorous nematodes recovered included Caenorhabitis elegans, Rhabditis sp., and Panagrolaimus species. Plant parasitic nematode OTUs included Diytlenchus dipsaci and Pratylenchus species. The bacterial and fungal OTUs isolated included Pseudomonas putida, and Stentrophomonas maltophilia, and fungal plant pathogens Fusarium solani, F. oxysporum f. sp. chrysanthemi, Rhizoctonia solani. Laboratory and growth chamber experiments were conducted to determine whether plant-pathogens inoculum is potentially disseminated and transmitted by H. aspersa. In laboratory experiments, individual snails were fed inoculum of Fusarium circinatum and Rhizoctonia solani as preserved mycelia on filter paper, with control snails being fed sterile filter paper. After H. aspersa consumed the filter paper with pathogen inoculum, viable fungus was recovered from 90% of fecal pellets for up to 9 days post feeding, while no pathogens were detected control snail fecal pellets. Vermiform, active plant-parasitic nematodes of Meloidogyne hapla, Diytlenchus dipsaci, and Aphelenchoides ritzemabosi were fed to snails as an aqueous nematode suspension placed on the surface of a single carrot disc (1.5 g). Viable vermiform nematodes of all the tested species were recovered from the fecal pellets of the snails post feeding. The nematodes passed through the snail digestive tract and recovered from fecal pellets over a time period of 2-8 days post feeding. In growth chamber experiments, active second-stage juveniles (J2) of root-knot nematodes M. hapla and M. incognita were fed to snails as an aqueous nematode suspension (a total of 1000 J2s) placed on the surface of a single carrot disc (1.5 g). Control snails received carrots without nematode inoculum. Subsequent to feeding H. aspersa the J2s, fecal pellets were recovered and placed at the base of individual tomato seedlings growing in a growth chamber. After two months, the tomato plants were destructively sampled and assessed for root-knot nematode infection. Control plants were not infected, but the plants exposed to fecal pellets from snails fed the root-knot nematode J2s were infected, showing symptoms of galled roots and erioglaucine-stained egg masses visible on the roots. A diverse range of nematode, fungal, and bacterial OTUs were recovered from field-collected H. aspersa in California. Many of the recovered OTUs were plant pathogens, and inoculum propagules of fungal and nematode plant pathogens can transit the H. aspersa digestive system in viable condition. Second-stage juveniles of the root-knot nematodes M. hapla and M. incognita survive transit of the H. aspersa digestive system, and are infective; being able to emerge from fecal pellets, move into soil, find host roots, and establish successful infections and reproduce. Plant pathogens are not infrequent associates of H. aspersa, and can potentially aid in pathogen dispersal and transmission. This new information should prompt further consideration of management and monitoring of mollusk pests in California agriculture.