In vitro production and biocontrol potential of nematodes associated with molluscs

Thesis (PhD)--Stellenbosch University, 2020.

ENGLISH ABSTRACT: Growing concern around the risks associated with the use of some molluscicide chemicals has prompted research into alternative control methods for mollusc pests. One of the most successful biocontrol methods available in Europe is the use of the mollusc-parasitic nematode, Phasmarhabditis hermaphrodita, which cannot be used or sold in South Africa under the terms of the Agricultural Pest Amendment Act, No. 18 of 1989. It is, therefore, necessary to isolate local mollusc-parasitic nematodes and to assess their biocontrol potential. Nematodes associated with slugs from the KwaZulu-Natal province of South Africa was surveyed. Caenorhabditis elegans was the only nematode isolated, and pathogenicity tests concluded that it was not capable of causing mortality to the slug species Deroceras invadens, even when associated with a pathogenic bacterium. During this survey, a number of endemic slug species were collected, including Chlamydephorus gibbonsi. A detailed description of C. gibbonsi’s feeding process is given. Following the quest to isolate other mollusc-nematodes across Africa, a collaborative project led to the isolation and description of Phasmarhabditis sp. (KEN1), a new nematode species from Kenya. Phasmarhabditis sp. (KEN1) can be characterised by the females having a conoid tail shape, the presence of males with a bursa bearing nine bilateral pairs of genital bursal papillae, as well as the infective juvenile (IJ) having the longest body length thus far recorded in the genus. The life cycles of five nematodes associated with molluscs were then studied and described. Based on the ease of mass-culturing, the species were ranked in the following order: C. elegans, P. hermaphrodita and Phasmarhabditis bohemica, with Phasamrhabditis papillosa and Phasmarhabditis sp. (KEN1) being equally difficult to massculture. The in vitro culturing of the nematodes P. papillosa, C. elegans, Phasmarhabditis sp. (KEN1) and P. bohemica was then studied using seven bacterial isolates from slugs and three associated with entomopathogenic nematodes. The results indicated that Kluyvera sp. would be a suitable bacterial candidate to support the growth of C. elegans and Phasmarhabditis sp. (KEN1), while Pseudomonas spp. would make suitable bacterial candidates for P. bohemica and P. papillosa. The nematode P. bohemica was then grown in association with the bacterial isolate Pseudomonas sp. (1), in liquid media cultures to test the effects of bacterial inoculum density and initial IJ inoculum density on the total nematode yield, the IJ yield and the IJ proportion in the cultures. The results showed that 1% bacteria inoculum and a higher initial inoculum concentration of 3000 IJs/ml led to higher total nematode and IJ yields. Lastly, a novel application method of metaldehyde was tested in an apple orchard on the snail Cornu aspersum. Baitchain, a new application method whereby bait pellets can be wrapped around the base of a tree was tested along with Sluggit, a metaldehyde bait pellet applied to the soil, to compare their effectiveness in protecting apple trees from C. aspersum. All treatments caused a significant reduction in snail numbers after 28 days, and all treatments, except Sluggit applied at a concentration of 15 g/kg metaldehyde, caused significant mortality.

AFRIKAANSE OPSOMMING: Groeiende kommer oor die risiko geassosieer met die gebruik van sommige chemiese slakdoders, het gelei tot soeke na alternatiewe beheermetodes vir slakspesies. Een van die mees suksesvolle biologiese beheer opsies is die gebruik van die slak-parasitiese nematode, Phasmarhabditis hermaphrodita, wat nie gebruik of verkoop kan word in Suid-Afrika nie. Dit is daarom belangrik dat plaaslike nematodes gevind word en hul biologiese beheer potensiaal ondersoek word. ʼn Opname was gedoen van nematodes geassosieer met slakke van die KwaZulu-Natal provinsie van Suid-Afrika. Caenorhabditis elegans was die enigste nematode spesie wat geïsoleer is en toetse het bevestig dat dit nie mortaliteit kan veroorsaak in die slak spesie Deroceras invadens nie. Een van die slak spesies wat versamel was gedurende die opname was Chlamydephorus gibbonsi. Die slak is endemies aan oostelike Suid-Afrika, maar opnames het getoon dat die spesie besig is om na die Wes-Kaap te versprei. ʼn Beskrywing van die proses waarin die slak voed op ʼn Amynthas spesie erdwurm was volledig beskryf. Phasmarhabditis sp. (KEN1), ʼn nuwe nematode spesie van Kenia, was ook beskryf en word uitgeken aan die keëlagtige stert van die wyfie, teenwoordige mannetjies met ʼn bursa met nege bilaterale, gepaarde genitale bursale papille, asook ʼn infektiewe larwe (IL) met die langste liggaamslengte tot dusver gevind in die genus. Die lewenssiklusse van vyf nematode spesies geassosieer met slakke was toe bestudeer en beskryf. Gebasseer op die resultate, kan die spesies gelys word van maklik tot moeilik om te massa-produseer soos volg: C. elegans, P. hermaphrodita en P. bohemica, met P. papillosa en Phasmarhabditis sp. (KEN1) wat ewe moeilik sou wees om te massa-produseer as gevolg van hul stadige lewenssiklus en hul voortplantingsmetode. Die in vitro produksie van die nematodes P. papillosa, C. elegans, Phasmarhabditis sp. (KEN1) en P. bohemica was ondersoek. Die resultate het getoon dat Kluyvera sp. ʼn gepaste bakteriële kandidaat sou wees vir C. elegans en Phasmarhabditis sp. (KEN1), terwyl Pseudomonas spesies gepaste bakteriële kandidate sou wees vir P. bohemica en P. papillosa. Die nematode P. bohemica was vermeerder in vloeistof medium in assosiasie met die bakterieë Pseudomonas sp. (1), om die effek van bakteriële en die invektiewe larwe (IL) inokulum digtheid te toets op die totale nematode opbrengs, IL opbrengs en IL proporsie van die totale nematodes. Die resultate het getoon dat 1 % bakteriële inokulum en ʼn hoër inokulum konsentrasie van 3000 ILs/ml gelei het tot hoër totale nematode en IL opbrengste. Laastens was ʼn nuwe metode vir die aanwending van metaldehied in ʼn appelboord getoets. Baitchain, ʼn produk wat bestaan uit slakpille gerangskik op ʼn koord, is ontwerp om om die stam van ʼn boom gebind te word. Die produk was getoets tesame met Sluggit, ʼn metaldehied slakpil wat op die grondoppervlak aangewend word, om te toets hoe effektief die behandelinge is daarin om die appelbome te beskerm teen die dopslak, Cornu aspersum. Alle behandelinge het ʼn beduidende afname veroorsaak in slak getalle na 28 dae en alle behandelinge, behalwe Sluggit teen ʼn konsentrasie van 15 g/kg metaldehied, het beduidende slakdood veroorsaak.

Doctoral