Entomopathogenic potential of bacteria associated with soil-borne nematodes and insect immune responses to their infection.

Soil-borne nematodes establish close associations with several bacterial species. Whether they confer benefits to their hosts has been investigated in only a few nematode-bacteria systems. Their ecological function, therefore, remains poorly understood. In this study, we isolated several bacterial species from rhabditid nematodes, molecularly identified them, evaluated their entomopathogenic potential on Galleria mellonella larvae, and measured immune responses of G. mellonella larvae to their infection. Bacteria were isolated from Acrobeloides sp., A. bodenheimeri, Heterorhabditis bacteriophora, Oscheius tipulae, and Pristionchus maupasi nematodes. They were identified as Acinetobacter sp., Alcaligenes sp., Bacillus cereus, Enterobacter sp., Kaistia sp., Lysinibacillus fusiformis, Morganella morganii subsp. morganii, Klebsiella quasipneumoniae subsp. quasipneumoniae, and Pseudomonas aeruginosa. All bacterial strains were found to be highly entomopathogenic as they killed at least 53.33% G. mellonella larvae within 72h post-infection, at a dose of 106 CFU/larvae. Among them, Lysinibacillus fusiformis, Enterobacter sp., Acinetobacter sp., and K. quasipneumoniae subsp. quasipneumoniae were the most entomopathogenic bacteria. Insects strongly responded to bacterial infection. However, their responses were apparently little effective to counteract bacterial infection. Our study, therefore, shows that bacteria associated with soil-borne nematodes have entomopathogenic capacities. From an applied perspective, our study motivates more research to determine the potential of these bacterial strains as biocontrol agents in environmentally friendly and sustainable agriculture.

Potential of Oscheius tipulae nematodes as biological control agents against Ceratitis capitata.

A survey to collect soil nematodes with potential to control Ceratitis capitata flies was carried out in different locations in Tunisia. Several nematode isolates were recovered, laboratory colonies were established, and their taxonomic identities were determined based on molecular methods. Among all the recovered nematode isolates, two of them, Oscheius tipulae TC2 and OC2, were evaluated for their capacity to control C. capitata flies and for their ability to kill and reproduce on Galleria mellonella larvae. Our results show a great potential of these two isolates as biocontrol agents as they kill C. capitata eggs and pupae and interfere with the metamorphosis of C. capitata larvae. More specifically, TC2 and OC2 nematodes killed 39 and 31% of C. capitata eggs, respectively, impaired the metamorphosis of up to 77% and up to 67% of C. capitata larvae, respectively, and killed up to 66% and up to 58% of C. capitata pupae, respectively. The efficacy of TC2 and OC2 nematodes was particularly high on C. capitata pupae, and significant insect mortalities were observed even at concentrations of 1 and 5 nematodes/pupae, respectively. We also found that TC2 and OC2 nematodes efficiently kill and reproduce in G. mellonella larvae, suggesting that these insects could be used for mass-multiplication of these nematodes. These results reveal the potential of O. tipulae to complement integrated pest management programs against C. capitata flies.

Parasites rather than phoronts: Teratorhabditis synpapillata nematodes reduce lifespan of their Rhynchophorus ferrugineus host in a life stage-dependent manner.

Rhynchophorus ferrugineus Olivier (Coleoptera: Curculionidae) red palm weevils are often reported in association with different organisms including nematodes. The significance of this interaction and whether nematodes can influence their life-history traits is unclear. We collected Rhynchophorus ferrugineus red palm weevils at different developmental stages and locations in Tunisia, observed and dissected them in search for nematodes and other interacting organisms, established laboratory colonies and identified the nematodes associated with them, and conducted nematode-insect interaction assays to determine the capacity of the nematodes to influence their life-history traits. We observed Beauveria bassiana fungi in larvae, nymph, and adults; Centrouropoda and Uroobovella acari associated with the adults, and Teratorhabditis synpapillata nematodes associated with larvae and adults. Nematode-insect interaction bioassays revealed that T. synpapillata nematodes reduce the lifespan of the insect larvae in a population-dependent manner, but do not influence the lifespan of adults. Our study uncovers an important factor that may determine population dynamics of this important palm pests and provides evidence to conclude that these organisms establish a parasitic relationship, rather than a phoretic relationship.