Influence of the ascarosides on the recovery, yield and dispersal of entomopathogenic nematodes

Connecting academia and industry: Advancing the use of entomopathogenic nematodes to tackle emerging challenges and opportunities in modern agriculture

The collaboration among academia, industry, and government is crucial for scientific progress and innovation. Academia generates fundamental knowledge, which industry translates into sound applications, considering government policies. This partnership is vital to feed progress and constant development and address global challenges like climate change and food security. Sustainable crop protection is a topical theme, with efforts to reduce pesticide reliance and promote alternatives to chemical pest management, and it continues to grow and be accepted worldwide. In this respect, biopesticides such as entomopathogenic nematodes (EPNs) offer a promising solution for pest and disease management as an eco-friendly alternative. However, EPN continue to face adoption barriers due to regulatory, commercialization and basic and applied knowledge gaps. Thus, stronger collaborations are needed to unlock their full potential, as highlighted in the 2024 congress organized in La Rioja (Spain) to commemorate the 100 years since the discovery of the first EPN. This review examines the gap between academia and industry, suggesting strategies to bridge it, thereby promoting the advancement of EPN in 21st-century agriculture. Despite decades of research demonstrating their efficacy, EPN commercialization remains limited by production, formulation, and application challenges. Universities and government research agencies have driven fundamental innovation through the discovery of new EPN-bacteria partnerships in new regions of the world, which are helping us understand their distribution and habitant adaptations which are required for their registration and in establishing global regulations. Research conducted both in academia and the private sector (both big and small start-up companies) has and continues to play a key role in the characterization of EPN and in assessing their performance for their subsequent formulation, product optimization, and commercialization. These are fundamental steps to reach the ultimate goal, which is to provide growers with reliable products that are cost-effective and sustainable. In this review, we summarize key findings that have led to the commercialization and application of EPN, spanning from the characterization of EPN and their symbiotic bacteria to production, formulation, and the requirements for their registration. We also highlight critical knowledge gaps and opportunities for collaboration between academia, government agencies, and industry. Strengthening these partnerships will drive EPN adoption in agriculture, establishing them as a desirable biocontrol solution.

Effect of Different Soils on Pheromone-Enhanced Movement of Entomopathogenic Nematodes

Entomopathogenic nematodes (EPNs) have a specialized infective juvenile stage (IJ) that is mobile and has the capability to seek insect hosts to penetrate their haemocoel. EPNs are primarily applied to soil as biological control agents; thus, the IJs must move through the soil to find and infect a host. Soil characteristics are known to be an important factor that can affect the efficiency of EPN movement behavior. Previous research has shown that exposure to ascaroside pheromones can enhance EPN movement and infectivity in soil. The ability of pheromones to enhance EPN efficacy was recently demonstrated under field conditions in a pecan orchard. However, prior to our research, it was unknown whether different soils have differential effects on pheromone enhanced EPN efficacy. In different soils, we tested the biocontrol efficacy of Steinernema carpocapsae, Steinernema feltiae and Heterorhabditis bacteriophora in soil columns with and without pheromone exposure. All nematodes were evaluated in separate columns filled with oven dried commercial play sand and two different soils from pecan orchards (from Byron, GA and Tifton, GA). The soils differed substantially in several aspects such as field capacity, organic matter, nutrients, and nematode movement capacity. Efficacy was determined by baiting the bottom section of each column with larvae of the yellow mealworm (Tenebrio molitor L.). Results indicated that pheromones enhanced EPN efficacy for all EPN species and soils tested compared to treatments without pheromones. The magnitude/extent that pheromones boosted EPN movement in all EPNs regardless of soil type did not differ. Soil did not affect EPN efficacy for H. bacteriophora but did affect S. carpocapsae and S. feltiae. For both S. carpocapsae and S. feltiae efficacy was highest in the sandy field soil (Tifton soil) followed by that of the loamy sand (Byron soil) and pure sand (commercial play sand). When comparing the efficacy of EPN species to each other, we observed that H. bacteriophora killed more bait insects exposed to soil in the bottom of the soil column than other EPNs. Our findings suggest that pheromones can be used to enhance EPN efficacy in diverse soils. Future research may explore pheromone effects on EPNs in additional substrates.

Genes involved in DMSO-mediated yield increase of entomopathogenic nematodes

Entomopathogenic nematodes (EPNs) associated with their symbiotic bacteria can effectively kill insect pests, in agriculture, forestry and floriculture. Industrial-scale production techniques for EPNs have been established, including solid and liquid monoculture systems. It is found that supplement of 0.01% dimethyl sulfoxide (DMSO) to the culture medium significantly enhances the recovery and infective juvenile yield of these nematodes. However, the specific nematode genes responding to DMSO is not yet clear. This study identified differentially expressed genes and associated enriched pathways by analyzing the transcriptomic response of Heterorhabditis bacteriophora H06 in the liquid medium containing 0.01% DMSO, and validated the function of the selected genes using RNA interference (RNAi). 11 upregulated and 295 downregulated genes were detected in the hermaphrodites of H. bacteriophora H06 at 3 days beforee egg formation, following DMSO treatment. The biological processes involved included regulation of biological processes, metabolism, binding, signal transduction, post-transcriptional modifications, metabolism, and protein folding. By using RNAi, knockdown of three genes, Hint module, PAN domain protein, and Dpy-13, significantly regulated the egg load (at least a 17% increase in eggs per hermaphroditic adult) and the yield of infective juveniles (at least a 48% increase), in an optimized 96-well plates containing the medium with 1.6% nutrient broth, 1% corn oil and 1.2% agar, without DMSO. qRT-PCR results confirmed that the expression levels of the knockdown genes decreased significantly, compared to the control. These results demonstrate that DMSO regulates nematode gravidity by suppressing at least three selected genes, thereby enhancing IJ yield, providing molecular cues for understanding how DMSO regulates the EPN yield, and a new technology for enhancing nematode commercial production.

Meloidogyne incognita genes involved in the repellent behavior in response to ascr#9

Meloidogyne incognita is one of the globally serious plant parasitic nematodes. New control measure is urgently needed to replace the common chemical control method. Ascarosides are pheromones regulating the nematodes' aggregation, avoidance, mating, dispersal and dauer recovery and formation. Ascr#9, one of the ascarosides, exhibits the potential to repel M. incognita. However, the nematode genes involved in the perception of ascr# 9 signal are totally unknown. In this study, the transcriptome of ascr#9-treated second stage M. incognita juveniles (J2s) was analyzed, 44 pathways were significantly affected, multiple ligand-receptor and mucin type O-glycan were induced, and olfactory transduction was disturbed. A total of 11 highly differentially expressed genes involved in neuroactive ligand-receptor interaction and FMRFamide-like peptide related process were identified and knocked down by RNAi. The dispersal rates of M. incognita with three knocked-down genes (flp-14, mgl-1 and ADOR-1) significantly decreased, respectively, when ascr#9 was present. The results demonstrate that flp-14, mgl-1, and ADOR-1 are involved in the dispersal behavior of M. incognita nematodes responding to ascr#9, which promotes the interaction study between ascarosides and M. incognita, and provides new ideas for the prevention and control of M. incognita by using pheromone ascarosides.

Ascarosides and Symbiotic Bacteria of Entomopathogenic Nematodes Regulate Host Immune Response in Galleria mellonella Larvae

Insects protect themselves through their immune systems. Entomopathogenic nematodes and their bacterial symbionts are widely used for the biocontrol of economically important pests. Ascarosides are pheromones that regulate nematode behaviors, such as aggregation, avoidance, mating, dispersal, and dauer recovery and formation. However, whether ascarosides influence the immune response of insects remains unexplored. In this study, we co-injected ascarosides and symbiotic Photorhabdus luminescens subsp. kayaii H06 bacteria derived from Heterorhabditis bacteriophora H06 into the last instar larvae of Galleria mellonella. We recorded larval mortality and analyzed the expressions of AMPs, ROS/RNS, and LPSs. Our results revealed a process in which ascarosides, acting as enhancers of the symbiotic bacteria, co-induced G. mellonella immunity by significantly increasing oxidative stress responses and secreting AMPs (gallerimycin, gloverin, and cecropin). This led to a reduction in color intensity and the symbiotic bacteria load, ultimately resulting in delayed host mortality compared to either ascarosides or symbiotic bacteria. These findings demonstrate the cross-kingdom regulation of insects and symbiotic bacteria by nematode pheromones. Furthermore, our results suggest that G. mellonella larvae may employ nematode pheromones secreted by IJs to modulate insect immunity during early infection, particularly in the presence of symbiotic bacteria, for enhancing resistance to invasive bacteria in the hemolymph.

Dauer juvenile recovery transcriptome of two contrasting EMS mutants of the entomopathogenic nematode Heterorhabditis bacteriophora

The entomopathogenic nematode Heterorhabditis bacteriophora, symbiotically associated with enterobacteria of the genus Photorhabdus, is a biological control agent against many insect pests. Dauer Juveniles (DJ) of this nematode are produced in industrial-scale bioreactors up to 100 m3 in liquid culture processes lasting approximately 11 days. A high DJ yield (> 200,000 DJ·mL-1) determines the success of the process. To start the mass production, a DJ inoculum proceeding from a previous monoxenic culture is added to pre-cultured (24 h) Photorhabdus bacteria. Within minutes after contact with the bacteria, DJ are expected to perceive signals that trigger their further development (DJ recovery) to reproductive hermaphrodites. A rapid, synchronized, and high DJ recovery is a key factor for an efficient culture process. In case of low percentage of DJ recovery, the final DJ yield is drastically reduced, and the amount of non-desired stages (males and non-fertilized females) hinders the DJ harvest. In a preliminary work, a huge DJ recovery phenotypic variability in H. bacteriophora ethyl methanesulphonate (EMS) mutants was determined. In the present study, two EMS-mutant lines (M31 and M88) with high and low recovery phenotypes were analyzed concerning their differences in gene expression during the first hours of contact with Photorhabdus supernatant containing food signals triggering recovery. A snapshot (RNA-seq analysis) of their transcriptome was captured at 0.5, 1, 3 and 6 h after exposure. Transcripts (3060) with significant regulation changes were identified in the two lines. To analyze the RNA-seq data over time, we (1) divided the expression profiles into clusters of similar regulation, (2) identified over and under-represented gene ontology categories for each cluster, (3) identified Caenorhabditis elegans homologous genes with recovery-related function, and (4) combined the information with available single nucleotide polymorphism (SNP) data. We observed that the expression dynamics of the contrasting mutants (M31 and M88) differ the most within the first 3 h after Photorhabdus supernatant exposure, and during this time, genes related to changes in the DJ cuticle and molting are more active in the high-recovery line (M31). Comparing the gene expression of DJ exposed to the insect food signal in the haemolymph, genes related to host immunosuppressive factors were not found in DJ upon bacterial supernatant exposure. No link between the position of SNPs associated with high recovery and changes in gene expression was determined for genes with high differential expression. Concerning specific transcripts, nine H. bacteriophora gene models with differential expression are provided as candidate genes for further studies.

Enhancing entomopathogenic nematode efficacy with Pheromones: A field study targeting the pecan weevil

Consistent efficacy is required for entomopathogenic nematodes to gain wider adoption as biocontrol agents. Recently, we demonstrated that when exposed to nematode pheromone blends, entomopathogenic nematodes showed increased dispersal, infectivity, and efficacy under laboratory and greenhouse conditions. Prior to this study, the impact of entomopathogenic nematode-pheromone combinations on field efficacy had yet to be studied. Steinernema feltiae is a commercially available entomopathogenic nematode that has been shown to increase mortality in insect pests such as the pecan weevil Curculio caryae. In this study, the pecan weevil was used as a model system to evaluate changes in S. feltiae efficacy when treated with a partially purified ascaroside pheromone blend. Following exposure to the pheromone blend, the efficacy of S. feltiae significantly increased as measured with decreased C. caryae survival despite unfavorable environmental conditions. The results of this study highlight a potential new avenue for using entomopathogenic nematodes in field conditions. With increased efficacy, using entomopathogenic nematodes will reduce reliance on conventional management methods in pecan production, translating into more environmentally acceptable practices.

Pheno- and genotyping in vitro dauer juvenile recovery in the nematode Heterorhabditis bacteriophora

The entomopathogenic nematode (EPN) Heterorhabditis bacteriophora is an effective biological-control agent of insect pests. The dauer juveniles (DJs) seek for, infect insects, and release cells of the carried symbiotic bacterium of the genus Photorhabdus. Inside the host, the DJs perceive signals from the insect's haemolymph that trigger the exit from the arrested stage and the further development to mature adults. This developmental step is called DJ recovery. In commercial production, a high and synchronous DJ recovery determines the success of liquid-culture mass production. To enhance the understanding about genetic components regulating DJ recovery, more than 160 mutant- and 25 wild type inbred lines (WT ILs) were characterized for DJ recovery induced by cell-free bacterial supernatant. The mutant lines exhibited a broader DJ recovery range than WT ILs (4.6-67.2% vs 1.6-35.7%). A subset of mutant lines presented high variability of virulence against mealworm (Tenebrio molitor) (from 22 to 78% mortality) and mean time survival under oxidative stress (70 mM H2O2; from 10 to 151 h). Genotyping by sequencing of 96 mutant lines resulted in more than 150 single nucleotide polymorphisms (SNPs), of which four results are strongly associated with the DJ recovery trait. The present results are the basis for future approaches in improving DJ recovery by breeding under in vitro liquid-culture mass production in H. bacteriophora. This generated platform of EMS-mutants is as well a versatile tool for the investigation of many further traits of interest in EPNs. KEYPOINTS: • Exposure to bacterial supernatants of Photorhabdus laumondii induces the recovery of Heterorhabditis bacteriophora dauer juveniles (DJs). Both, the bacteria and the nematode partner, influence this response. However, the complete identity of its regulators is not known. • We dissected the genetic component of DJ recovery regulation in H. bacteriophora nematodes by generating a large array of EMS mutant lines and characterizing their recovery pheno- and genotypes. • We determined sets of mutants with contrasting DJ recovery and genotyped a subset of the EMS-mutant lines via genotyping by sequencing (GBS) and identified SNPs with significant correlation to the recovery trait.

Enhancing mass production of Heterorhabditis bacteriophora: influence of different bacterial symbionts (Photorhabdus spp.) and inoculum age on dauer juvenile recovery

The entomopathogenic nematode Heterorhabditis bacteriophora (Nematoda: Rhabditidae) is used in biological insect control. Their dauer juveniles (DJs) are free-living and developmentally arrested, invading host insects. They carry cells of their bacterial symbiont Photorhabdus spp. in the intestine. Once inside the insect´s hemolymph the DJs perceive a food signal, triggering them to exit the DJ stage and regurgitate the Photorhabdus cells into the insect's haemocoel, which kill the host and later provide essential nutrients for nematode reproduction. The exit from the DJ stage is called "recovery". For commercial pest control, nematodes are industrially produced in monoxenic liquid cultures. Artificial media are incubated with Photorhabdus before DJs are added. In absence of the insect's food signal, DJs depend on unknown bacterial food signals to trigger exit of the DJ stage. A synchronized and high DJ recovery determines the success of the industrial in vitro production and can significantly vary between nematode strains, inbred lines and mutants. In this study, fourteen bacterial strains from H. bacteriophora were isolated and identified as P. laumondii, P. kayaii and P. thracensis. Although the influence of bacterial supernatants on the DJ recovery of three inbred lines and two mutants differed significantly, the bacterial impact on recovery has a subordinate role whereas nematode factors have a superior influence. Recovery of inbred lines decreased with age of the DJs. One mutant (M31) had very high recovery in bacterial supernatant and spontaneous recovery in Ringer solution. Another mutant (M88) was recovery defective.

Potential of entomopathogenic nematode-infected insect cadavers for the biocontrol of the red imported fire ant Solenopsis invicta

BACKGROUND

Entomopathogenic nematodes (EPNs) in an aqueous suspension treatment showed potential in the biocontrol of the red imported fire ant Solenopsis invicta, whereas colony relocation following this inundative application had restricted overall effectiveness. As a novel strategy, EPN pre-infected insect cadavers carrying the next generation of nematode-infective juveniles (IJs) might be used to efficiently manage insect pests. However, this strategy has not been tested on S. invicta. This study evaluated EPNs emerging from wax moth Galleria mellonella cadavers to facilitate infection of S. invicta compared to those used in aqueous suspensions.

RESULTS

Among seven EPN species, Steinernema riobrave and Heterorhabditis bacteriophora had the highest insecticidal efficiency in aqueous treatment. Cadavers of G. mellonella colonized by either one of two EPN species were not damaged by worker ants, ensuring that the IJs could develop within and emerge successfully from the cadavers. Likewise, compared to an aqueous suspension treatment with an equivalent number of IJs, treatment with an S. riobrave-infected cadaver increased the mortality of S. invicta by ≈10%, whereas infection by H. bacteriophora did not differ between treatments. However, the coexistence of S. riobrave- and H. bacteriophora-infected cadavers adversely affected the control of S. invicta, likely as a result of competition resulting from the increased dispersal of each emerging EPN species.

CONCLUSION

Using EPN-infected G. mellonella cadavers as a strategy increased the mortality of S. invicta in the laboratory. This study provides positive evidence for the future applications of S. riobrave-infected cadavers in the biocontrol of red imported fire ants. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Nematode Pheromones: Structures and Functions

Pheromones are chemical signals secreted by one individual that can affect the behaviors of other individuals within the same species. Ascaroside is an evolutionarily conserved family of nematode pheromones that play an integral role in the development, lifespan, propagation, and stress response of nematodes. Their general structure comprises the dideoxysugar ascarylose and fatty-acid-like side chains. Ascarosides can vary structurally and functionally according to the lengths of their side chains and how they are derivatized with different moieties. In this review, we mainly describe the chemical structures of ascarosides and their different effects on the development, mating, and aggregation of nematodes, as well as how they are synthesized and regulated. In addition, we discuss their influences on other species in various aspects. This review provides a reference for the functions and structures of ascarosides and enables their better application.

Dimethyl sulfoxide and ascarosides improve the growth and yields of entomopathogenic nematodes in liquid cultures

Increasing the infective juvenile (IJ) yields of entomopathogenic nematodes in monoxenic culture systems would reduce their production cost for the market. Ascarosides act as universal nematode pheromones with developmental and behavioral effects of nematodes. Dimethyl sulfoxide (DMSO) is unexpectedly found to enhance the IJ yields of entomopathogenic nematodes on fortified nutrient broth plates. In this study, the influence of selected ascarosides (ascr#7, ascr#9 and ascr#11) and DMSO in three concentrations on the IJ yields of S. carpocapsae All and H. bacteriophora H06 in liquid culture flasks was determined, and the critical development parameters (IJ recovery rate, number of hermaphrodites, number of visible eggs in a hermaphrodite) were examined for H. bacteriophora H06. The results demonstrated that IJ yields were significantly improved in the liquid medium containing 0.01 % DMSO, and 0.02 nM ascr#11 for S. carpocapsae All, and 0.1 % and 0.01 % DMSO and 0.02 pM ascr#11 for H. bacteriophora H06 in proper concentrations. Furthermore, it was discovered that increased recovery rate, hermaphrodite numbers and eggs in the hermaphrodites may contribute to the improved IJ yields of H. bacteriophora H06 in DMSO-supplemented liquid medium. Compared with the control flasks, the IJ yields from the flasks containing 0.01 % DMSO were 15 % and 35 % higher for S. carpocapsae All and H. bacteriophora H06 respectively in 15 days. The cost for ascarosides and DMSO is almost negligible. The results would provide practical technology for low-cost commercial production of these nematodes for pest management program.