Sustainable agricultural intensification employs alternatives to synthetic insecticides for pest management, but these are not always a direct replacement. Botanical insecticides, for example, have rapid knockdown but are highly labile and while biological pesticides are more persistent, they are slow acting. To mitigate these shortcomings, we combined the entomopathogenic fungus (EPF) Metarhizium anisopliae with pyrethrum and evaluated their efficacy against the bean aphid, Aphis fabae. To ascertain higher trophic effects, we presented these treatments to the parasitoid, Aphidius colemani, on an aphid infested plant in a Y-tube olfactometer and measured their preferences. Aphid mortality was significantly higher than controls when exposed to EPF or pyrethrum but was greater still when exposed to a combination of both treatments, indicating an additive effect. This highlights the potential for applications of pyrethrum at lower doses, or the use of less refined products with lower production costs to achieve control. While parasitoids were deterred by aphid infested plants treated with EPF, no preference was observed with the combination pesticide, which provides insight into the importance that both application technique and timing may play in the success of this new technology. These results indicate the potential for biorational pesticides that combine botanicals with EPF.

A novel bacterial species, Photorhabdus viridis sp. nov., represented by strain Green^T, isolated from Heterorhabditis zealandica MJ2C entomopathogenic nematodes, is described. Phylogenetic reconstructions using 16S rRNA gene sequences show that strain Green^T is closely related to P. thracensis DSM 15199 ^T. The 16rRNA gene sequences of these two strains are 98.8% identical. Phylogenetic reconstructions using whole-genome sequences show that strain Green^T is closely related to P. tasmaniensis DSM 22387 ^T, P. thracensis DSM 15199 ^T, and P. temperata DSM 14550 ^T. Digital DNA-DNA hybridization (dDDH) values between strain Green^T and its three more close relative species, P. tasmaniensis DSM 22387 ^T, P. thracensis DSM 15199 ^T, and P. temperata DSM 14550 ^T, are 49%, 59%, and 59%, respectively. In addition, average nucleotide identity (ANI) values between Green^T and P. tasmaniensis DSM 22387 ^T, P. thracensis DSM 15199 ^T, and P. temperata DSM 14550 ^T are 92.4%, 94.4%, and 94.6%, respectively. The novel species also differs in their biochemical capacities from the biochemical capacities of their more closely related taxa. The following biochemical tests may be particularly useful in this context: Arginine dihydrolase, gelatinase, and glucose and mannitol oxidation. Given the clear phylogenetic separation, the sequence divergence values, and the phenotypic differences, we conclude that strain Green^T represents a novel bacterial species, for which we propose the name Photorhabdus viridis sp. nov. with Green^T (= CCM 9407 ^T = CCOS 2117 ^T = MJ2C^T) as the type strain. Our study contributes to a better understanding of the taxonomy and biodiversity of an important bacterial group with great biotechnological and agricultural potential.

Photorhabdus bacteria are potent insect-killing microbes associated with entomopathogenic nematodes and offer opportunities for environmentally benign pest control. They can be applied as foliar sprays or soil drenches without their nematode vector, resulting in massive amounts of Photorhabdus cells and their (toxic) metabolites introduced into the soil. However, their effects beyond the target organisms are unknown. To fill this knowledge gap, we investigated the soil legacy effects of Photorhabdus cells and their metabolites on soil microbial communities, plant performance and resistance to herbivores. To this end, we first conditioned soils with i) mechanically killed (MK) or Photorhabdus -infected insect larvae, ii) aqueous extracts of MK or Photorhabdus -infected insect larvae, iii) cell-free Photorhabdus supernatants, iv) autoclaved soil complemented with live soil previously conditioned with MK or Photorhabdus -infected insect larvae. We then grew maize plants in these soils and measured plant biomass, profiled soil microbial communities and plant metabolites, and evaluated plant resistance against two pest insects Diabrotica balteata and Spodoptera frugiperda . We found that conditioned soils increased plant biomass by 10–26% relative to controls and significantly altered soil bacterial and nematode communities, and to a lesser extent, fungal communities. Re-inoculating conditioned soil microbiota into autoclaved soils recreated the plant growth-promoting effects, indicating microbial-mediated mechanisms. Additionally, plants grown in soils conditioned with Photorhabdus -infected insect cadavers were often more resistant to herbivorous insect attack, in a strain-specific manner. On average, D. balteata and S. frugiperda larvae gained 10–20% and 10–59% less weight, respectively, when fed on plants grown in conditioned soils than on plants grown in control soils. The plant metabolic profiles of plant leaves and roots also varied with resistance levels. We conclude that Photorhabdus metabolites modulate soil microbial communities towards a structure that enhances plant growth and triggers systemic responses against herbivores.

Les bactéries Photorhabdus sont des microbes puissants qui tuent les insectes et offrent une excellente opportunité pour lutter contre les ravageurs de manière écologique. Cependant, l'application de bactéries Photorhabdus par pulvérisation foliaire ou arrosage du sol sans leur vecteur nématode entomopathogène introduit des cellules bactériennes et des toxines dans le sol. Or, les effets à long terme des bactéries Photorhabdus et de leurs métabolites sur les plantes et autres organismes sont inconnus. Dans cette étude, j'ai examiné les effets résiduels des métabolites de Photorhabdus sur la communauté microbienne du sol, la physiologie des plantes, leur performance et leur résistance aux attaques d'insectes. Pour ce faire, j'ai conditionné les sols avec : i) des cadavres d'insectes infectés par Photorhabdus ou des larves tuées mécaniquement (MK), ii) des extraits aqueux de toxines provenant de cadavres d'insectes infectés par Photorhabdus ou de larves MK, iii) des surnageants de culture de Photorhabdus sans cellules, iv) du sol autoclavé complété avec 10 % de sol vivant préalablement conditionné avec des cadavres d'insectes infectés par Photorhabdus ou des larves MK, et j'ai utilisé du sol non conditionné comme témoin. J'ai ensuite mesuré les caractéristiques de croissance des plantes et l'accumulation de biomasse chez les plantes cultivées sur les différents traitements de conditionnement du sol. Ensuite, j'ai utilisé le séquençage d'amplicons pour déterminer comment le conditionnement du sol affectait les communautés bactériennes, nématodes et fongiques. Enfin, j'ai évalué les performances des larves de Diabrotica balteata et Spodoptera frugiperda se nourrissant de plantes cultivées sur un sol conditionné, et j'ai utilisé la métabolomique pour établir le profil des réponses des plantes au niveau métabolique. J'ai démontré que tous les traitements d'amendement du sol amélioraient considérablement la croissance des plantes, de 10 à 26 % par rapport aux plantes témoins. Cependant, aucune différence significative n'a été observée en termes de hauteur, de longueur et de largeur des feuilles, ni de nombre de racines pivotantes entre les plantes expérimentales et les plantes témoins. Des effets similaires sur la croissance des plantes ont été observés chez les plantes cultivées sur un sol amendé, soulignant le rôle des micro-organismes du sol dans ces résultats. Après analyse de la communauté microbienne du sol, l'amendement du sol avec des cadavres d'insectes infectés par Photorhabdus ou MK a considérablement modifié les communautés bactériennes et nématodes, tandis que la communauté fongique est restée stable. Il est à noter que les bactéries bénéfiques et les espèces nématodes étaient plus abondantes dans le sol amendé que dans le sol témoin, ce qui explique probablement l'amélioration de la croissance des plantes dans le sol amendé. J'ai également montré que les plantes cultivées sur des sols amendés avec des cadavres d'insectes infectés par Photorhabdus ont considérablement réduit l'alimentation des larves d'insectes d'une manière spécifique à chaque souche. Plus précisément, le poids des larves de D. balteata et S. frugiperda a été réduit respectivement de 10 à 20 % et de 10 à 59 %, soit moins que dans le sol témoin. De plus, les plantes résistantes ont accumulé respectivement douze et huit métabolites distincts dans leurs racines et leurs feuilles, dont les effets biologiques contre les insectes et les agents pathogènes sont connus. Sur la base de ces résultats, je conclus que l'amendement du sol avec des métabolites de Photorhabdus améliore la croissance des plantes, module la communauté microbienne vers une structure favorable à la croissance des plantes et que l'amendement avec des souches sélectionnées de Photorhabdus déclenche des réponses systémiques chez les plantes contre les herbivores qui se nourrissent des racines et des feuilles. Abstract : Photorhabdus bacteria are potent insect-killing microbes offering a prime opportunity for environmentally benign pest control. However, applying Photorhabdus bacteria by foliar sprays or soil drenching without its entomopathogenic nematode vector introduces bacterial cells and toxins into the soil. Yet, the soil legacy effects of Photorhabdus bacteria and their metabolites on plants and other organisms are unknown. In this study, I investigated the soil legacy effects of Photorhabdus metabolites on soil microbial community, plant physiology, performance and resistance against insect attack. To achieve this, I conditioned soils with: i) Photorhabdus-infected insect cadavers or mechanically killed (MK) larvae, ii) water extracts of toxins derived from Photorhabdus-infected insect cadavers, or from MK larvae, iii) cell-free Photorhabdus culture supernatants, iv) autoclaved soil complemented with 10% of live soil previously conditioned with Photorhabdus-infected insect cadavers or MK, and used non-conditioned soil as the control. I then measured plant growth traits and biomass accumulation in plants grown on the different soil conditioning treatments. Next, I employed amplicon sequencing to determine how soil conditioning affected the bacterial, nematode, and fungal communities. Finally, I evaluated the performance of Diabrotica balteata and Spodoptera frugiperda larvae feeding on plants grown on conditioned soil, and I used...

Plant-nematode interactions represent a dynamic interplay between parasitic nematodes and their host plants, influencing plant health and agricultural productivity worldwide. This chapter provides a comprehensive overview of the mechanisms underlying plant defense against nematode infestation. It begins with an exploration of nematode parasitism strategies, including sedentary endoparasites and migratory ectoparasites, the discussion delves into the molecular and biochemical mechanisms employed by plants to recognize nematode invasion and mount defense responses. Key topics include the role of plant hormones such as jasmonic acid and salicylic acid in signaling pathways, the activation of defense-related genes, and the induction of physical barriers to nematode penetration. Furthermore, recent advances in understanding plant-nematode interactions, such as the discovery of nematode effectors and their manipulation of plant immunity, are highlighted. Additionally, the chapter examines the potential application of biotechnological approaches, such as breeding for nematode resistance and the use of biocontrol agents, in managing nematode infestations sustainably. By elucidating the intricate mechanisms of plant defense against nematodes, this chapter aims to contribute to the development of effective strategies for enhancing crop resilience and ensuring global food security. By synthesizing current knowledge and research findings, this chapter contributes to a comprehensive understanding of plant-nematode interactions and provides insights into novel avenues for enhancing plant resistance to nematode pests. Ultimately, elucidating the intricacies of plant defense mechanisms against nematodes holds promise for sustainable agriculture practices and the development of resilient crop varieties in the face of evolving pest pressures.