Summary: Plants respond to herbivory by reconfiguring hormonal networks, increasing secondary metabolite production and decreasing growth. Furthermore, some plants display a decrease in leaf energy reserves in the form of soluble sugars and starch, leading to the hypothesis that herbivory‐induced secondary metabolite production and growth reduction may be linked through a carbohydrate‐based resource trade‐off. In order to test the above hypothesis, we measured leaf carbohydrates and plant growth in seven genetically engineered Nicotiana attenuata genotypes that are deficient in one or several major herbivore‐induced, jasmonate‐dependent defensive secondary metabolites and proteins. Furthermore, we manipulated gibberellin and jasmonate signaling, and quantified the impact of these phytohormones on secondary metabolite production, sugar accumulation and growth. Simulated herbivore attack by Manduca sexta specifically reduced leaf sugar concentrations and growth in a jasmonate‐dependent manner. These effects were similar or even stronger in defenseless genotypes with intact jasmonate signaling. Gibberellin complementation rescued carbohydrate accumulation and growth in induced plants without impairing the induction of defensive secondary metabolites. These results are consistent with a hormonal antagonism model rather than a resource–cost model to explain the negative relationship between herbivory‐induced defenses, leaf energy reserves and growth.

Highly adapted herbivores can phenocopy two-component systems by stabilizing, sequestering and reactivating plant toxins. However, whether these traits protect herbivores against their enemies is poorly understood. We demonstrate that the western corn rootworm Diabrotica virgifera virgifera, the most damaging maize pest on the planet, specifically accumulates the root-derived benzoxazinoid glucosides HDMBOA-Glc and MBOA-Glc. MBOA-Glc is produced by D. virgifera through stabilization of the benzoxazinoid breakdown product MBOA by N-glycosylation. The larvae can hydrolyze HDMBOA-Glc, but not MBOA-Glc, to produce toxic MBOA upon predator attack. Accumulation of benzoxazinoids renders D. virgifera highly resistant to nematodes which inject and feed on entomopathogenic symbiotic bacteria. While HDMBOA-Glc and MBOA reduce the growth and infectivity of both the nematodes and the bacteria, MBOA-Glc repels infective juvenile nematodes. Our results illustrate how herbivores combine stabilized and reactivated plant toxins to defend themselves against a deadly symbiosis between the third and the fourth trophic level enemies.

Significance: Certain adapted insect herbivores utilize plant toxins for self-defense against their own enemies. These adaptations structure ecosystems and limit our capacity to use biological control agents to manage specialized agricultural pests. We show that entomopathogenic nematodes that are exposed to the western corn rootworm, an important agricultural pest that sequesters defense metabolites from maize, can evolve resistance to these defenses. Resisting the plant defense metabolites likely allows the nematodes to infect and kill the western corn rootworm more efficiently. These findings illustrate how predators can counter the plant-based resistance strategies of specialized insect herbivores. Breeding or engineering biological control agents that resist plant defense metabolites may improve their capacity to kill important agricultural pests such as the western corn rootworm.

Three Gram-stain-negative, rod-shaped, non-spore-forming bacteria, BA1^T, Q614^T and PB68.1^T, isolated from the digestive system of Heterorhabditis entomopathogenic nematodes, were biochemically and molecularly characterized to clarify their taxonomic affiliations. The 16S rRNA gene sequences of these strains suggest that they belong to the Gammaproteobacteria, to the family Morganellacea, and to the genus Photorhabdus . Deeper analyses using whole genome-based phylogenetic reconstructions suggest that BA1^T is closely related to Photorhabdus akhursti, that Q614^T is closely related to Photorhabdus heterorhabditis, and that PB68.1^T is closely related to Photorhabdus australis. In silico genomic comparisons confirm these observations: BA1^T and P. akhursti 15138^T share 68.8 % digital DNA–DNA hybridization (dDDH), Q614^T and P. heterorhabditis SF41^T share 75.4 % dDDH, and PB68.1^T and P. australis DSM 17609^T share 76.6 % dDDH. Physiological and biochemical characterizations reveal that these three strains also differ from all validly described Photorhabdus species and from their more closely related taxa, contrary to what was previously suggested. We therefore propose to classify BA1^T as a new species within the genus Photorhabdus , Q614^T as a new subspecies within P. heterorhabditis, and PB68.1^T as a new subspecies within P. australis . Hence, the following names are proposed for these strains: Photorhabdus aegyptia sp. nov. with the type strain BA1^T(=DSM 111180^T=CCOS 1943^T=LMG 31957^T), Photorhabdus heterorhabditis subsp. aluminescens subsp. nov. with the type strain Q614^T (=DSM 111144^T=CCOS 1944^T=LMG 31959^T) and Photorhabdus australis subsp. thailandensis subsp. nov. with the type strain PB68.1^T (=DSM 111145^T=CCOS 1942^T). These propositions automatically create Photorhabdus heterorhabditis subsp. heterorhabditis subsp. nov. with SF41^T as the type strain (currently classified as P. heterorhabditis ) and Photorhabdus australis subsp. australis subsp. nov. with DSM17609^T as the type strain (currently classified as P. australis ).

Insect herbivores use different cues to locate host plants. The importance of CO₂ in this context is not well understood. We manipulated CO₂ perception in western corn rootworm (WCR) larvae through RNAi and studied how CO₂ perception impacts their interaction with their host plant. The expression of a carbon dioxide receptor, DvvGr2, is specifically required for dose-dependent larval responses to CO₂. Silencing CO₂ perception or scrubbing plant-associated CO₂ has no effect on the ability of WCR larvae to locate host plants at short distances (<9 cm), but impairs host location at greater distances. WCR larvae preferentially orient and prefer plants that grow in well-fertilized soils compared to plants that grow in nutrient-poor soils, a behaviour that has direct consequences for larval growth and depends on the ability of the larvae to perceive root-emitted CO₂. This study unravels how CO₂ can mediate plant–herbivore interactions by serving as a distance-dependent host location cue.