Response of a Predatory ant to Volatiles Emitted by Aphid- and Caterpillar-Infested Cucumber and Potato Plants

Dynamic distress calls: volatile info chemicals induce and regulate defense responses during herbivory

Plants are continuously threatened by a plethora of biotic stresses caused by microbes, pathogens, and pests, which often act as the major constraint in crop productivity. To overcome such attacks, plants have evolved with an array of constitutive and induced defense mechanisms- morphological, biochemical, and molecular. Volatile organic compounds (VOCs) are a class of specialized metabolites that are naturally emitted by plants and play an important role in plant communication and signaling. During herbivory and mechanical damage, plants also emit an exclusive blend of volatiles often referred to as herbivore-induced plant volatiles (HIPVs). The composition of this unique aroma bouquet is dependent upon the plant species, developmental stage, environment, and herbivore species. HIPVs emitted from infested and non-infested plant parts can prime plant defense responses by various mechanisms such as redox, systemic and jasmonate signaling, activation of mitogen-activated protein (MAP) kinases, and transcription factors; mediate histone modifications; and can also modulate the interactions with natural enemies via direct and indirect mechanisms. These specific volatile cues mediate allelopathic interactions leading to altered transcription of defense-related genes, viz., proteinase inhibitors, amylase inhibitors in neighboring plants, and enhanced levels of defense-related secondary metabolites like terpenoids and phenolic compounds. These factors act as deterrents to feeding insects, attract parasitoids, and provoke behavioral changes in plants and their neighboring species. This review presents an overview of the plasticity identified in HIPVs and their role as regulators of plant defense in Solanaceous plants. The selective emission of green leaf volatiles (GLVs) including hexanal and its derivatives, terpenes, methyl salicylate, and methyl jasmonate (MeJa) inducing direct and indirect defense responses during an attack from phloem-sucking and leaf-chewing pests is discussed. Furthermore, we also focus on the recent developments in the field of metabolic engineering focused on modulation of the volatile bouquet to improve plant defenses.

Plant Secondary Metabolites: The Weapons for Biotic Stress Management

The rise in global temperature also favors the multiplication of pests and pathogens, which calls into question global food security. Plants have developed special coping mechanisms since they are sessile and lack an immune system. These mechanisms use a variety of secondary metabolites as weapons to avoid obstacles, adapt to their changing environment, and survive in less-than-ideal circumstances. Plant secondary metabolites include phenolic compounds, alkaloids, glycosides, and terpenoids, which are stored in specialized structures such as latex, trichomes, resin ducts, etc. Secondary metabolites help the plants to be safe from biotic stressors, either by repelling them or attracting their enemies, or exerting toxic effects on them. Modern omics technologies enable the elucidation of the structural and functional properties of these metabolites along with their biosynthesis. A better understanding of the enzymatic regulations and molecular mechanisms aids in the exploitation of secondary metabolites in modern pest management approaches such as biopesticides and integrated pest management. The current review provides an overview of the major plant secondary metabolites that play significant roles in enhancing biotic stress tolerance. It examines their involvement in both indirect and direct defense mechanisms, as well as their storage within plant tissues. Additionally, this review explores the importance of metabolomics approaches in elucidating the significance of secondary metabolites in biotic stress tolerance. The application of metabolic engineering in breeding for biotic stress resistance is discussed, along with the exploitation of secondary metabolites for sustainable pest management.

Plant viruses induce plant volatiles that are detected by aphid parasitoids

Aphis gossypii (Sternorrhyncha: Aphididae) aphids are vectors of important plant viruses among which cucumber mosaic virus (CMV) and potato virus Y (PVY). Virus-infected plants attract aphid vectors and affect their behavior and growth performance either positively or negatively depending on mode of transmission. Viruses cause changes in the composition and the amount of volatile organic compounds (VOCs) released by the plant that attract aphids. The aphid parasitoid Aphidius colemani (Hymenoptera: Aphelinidae) has been shown to have higher parasitism and survival rates on aphids fed on virus-infected than aphids fed on non-infected plants. We hypothesized that parasitoids distinguish virus-infected plants and are attracted to them regardless of the presence of their aphid hosts. Herein, we examined the attraction of the A. colemani parasitoid to infected pepper plants with each of CMV or PVY without the presence of aphids. The dynamic headspace technique was used to collect VOCs from non-infected and CMV or PVY-infected pepper plants. Identification was performed with gas chromatography-mass spectrometry (GC-MS). The response of the parasitoids on virus-infected vs non-infected pepper plants was tested by Y-tube olfactometer assays. The results revealed that parasitoids displayed a preference to CMV and PVY infected plants compared to those that were not infected.

Parasitism causes changes in caterpillar odours and associated bacterial communities with consequences for host-location by a hyperparasitoid

Microorganisms living in and on macroorganisms may produce microbial volatile compounds (mVOCs) that characterise organismal odours. The mVOCs might thereby provide a reliable cue to carnivorous enemies in locating their host or prey. Parasitism by parasitoid wasps might alter the microbiome of their caterpillar host, affecting organismal odours and interactions with insects of higher trophic levels such as hyperparasitoids. Hyperparasitoids parasitise larvae or pupae of parasitoids, which are often concealed or inconspicuous. Odours of parasitised caterpillars aid them to locate their host, but the origin of these odours and its relationship to the caterpillar microbiome are unknown. Here, we analysed the odours and microbiome of the large cabbage white caterpillar Pieris brassicae in relation to parasitism by its endoparasitoid Cotesia glomerata. We identified how bacterial presence in and on the caterpillars is correlated with caterpillar odours and tested the attractiveness of parasitised and unparasitised caterpillars to the hyperparasitoid Baryscapus galactopus. We manipulated the presence of the external microbiome and the transient internal microbiome of caterpillars to identify the microbial origin of odours. We found that parasitism by C. glomerata led to the production of five characteristic volatile products and significantly affected the internal and external microbiome of the caterpillar, which were both found to have a significant correlation with caterpillar odours. The preference of the hyperparasitoid was correlated with the presence of the external microbiome. Likely, the changes in external microbiome and body odour after parasitism were driven by the resident internal microbiome of caterpillars, where the bacterium Wolbachia sp. was only present after parasitism. Micro-injection of Wolbachia in unparasitised caterpillars increased hyperparasitoid attraction to the caterpillars compared to untreated caterpillars, while no differences were found compared to parasitised caterpillars. In conclusion, our results indicate that host-parasite interactions can affect multi-trophic interactions and hyperparasitoid olfaction through alterations of the microbiome.

Predatory Abilities of Two Mediterranean Ants on the Eggs and Larvae of the Codling Moth Cydia pomonella

The predatory ability of ants (Hymenoptera, Formicidae) against insect pests can offer an important service to agricultural activities and may sometimes be directly exploited in biological control strategies. The codling moth Cydia pomonella (Lepidoptera, Tortricidae) is a major agricultural pest of fruit orchards, whose biological control is complicated by the fact that the larvae spend most of their life protected within the fruits they damage. In a recent experiment in Europe, pear trees in which ant activity was artificially increased by the addition of sugary liquid dispensers (artificial nectaries) suffered less damage caused by the larvae to their fruits. While some ants were already known to prey upon the mature larvae or pupae of C. pomonella in the soil, prevention of fruit damage would require predation upon eggs or newly hatched larvae, which have not yet excavated into the fruits. We verified whether two different Mediterranean ants frequently observed in fruit orchards, Crematogaster scutellaris and Tapinoma magnum, were able to prey upon C. pomonella eggs and larvae in laboratory conditions. Our experiments demonstrated that both species similarly attacked and killed young C. pomonella larvae. On the other hand, the eggs mostly attracted the attention of T. magnum but were never damaged. Further field assessments are required to understand whether ants may also interfere with oviposition by adults or whether larger ant species, although generally rarer in orchards, may also prey upon eggs.

Floral and Bird Excreta Semiochemicals Attract Western Carpenter Ants

Ant colonies have vast and diverse nutritional needs but forager ants have limited mobility to meet these needs. Forager ants would accrue significant energy savings if they were able to sense and orient toward odor plumes of both carbohydrate and protein food sources. Moreover, if worker ants, like other flightless insects, had reduced olfactory acuity, they would not recognize the specific odor signatures of diverse carbohydrate and protein sources, but they may be able to orient toward those odorants that are shared between (macronutrient) food sources. Using the Western carpenter ant, Camponotus modoc, as a model species, we tested the hypotheses that (1) food sources rich in carbohydrates (aphid honeydew, floral nectar) and rich in proteins (bird excreta, house mouse carrion, cow liver infested or not with fly maggots) all prompt long-distance, anemotactic attraction of worker ants, and (2) attraction of ants to plant inflorescences (fireweed, Chamaenerion angustifolium; thimbleberry, Rubus parviflorus; and hardhack, Spiraea douglasii) is mediated by shared floral odorants. In moving-air Y-tube olfactometer bioassays, ants were attracted to two of four carbohydrate sources (thimbleberry and fireweed), and one of four protein sources (bird excreta). Headspace volatiles of these three attractive sources were analyzed by gas chromatography-mass spectrometry, and synthetic odor blends of thimbleberry (7 components), fireweed (23 components), and bird excreta (38 components) were prepared. In Y-tube olfactometer bioassays, synthetic blends of thimbleberry and fireweed but not of bird excreta attracted ants, indicating that only the two floral blends contained all essential attractants. A blend of components shared between thimbleberry and fireweed was not attractive to ants. Our data support the conclusion that C. modoc worker ants can sense and orient toward both carbohydrate and protein food sources. As ants were selective in their responses to carbohydrate and protein resources, it seems that they can discern between specific food odor profiles and that they have good, rather than poor, olfactory acuity.

The discrepancy between fire ant recruitment to and performance on rodent carrion

Ants have not been considered important in the process of vertebrate carrion decomposition, but a recent literature review reported over 150 carrion-visiting ant species. Though many ant species have been observed to remove carrion tissue and consume carrion-exuded liquids, the significance of ant recruitment to vertebrate carrion is poorly understood. We conducted a combination of field and laboratory experiments to quantify red imported fire ant recruitment to rodent carrion and determine whether consuming rodent carrion is beneficial to ant colony performance. In the field, 100% of rat carcasses were rapidly colonized by fire ants at high abundances. In our laboratory experiment, the performance of mice-fed fire ant colonies was poor when compared to colonies that were fed mice and insects or insects only. Our results suggest that there is a discrepancy between high levels of fire ant recruitment to vertebrate carrion and the poor colony performance when fed carrion. We hypothesize that fire ants are attracted to vertebrate carrion not because it is a high-quality food, but rather because it hosts large numbers of other invertebrates that can serve as prey for fire ants, potentially showcasing an interesting case of tritrophic interaction in carrion ecology.

New Tools for Conservation Biological Control: Testing Ant-Attracting Artificial Nectaries to Employ Ants as Plant Defenders

Knowledge of the role of ants in many agroecosystems is relatively scarce, and in temperate regions the possibility to exploit ants as biocontrol agents for crop protection is still largely unexplored. Drawing inspiration from mutualistic ant–plant relationships mediated by extrafloral nectaries (EFNs), we tested the use of artificial nectaries (ANs) in order to increase ant activity on pear trees and to evaluate the effects on the arthropods, plant health and fruit production. While EFNs secrete a complex solution mainly composed of sugars and amino acids, ANs were filled with water and sucrose only. The results suggest that ANs can be used as manipulative instruments to increase ant activity over long periods of time. High ant activity was significantly linked to lower incidence of the pathogen fungus Venturia pyrina (pear scab) on pear leaves, and of the presence of Cydia pomonella (codling moth) caterpillars on pear fruit production. These results further encourage exploring underrated possibilities in the development of new tools for conservation biological control (CBC).

Bacterial Semiochemicals and Transkingdom Interactions with Insects and Plants

A peculiar feature of all living beings is their capability to communicate. With the discovery of the quorum sensing phenomenon in bioluminescent bacteria in the late 1960s, it became clear that intraspecies and interspecies communications and social behaviors also occur in simple microorganisms such as bacteria. However, at that time, it was difficult to imagine how such small organisms-invisible to the naked eye-could influence the behavior and wellbeing of the larger, more complex and visible organisms they colonize. Now that we know this information, the challenge is to identify the myriad of bacterial chemical signals and communication networks that regulate the life of what can be defined, in a whole, as a meta-organism. In this review, we described the transkingdom crosstalk between bacteria, insects, and plants from an ecological perspective, providing some paradigmatic examples. Second, we reviewed what is known about the genetic and biochemical bases of the bacterial chemical communication with other organisms and how explore the semiochemical potential of a bacterium can be explored. Finally, we illustrated how bacterial semiochemicals managing the transkingdom communication may be exploited from a biotechnological point of view.

Multifunctionality of herbivory-induced plant volatiles in chemical communication in tritrophic interactions

Uninfested plants emit only trace quantities of volatiles (constitutively emitted plant volatiles). In contrast, some plants emit relatively large quantities of volatiles in response to herbivory (herbivory-Induced plant volatiles: HIPVs). Organisms belonging to different trophic levels use plant volatiles in context-dependent manners; consequently, volatiles can be adaptive, non-adaptive, or maladaptive to the emitter plants. In this review, we focus on the multifunctional aspects of HIPVs, which vary qualitatively and quantitatively in emitting plant species and infesting herbivore species, in plant-carnivore interactions, plant-herbivore interactions, and plant-omnivore interactions. Additionally, we review the evidence of plant-plant communication and its effects on tritrophic interactions involving plants, herbivores, and carnivores. Prospects on interactions mediated by plant volatiles induced by herbivorous arthropods are discussed.

Plant chemical mediation of ant behavior

Ants are ecologically dominant members of terrestrial communities. Ant foraging is often strongly associated with plants and depends upon associative learning of chemicals in the environment. As a result, plant chemicals can affect ant behaviors and, in so doing, have strong multi-trophic indirect effects. Plant chemicals mediate ant behaviors in the contexts of floral visitation, seed dispersal and predation, leaf cutting, interactions with ant-mutualist host plants, interactions with mutualist and prey insects in plant canopies, and plant predation of ants by carnivorous plants. Here, we review what is known about these differing contexts in which plant chemicals influence ant behavior, the mechanisms by which ants are affected by plant chemicals, and future directions within these topics.

Antennal Olfactory Physiology and Behavior of Males of the Ponerine Ant Harpegnathos saltator

In comparison to the large amount of study on the communication abilities of females in ant societies and their associated chemical ecology and sensory physiology, such study of male ants has been largely ignored; accordingly, little is known about their olfactory sensory capabilities. To address this, we explored peripheral odor sensitivities in male Harpegnathos saltator by measuring the electrophysiological activity of olfactory sensory neurons within antennal trichoid and coeloconic sensilla using an extracellular recording technique. In an initial trial of 46 compounds, sensilla trichodea responded strongly to two alarm pheromone components, while a limited number of non-hydrocarbon odorants elicited strong responses in sensilla coeloconica. Both sensillar types responded indifferently to 31 cuticular hydrocarbons (CHCs) and synthetic long-chain hydrocarbons (HCs) typically found on insect cuticle. In a search for sensilla responding to CHCs and other compounds, we found some sensilla that responded to synthetic HCs and CHCs from virgin queen postpharyngeal glands that are potentially used in close range mate recognition. Olfactometer bioassays of male ants to 15 non-HCs correlated sensory responsiveness to the respective behavioral responses. Comparing olfactory responses between H. saltator males and females, we found that sensilla coeloconica and basiconica of workers showed greater responses and broader selectivity to all compounds. The rarity of CHC-responding trichoid sensilla in Harpegnathos males suggests a more specific role in sexual communication compared to that in females, which use CHCs in a broader communication context.