Cascading effects of combining synthetic herbivore-induced plant volatiles with companion plants to manipulate natural enemies in an agro-ecosystem

Evaluation of olfactory and visual cues for conservation biological control of crape myrtle bark scale in urban landscapes

BACKGROUND

Acanthacoccus lagerstroemiae (crape myrtle bark scale, CMBS) is an exotic scale insect that feeds on the sap of crape myrtle trees. Heavy infestations of CMBS reduce flowering and honeydew promotes sooty mold growth on the leaves and branches, reducing the aesthetic value of crape myrtle trees in urban landscapes. Lady beetles (Coleoptera: Coccinellidae) are generalist predators that feed on CMBS. Several laboratory and field studies have demonstrated the attraction of lady beetles to olfactory and visual cues. We evaluated lady beetles' responses to olfactory lures and yellow visual attractants on infested potted and landscape trees to increase natural enemy recruitment and reduce dependence on chemical control methods.

RESULTS

Gas chromatography mass spectrometry analysis showed that limonene is the dominant volatile compound released by crape myrtles infested with CMBS. Limonene and methyl salicylate lures, alone or in combination, failed to reduce CMBS or recruit additional lady beetles to infested trees in all experiments. Yellow rectangular panels placed 1 m above the base of an infested tree recruited up to twofold more lady beetles than control trees; however, this was not statistically significant. A significant reduction in CMBS was observed on infested trees with yellow rectangular panels placed in the canopy.

CONCLUSION

Yellow rectangular panels are more likely to recruit lady beetles than limonene or methyl salicylate lures in an urban landscape. Management of CMBS is currently achieved using systemic insecticides. This study provides a basis for conservation biological control of CMBS through the recruitment of lady beetles, a significant advancement toward integrated management of this exotic pest. © 2024 Society of Chemical Industry.

What Do We Know About Capsicum Volatilome?

The Capsicum genus includes several cultivated species that release complex blends of volatile organic compounds (VOCs) associated with their unique aroma. These VOCs are essential info-chemicals in ecological interactions. In this review, we describe how the volatilomic profiling naturally varies based on specific plant organs and genotypes as well as how non-beneficial organisms affect VOCs biosynthesis and accumulation in pepper plants. Also, we show evidence about VOCs variation under the pressure of different abiotic factors such as water stress, soil type and nutrient availability. The contribution of specific metabolic pathways and gene expression related to the biosynthesis of particular VOCs is addressed. We highlighted the utility of VOCs as chemical markers for quality control in the food industry, breeding programs to generate resistant plants and to improve aroma innovation. Herein we present a database containing 2734 VOCs, revealing 113 as the basic core of the volatilome from five Capsicum species.

Immunity priming and biostimulation by airborne nonanal increase yield of field-grown common bean plants

Introduction

Stress-induced volatile organic compounds (VOCs) that induce plant immunity bear potential for biocontrol. Here, we explore the potential of nonanal to enhance the seed yield of common bean (Phaseolus vulgaris) under open field conditions that are realistic for smallholder farmers.

Methods and results

Using plastic cups with a nonanal-containing lanolin paste as low-cost dispensers, we observed that exposure of Flor de Junio Marcela (FJM) plants over 48h to airborne nonanal was followed by a 3-fold higher expression of pathogenesis-related (PR) genes PR1 and PR4. Both genes further increased their expression in response to subsequent challenge with the fungal pathogen Colletotrichum lindemuthianum. Therefore, we conclude that nonanal causes resistance gene priming. This effect was associated with ca. 2.5-fold lower infection rates and a 2-fold higher seed yield. Offspring of nonanal-exposed FJM plants exhibited a 10% higher emergence rate and a priming of PR1- and PR4-expression, which was associated with decreased infection by C. lindemuthianum and, ultimately, a ca. 3-fold increase in seed yield by anthracnose-infected offspring of nonanal-exposed plants. Seeds of nonanal-exposed and of challenged plants contained significantly more phenolic compounds (increase by ca 40%) and increased antioxidant and radical scavenging activity. Comparative studies including five widely used bean cultivars revealed 2-fold to 3-fold higher seed yield for nonanal-exposed plants. Finally, a cost-benefit analysis indicated a potential economic net profit of nonanal exposure for some, but not all cultivars.

Outlook

We consider nonanal as a promising candidate for an affordable tool that allows low-income smallholder farmers to increase the yield of an important staple-crop without using pesticides.

Chemical ecology in conservation biocontrol: new perspectives for plant protection

Threats to food security require novel sustainable agriculture practices to manage insect pests. One strategy is conservation biological control (CBC), which relies on pest control services provided by local populations of arthropod natural enemies. Research has explored manipulative use of chemical information from plants and insects that act as attractant cues for natural enemies (predators and parasitoids) and repellents of pests. In this review, we reflect on past strategies using chemical ecology in CBC, such as herbivore-induced plant volatiles and the push-pull technique, and propose future directions, including leveraging induced plant defenses in crop plants, repellent insect-based signaling, and genetically engineered crops. Further, we discuss how climate change may disrupt CBC and stress the importance of context dependency and yield outcomes.

The effects of plant domestication on the foraging and performance of parasitoids

Domestication-related changes in the chemical traits of crop plants affect parasitoid foraging success, development, and survival. For example, herbivore-induced changes in the production of volatiles by domesticated plants can enhance or reduce parasitoid attraction. While the trade-off between nutrient content and chemical defense in cultivated plants can increase the suitability of hosts for parasitoids, their increased health and size can positively affect their immune response against parasitoids. Overall, plant domestication is expected to significantly affect their relationship with parasitoids due to altered plant morphology, physical characteristics, chemical defenses, and new plant associations. This review highlights the need for research on the effects of plant domestication on host-parasitoid interactions in the interest of better controlling insect pests.

Attracted to feed, not to be fed upon – on the biology of Toxomerus basalis (Walker, 1836), the kleptoparasitic ‘sundew flower fly’ (Diptera: Syrphidae)

The complete life history of the kleptoparasitic ‘sundew flower fly’, Toxomerus basalis, is presented and illustrated. Adults of this species are photographed alive for the first time, including video recordings of larval and adult behaviour. Adult flies of both sexes visit Drosera (sundews) and show territorial behaviour around the plants, avoiding the dangerous sticky traps and demonstrating recognition of their larval host plant. Females lay eggs directly on non-sticky parts of the Drosera host plants, such as on the lower surface of the leaves and flower stalks, but apparently also on other plants growing in close proximity with the sundews.

Methyl Salicylate Can Benefit Ornamental Pest Control, and Does Not Alter Per Capita Predator Consumption at Close-Range

Methyl salicylate (MeSA) is an herbivore-induced plant volatile widely tested for attracting natural enemies for pest control. MeSA is commercially sold as slow-release lures or as a spray. While MeSA application has increased the abundance of natural enemies in numerous food crops, its ability to reduce pests for crop protection is not as frequently demonstrated. Our first objective was to test MeSA lures in ornamental fields where few studies have been done, and monitor natural enemies, pests, and crop protection. A 2-year study in spruce container yards revealed more aphid parasitoids (Pseudopraon sp.), fewer aphids (Mindarus obliquus) on shoot tips, and less shoot tip damage in MeSA plots during the first year. A 2-year study in red maple fields revealed more predatory lady beetles and rove beetles, and parasitic Ceraphronidae, Diapriidae, and Chalcidoidea in one or both years with MeSA. Fewer pest thrips were also captured in MeSA plots, though it is not clear whether this was due to enhanced predation or reduced colonization. Maple growth as measured by stem diameter change did not differ with MeSA use. A 2-year study examining predation on sentinel Halyomorpha halys eggs in various mature ornamental stock blocks found no increase in predation except for 1 month, though green lacewings, lady beetles, and predatory thrips occurred more in MeSA plots in the first year. While MeSA is expected to enhance biological control by herding in natural enemies, the impacts that applied volatiles have on predator efficiency is mostly unknown. Thus, our second objective examined how volatiles would impact feeding rates at close-range. Adult carabid Pterostichus melanarius, adult coccinellids Coccinella septempunctata and Harmonia axyridis, and larval lacewing Chrysoperla rufilabris consumed their prey at similar rates in the presence/absence of MeSA when food was presented directly in a 28 cm2 or 30 ml arena, or when foraging in a 520 cm2 outdoor soil arena or 946 ml arena with aphids on leaves.

Exploring the Kairomone-Based Foraging Behaviour of Natural Enemies to Enhance Biological Control: A Review

Kairomones are chemical signals that mediate interspecific interactions beneficial to organisms that detect the cues. These attractants can be individual compounds or mixtures of herbivore-induced plant volatiles (HIPVs) or herbivore chemicals such as pheromones, i.e., chemicals mediating intraspecific communication between herbivores. Natural enemies eavesdrop on kairomones during their foraging behaviour, i.e., location of oviposition sites and feeding resources in nature. Kairomone mixtures are likely to elicit stronger olfactory responses in natural enemies than single kairomones. Kairomone-based lures are used to enhance biological control strategies via the attraction and retention of natural enemies to reduce insect pest populations and crop damage in an environmentally friendly way. In this review, we focus on ways to improve the efficiency of kairomone use in crop fields. First, we highlight kairomone sources in tri-trophic systems and discuss how these attractants are used by natural enemies searching for hosts or prey. Then we summarise examples of field application of kairomones (pheromones vs. HIPVs) in recruiting natural enemies. We highlight the need for future field studies to focus on the application of kairomone blends rather than single kairomones which currently dominate the literature on field attractants for natural enemies. We further discuss ways for improving kairomone use through attract and reward technique, olfactory associative learning, and optimisation of kairomone lure formulations. Finally, we discuss why the effectiveness of kairomone use for enhancing biological control strategies should move from demonstration of increase in the number of attracted natural enemies, to reducing pest populations and crop damage below economic threshold levels and increasing crop yield.

Synergistic Effects of Volatiles from Host-Infested Plants on Host-Searching Behavior in the Parasitoid Wasp Lytopylus rufipes (Hymenoptera: Braconidae)

Herbivore-induced plant volatiles (HIPVs) are important cues for natural enemies to find their hosts. HIPVs are usually present as blends and the effects of combinations of individual components are less studied. Here, we investigated plant volatiles in a tritrophic system, comprising the parasitoid wasp Lytopylus rufipes Nees (Hymenoptera: Braconidae), the Oriental fruit moth Grapholita molesta (Busck) (Lepidoptera: Tortricidae), and Japanese pear, Pyrus pyrifolia 'Kosui', so as to elucidate the effects of single components and blends on wasp behaviors. Bioassays in a four-arm olfactometer, using either shoots or their isolated volatiles collected on adsorbent, revealed that female wasps preferred volatiles from host-infested shoots over those from intact shoots. Analyses identified (Z)-3-hexenyl acetate (H), linalool (L), (E)-β-ocimene (O), (E)-3,8-dimethyl-1,4,7-nonatriene (D), and (E,E)-α-farnesene (F). Among them, only F was induced by infestation with G. molesta. When tested singly, only O and D elicited positive responses by L. rufipes. Binary blends of HO and DF elicited a positive response, but that of HD elicited a negative one, even though D alone elicited a positive response. Remarkably, wasps did not prefer either the ODF or HL blends, but showed a highest positive response to a quinary blend (HLODF). These results show that synergism among volatiles released from host-infested plants is necessary for eliciting high behavioral responses in L. rufipes, enabling L. rufipes to find its host efficiently.

Genotypic Variation and Phenotypic Plasticity in Gene Expression and Emissions of Herbivore-Induced Volatiles, and their Potential Tritrophic Implications, in Cranberries

Herbivorous insects are important problems in cranberry (Vaccinium macrocarpon Ait.) production. The use of chemical pesticides is common practice, but beneficial insects such as natural enemies of herbivores (e.g. predators and parasitoids) could be affected as well. Therefore, we studied the defensive mechanisms that cranberry plants use to combat pests, focusing on herbivore-induced plant volatiles (HIPVs), which can be used to recruit predators and parasitoids foraging for prey or hosts. Then, we used synthetic HIPVs to test the attraction of natural enemies. In a greenhouse, we assessed nine cranberry genotypes for expression of genes involved in HIPV biosynthesis and/or emission of HIPVs. In an experimental field, we assessed whether baiting traps with individual or combinations of HIPVs increased attractiveness to natural enemies. The results showed that different cranberry genotypes vary in their emission of monoterpenes and sesquiterpenes but not in their expression of two genes associated with terpene biosynthesis, α-humulene/β-caryophyllene synthase and (3S,6E)-nerolidol/R-linalool synthase. Induction with methyl jasmonate or herbivore (gypsy moth, Lymantria dispar L.) feeding increased the expression of these genes and emission of HIPVs. The HIPV methyl salicylate (MeSA), alone or in combination with other HIPVs, increased syrphid attraction by 6-fold in the field, while (Z)-3-hexenyl acetate and MeSA repelled ladybeetles and megaspilids, respectively. Linalool and β-caryophyllene elicited no behavioral responses of natural enemies. Elucidating the mechanisms of pest resistance, as well as experimentally augmenting plant defenses such as HIPVs, may contribute to the development of more sustainable pest management practices in crops, including cranberries.