Plant-associated CO2 mediates long-distance host location and foraging behaviour of a root herbivore

Life-stage dependent behavior mimics chemosensory repertoire diversity in a belowground, specialist herbivore

Insects rely on the translation of environmental chemical cues into behaviors necessary for survival and reproduction. Specific chemosensory receptors belonging to the odorant and gustatory receptor groups detect odorant and gustatory cues, respectively, making them crucial to these processes. How odorant (OR) and gustatory (GR) receptor expression profiles change in combination with changing life strategies is not well understood. Using genomic and transcriptomic resources, we annotated the OR and GR expression profiles across all life stages of the western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte, a major pest of corn in the United States and Europe. Genomic analyses identified 193 ORs and 189 GRs, of which 125 and 116 were found to be expressed, respectively, in one or more WCR life stages. WCR larvae are subterranean and feed on roots before emerging as adults aboveground. Expression profile analyses revealed first instar larvae possess a unique OR and GR repertoire distinct from other instars and adults, suggesting a role in host plant finding. Similarly, a subset of ORs and GRs differed in their expression levels between adult male and female antennae. By comparing the phylogenetic relationship of ORs and GRs, we identified several receptors with potentially important roles in WCR foraging and reproductive behavior. Together, this study provides support for future investigations into the ecology and evolution of chemoreception in insects.

Social evolution and absence of olfactory function in larval honey bees

Social evolution made larval honey bees dependent on adult colony members for feeding. We therefore predicted they have diminished olfactory capabilities, and based on organismal resource conservation theory, also have downregulated olfactory receptor (OR) gene expression. Behavioral assays demonstrated that larvae cannot find food via olfaction and expressed very low levels of Orco , an essential gene for OR function. By contrast, larvae showed higher expression of Ir25a, an essential gene for multiple forms of sensory perception including gustation. These results suggest larvae rely on taste for feeding. In addition, considering that adult bees use OR-based olfaction extensively, they demonstrate strong developmental regulation of the OR system. Comparative transcriptomics of social and non-social insects further highlight the role of social evolution in shaping this sensory trait.

Wireworms (Coleoptera: Elateridae) Use Root Volatiles and CO2 to Discriminate Among Host Plants

In the absence of visual signals, subterranean arthropods rely on olfactory and tactile cues to navigate toward resources. Here, in a series of pairwise dual-choice bioassays, we investigated the Limonius californicus (Col., Elateridae) larva response to wheat, pea, and bean seedlings in sand-filled olfactometers. We then quantified volatile organic compounds (VOC) emitted from roots. Wireworm preference for beans compared to wheat was attributed to the higher CO2 emission. Wireworm preference for peas compared to wheat was attributed to the higher amounts (µg/hr) of hexanal emitted from pea roots. Wireworms preferred synthetic hexanal over clean air control and the higher amount of hexanal (200 µg) over the lower amount of 20 µg. In the presence of CO2 at both ends of the olfactometer, wireworms did not respond preferentially toward hexanal. 2-Hexenal was also attractive to wireworms relative to the control, but wireworms did not discriminate between hexanal and 2-hexenal. While our results confirmed wireworms' positive response to the presence of CO2 and some VOCs in isolation, their host choice appears to be driven by the combination and the concentrations of the present cues, allowing the insect to distinguish among host plants.

Reticulitermes flavipes (Blattodea: Rhinotermitidae) Response to Wood Mulch and Workers Mediated by Attraction to Carbon Dioxide

The eastern subterranean termite, Reticulitermes flavipes, is challenged by the significant energy expenditures of tunnel construction for resource discovery. Subterranean termites use idiothetic mechanisms to explore large spaces, while the use of resource-specific cues for localized search is disputed. Here, termite response to wood mulch, termite workers, extracts of wood mulch, and CO2 alone were tested using a bioassay design that distinguished between attraction and arrestment. Termites showed significant attraction to wood mulch with workers or to wood mulch alone. They did not respond to workers alone at the initial dose tested, but were attracted to workers at higher densities. Termites did not respond to water or the acetone extracts of wood mulch, but did show a partial response to hexane extract compared to intact wood mulch. More significantly, when CO2 was removed from the emissions of wood mulch and workers using soda lime, attraction was eliminated. Furthermore, termites showed a quadratic response to CO2 concentration that peaked at ca. 14,000 ppm. The response to CO2 alone predicted by the model matched termite response to mulch + workers when compared at the level of CO2 they emitted. The results suggest that CO2 is both necessary and sufficient to explain the attraction response of R. flavipes to mulch and workers we observed. It is argued that orientation to food cues complements the previously demonstrated idiothetic program to maximize the efficiency of resource location.

The Genomics Revolution Drives a New Era in Entomology

Thanks to the fast development of sequencing techniques and bioinformatics tools, sequencing the genome of an insect species for specific research purposes has become an increasingly popular practice. Insect genomes not only provide sets of gene sequences but also represent a change in focus from reductionism to systemic biology in the field of entomology. Using insect genomes, researchers are able to identify and study the functions of all members of a gene family, pathway, or gene network associated with a trait of interest. Comparative genomics studies provide new insights into insect evolution, addressing long-lasting controversies in taxonomy. It is also now feasible to uncover the genetic basis of important traits by identifying variants using genome resequencing data of individual insects, followed by genome-wide association analysis. Here, we review the current progress in insect genome sequencing projects and the application of insect genomes in uncovering the phylogenetic relationships between insects and unraveling the mechanisms of important life-history traits. We also summarize the challenges in genome data sharing and possible solutions. Finally, we provide guidance for fully and deeply mining insect genome data.

The polyvalent sequestration ability of an economically important beetle

Many specialized herbivorous insects sequester single classes of toxic secondary metabolites from their host plants as protection against natural enemies. If and how herbivores can use multiple classes of plant toxins across the large chemical diversity of plants for self-protection is unknown. We show that the polyphagous adults of the beetle Diabrotica virgifera are capable of selectively accumulating benzoxazinoids, cucurbitacins, and glucosinolates but not cyanogenic glycosides. Female beetles transfer the sequestered defense metabolites into their eggs, protecting them against generalist predators. Eggs containing a mixture of toxins are better protected than eggs with individual toxins. This work shows how herbivores can exploit plant chemical diversity to their own benefit as a novel adaptive mechanism that contributes to the structuring of multitrophic interaction networks.

Identification and functional characterization of chemosensory genes in olfactory and taste organs of Spodoptera litura (Lepidoptera: Noctuidae)

The tobacco cutworm Spodoptera litura is one of the most destructive polyphagous crop pests. Olfaction and taste play a crucial role in its host plant selection and sexual communication, but the expression profile of chemosensory genes remains unclear. In this study, we identified 185 chemosensory genes from 7 organs in S. litura by transcriptome sequencing, of which 72 genes were published for the first time, including 27 odorant receptors (ORs), 26 gustatory receptors (GRs), 1 ionotropic receptor (IR), 16 odorant-binding proteins (OBPs), and 2 chemosensory proteins (CSPs). Phylogenetic analyses revealed that ORs, IRs, OBPs, and sensory neuron membrane proteins (SNMPs) were mainly expressed in antennae and sequence-conserved among Noctuidae species. The most differentially expressed genes (DEGs) between sexes were ORs and OBPs, and no DEGs were found in GRs. GR transcripts were enriched in proboscis, and the expression of sugar receptors was the highest. Carbon dioxide receptors, sugar receptor-SliuGR6, and bitter GRs-SlituGR43 and SlituGR66 had higher sequence identities between Noctuidae species. CSPs were broadly expressed in various organs, and SlituCSP13 was a DEG in adult antennae. The functional analysis in the Drosophila OR67d expression system found that SlituOR50, a receptor highly expressed in female antennae, is selectively tuned to farnesyl acetate. The results provide a solid foundation for understanding the molecular mechanisms by which chemosensory genes operate to elicit behavioral responses in polyphagous insects.

Transcriptomic and Gene Expression Analysis of Chemosensory Genes from White Grubs of Hylamorpha elegans (Coleoptera: Scarabaeidae), a Subterranean Pest in South America

Olfaction and gustation processes play key roles in the life cycle of insects, such as finding and accepting food sources, oviposition sites, and mates, among other fundamental aspects of insect development. In this context, chemosensory genes found in sensory organs (e.g., antennae and maxillary palps) are crucial for understanding insect behaviour, particularly the phytophagous behaviour of insect pests that attack economically important crops. An example is the scarab beetle Hylamorpha elegans, which feeds on the roots of several crops important for livestock in its larval stage. In this study, chemosensory gene candidates of H. elegans white grubs identified through the head transcriptome and phylogenetic and tissue-biased gene expression (antennae, head without antennae, and legs) have been reported. Overall, 47 chemosensory genes were identified (2 ORs, 1 GR, 11 IRs, 9 CSPs, and 24 OBPs). Gene expression analysis revealed the predominant presence of IRs in the legs, whereas ORs and the GR were present in the heads and/or antennae. Particularly, HeleOBP9 and HeleCSP2 were significantly expressed in the head but not in the antennae or legs; these and other genes are discussed as potential targets in the context of H. elegans management.

Host search behaviors of specialist and generalist root feeding herbivores (Diabrotica spp.) on host and non-host plants

Western, northern, Mexican, and southern corn rootworms (WCR, NCR, MCR, and SCR) are serious corn pests. We evaluated host search behavior of these pests on six plant species using a video tracking system. After a 5-min exposure to plant roots, behavioral parameters were automatically recorded and used to quantify the search behavior. The search behavior was not observed for sorghum since no neonates survived after contacting sorghum roots. After exposures to corn roots, all neonates exhibited the localized search behaviors (i.e., shortening total distance traveled, lowering movement speed, increasing turn angle, moving farther from origin) which are used to stay in and search within root systems. When larvae contacted roots of wheat, barley, oats, soybean, or controls, they expanded the search area by extending the travel path, increasing velocity, and reducing turn angles and total distance moved. The intensity of the search expansion is highly associated with the host preferences known for the four rootworm species and subspecies. Neonates of each corn rootworm exhibited distinct search behaviors. In fact, NCR larvae had the highest speed, the greatest travel path, and the lowest turn angle, whereas MCR larvae had the highest turn angle and moved faster than WCR and SCR larvae.

Dynamic environmental interactions shaped by vegetative plant volatiles

Covering: up to November 2022Plants shape terrestrial ecosystems through physical and chemical interactions. Plant-derived volatile organic compounds in particular influence the behavior and performance of other organisms. In this review, we discuss how vegetative plant volatiles derived from leaves, stems and roots are produced and released into the environment, how their production and release is modified by abiotic and biotic factors, and how they influence other organisms. Vegetative plant volatiles are derived from different biosynthesis and degradation pathways and are released via distinct routes. Both biosynthesis and release are regulated by other organisms as well as abiotic factors. In turn, vegetative plant volatiles modify the physiology and the behavior of a wide range of organisms, from microbes to mammals. Several concepts and frameworks can help to explain and predict the evolution and ecology of vegetative plant volatile emission patterns of specific pathways: multifunctionality of specialized metabolites, chemical communication displays and the information arms race, and volatile physiochemistry. We discuss how these frameworks can be leveraged to understand the evolution and expression patterns of vegetative plant volatiles. The multifaceted roles of vegetative plant volatiles provide fertile grounds to understand ecosystem dynamics and harness their power for sustainable agriculture.

A draft Diabrotica virgifera virgifera genome: insights into control and host plant adaption by a major maize pest insect

BACKGROUND

Adaptations by arthropod pests to host plant defenses of crops determine their impacts on agricultural production. The larval host range of western corn rootworm, Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae), is restricted to maize and a few grasses. Resistance of D. v. virgifera to crop rotation practices and multiple insecticides contributes to its status as the most damaging pest of cultivated maize in North America and Europe. The extent to which adaptations by this pest contributes to host plant specialization remains unknown.

RESULTS

A 2.42 Gb draft D. v. virgifera genome, Dvir_v2.0, was assembled from short shotgun reads and scaffolded using long-insert mate-pair, transcriptome and linked read data. K-mer analysis predicted a repeat content of ≥ 61.5%. Ortholog assignments for Dvir_2.0 RefSeq models predict a greater number of species-specific gene duplications, including expansions in ATP binding cassette transporter and chemosensory gene families, than in other Coleoptera. A majority of annotated D. v. virgifera cytochrome P450s belong to CYP4, 6, and 9 clades. A total of 5,404 transcripts were differentially-expressed between D. v. virgifera larvae fed maize roots compared to alternative host (Miscanthus), a marginal host (Panicum virgatum), a poor host (Sorghum bicolor) and starvation treatments; Among differentially-expressed transcripts, 1,908 were shared across treatments and the least number were between Miscanthus compared to maize. Differentially-expressed transcripts were enriched for putative spliceosome, proteosome, and intracellular transport functions. General stress pathway functions were unique and enriched among up-regulated transcripts in marginal host, poor host, and starvation responses compared to responses on primary (maize) and alternate hosts.

CONCLUSIONS

Manual annotation of D. v. virgifera Dvir_2.0 RefSeq models predicted expansion of paralogs with gene families putatively involved in insecticide resistance and chemosensory perception. Our study also suggests that adaptations of D. v. virgifera larvae to feeding on an alternate host plant invoke fewer transcriptional changes compared to marginal or poor hosts. The shared up-regulation of stress response pathways between marginal host and poor host, and starvation treatments may reflect nutrient deprivation. This study provides insight into transcriptomic responses of larval feeding on different host plants and resources for genomic research on this economically significant pest of maize.

Molecular ecology of plant volatiles in interactions with insect herbivores

Plant-derived volatile organic compounds (VOCs) play pivotal roles in interactions with insect herbivores. Individual VOCs can be directly toxic or deterrent, serve as signal molecules to attract natural enemies, and/or be perceived by distal plant tissues as a priming signal to prepare for expected herbivory. Environmental conditions, as well as the specific plant-insect interaction being investigated, strongly influence the observed functions of VOC blends. The complexity of plant-insect chemical communication via VOCs is further enriched by the sophisticated molecular perception mechanisms of insects, which can respond to one or more VOCs and thereby influence insect behavior in a manner that has yet to be fully elucidated. Despite numerous gaps in the current understanding of VOC-mediated plant-insect interactions, successful pest management strategies such as push-pull systems, synthetic odorant traps, and crop cultivars with modified VOC profiles have been developed to supplement chemical pesticide applications and enable more sustainable agricultural practices. Future studies in this field would benefit from examining the responses of both plants and insects in the same experiment to gain a more complete view of these interactive systems. Furthermore, a molecular evolutionary study of key genetic elements of the ecological interaction phenotypes could provide new insights into VOC-mediated plant communication with insect herbivores.

Formalin-casein enhances water absorbency of calcium alginate beads and activity of encapsulated Metarhizium brunneum and Saccharomyces cerevisiae

The control of root-feeding wireworms has become more challenging as synthetic soil insecticides have been progressively phased out due to environmental risk concerns. Innovative microbial control alternatives such as the so-called attract-and-kill strategy depend on the rapid and successful development of dried encapsulated microorganisms, which is initiated by rehydration. Casein is a functional additive that is already used in food or pharmaceutical industry due to its water binding capacity. Cross-linked forms such as formalin-casein (FC), exhibit altered network structures. To determine whether FC influences the rehydration of alginate beads in order to increase the efficacy of an attract-and-kill formulation for wireworm pest control, we incorporated either casein or FC in different alginate/starch formulations. We investigated the porous properties of alginate/starch beads and subsequently evaluated the activities of the encapsulated entomopathogenic fungus Metarhizium brunneum and the CO₂ producing yeast Saccharomyces cerevisiae. Adding caseins altered the porous structure of beads. FC decreased the bead density from (1.0197 ± 0.0008) g/mL to (1.0144 ± 0.0008) g/mL and the pore diameter by 31%. In contrast to casein, FC enhanced the water absorbency of alginate/starch beads by 40%. Furthermore, incorporating FC quadrupled the spore density on beads containing M. brunneum and S. cerevisiae, and simultaneous venting increased the spore density even by a factor of 18. Moreover, FC increased the total CO₂ produced by M. brunneum and S. cerevisiae by 29%. Thus, our findings suggest that rehydration is enhanced by larger capillaries, resulting in an increased water absorption capacity. Our data further suggest that gas exchange is improved by FC. Therefore, our results indicate that FC enhances the fungal activity of both fungi M. brunneum and S. cerevisiae, presumably leading to an enhanced attract-and-kill efficacy for pest control.

The effect of organic farming on water reusability, sustainable ecosystem, and food toxicity

Water is a fundamental necessity for people's well-being and the ecosystem's sustainability; however, its toxicity due to agrochemicals usage for food production leads to the deterioration of water quality. The poor water quality diminishes its reusability, thus limiting efficient water usage. Organic farming is one of the best ways that does not only reduce the deterioration of water quality but also decrease food toxicity. In organic farming, the crop is grown with no/less chemical usage. Besides, organic farming maintains biodiversity and reduces the anthropogenic footprint on soil, air, water, wildlife, and especially on the farming communities. Fields that are organically managed continuously for years have fewer pest populations and were attributed to increased biodiversity and abundance of multi-trophic interactions as well as to changes in plant metabolites. Fewer insect pests (pathogen vectors), in turn, would result in fewer crop diseases and increase crop production. This review highlights that organic farming may play a critical role in the reduction of pests and pathogens, which eventually would reduce the need for chemical reagents to protect crops, improving yield quality and water reusability.