Solid-phase microextraction-based cuticular hydrocarbon profiling for intraspecific delimitation in Acyrthosiphon pisum

Cuticular Hydrocarbons of Six Geographic Populations of Ips subelongauts in Northeastern China: Similarities and Evolutionary Hints

The Asian larch bark beetle, I. subelongatus Motschulsky, is a severe pest of various Larix species in its natural range. This study reports the degree of similarity among cuticular hydrocarbon (CHC) profiles of six populations of I. subelongatus in northeastern China. Thirty individual or isomeric mixtures of hydrocarbons were identified by solid-phase microextraction (SPME) coupled with gas chromatography-mass spectrometry (GC-MS). The hydrocarbon components consist of straight-chain alkanes, alkenes, and methyl-branched hydrocarbons with carbon chain lengths ranging from 24 to 31. Among these, four CHCs (n-C25, 9-C27:1, n-C27, and 3-meC27) can serve as chemotaxonomic markers to identify I. subelongatus. No qualitative differences were detected between males and females in any of the six populations, but significant quantitative differences were observed in some CHCs. Phylogenetic analysis based on CHC profiles showed only minor differences compared to analysis based on partial mtDNA COI sequences regarding bark beetle species affinity. These results establish a rapid chemotaxonomic method and provide a basis for further investigations into the functions of CHCs in I. subelongatus.

Rapid Identification of Aphid Species by Headspace GC-MS and Discriminant Analysis

Aphids are a ubiquitous group of pests in agriculture that cause serious losses. For sustainable aphid identification, it is necessary to develop a precise and fast aphid identification tool. A new simple chemotaxonomy approach to rapidly identify aphids was implemented. The method was calibrated in comparison to the established phylogenetic analysis. For chemotaxonomic analysis, aphids were crushed, their headspace compounds were collected through closed-loop stripping (CLS) and analysed using gas chromatography-mass spectrometry (GC-MS). GC-MS data were then subjected to a discriminant analysis using CAP12.exe software, which identified key biomarkers that distinguish aphid species. A dichotomous key taking into account the presence and absence of a set of species-specific biomarkers was derived from the discriminant analysis which enabled rapid and reliable identification of aphid species. As the method overcomes the limits of morphological identification, it works with aphids at all life stages and in both genders. Thus, our method enables entomologists to assign aphids to growth stages and identify the life history of the investigated aphids, i.e., the food plant(s) they fed on. Our experiments clearly showed that the method could be used as a software to automatically identify aphids.

Rapid Changes in Composition and Contents of Cuticular Hydrocarbons in Sitobion avenae (Hemiptera: Aphididae) Clones Adapting to Desiccation Stress

Cuticular hydrocarbons (CHCs) are diverse in insects, and include variable classes of cuticular lipids, contributing to waterproofing for insects under desiccation environments. However, this waterproofing function of CHCs is still not well characterized in aphids. In this study, we compared CHC profiles for desiccation-resistant and nonresistant genotypes of the grain aphid, Sitobion avenae (Fabricius), in responses to desiccation. Our result showed that a total of 27 CHCs were detected in S. avenae, and linear alkanes (e.g., n-C29) were found to be the predominant components. Long-chain monomethyl alkanes were found to associate closely with water loss rates in S. avenae in most cases. Resistant genotypes of both wing morphs had higher contents of short-chain n-alkanes under control than nonresistant genotypes, showing the importance of short-chain n-alkanes in constitutive desiccation resistance. Among these, n-C25 might provide a CHC signature to distinguish between desiccation-resistant and nonresistant individuals. Compared with linear alkanes, methyl-branched CHCs appeared to display higher plasticity in rapid responses to desiccation, especially for 2-MeC26, implying that methyl-branched CHCs could be more sensitive to desiccation, and play more important roles in induced desiccation-resistance. Thus, both constitutive and induced CHCs (linear or methyl-branched) can contribute to adaptive responses of S. avenae populations under desiccation environments. Our results provide substantial evidence for adaptive changes of desiccation resistance and associated CHCs in S. avenae, and have significant implications for aphid evolution and management in the context of global climate change.

Drosophila melanogaster Stress Odorant (dSO) Displays the Characteristics of an Interspecific Alarm Cue

Organisms depend on visual, auditory, and olfactory cues to signal the presence of danger that could impact survival and reproduction. Drosophila melanogaster emits an olfactory alarm signal, termed the Drosophila stress odorant (dSO), in response to mechanical agitation or electric shock. While it has been shown that conspecifics avoid areas previously occupied by stressed individuals, the contextual underpinnings of the emission of, and response to dSO, have received little attention. Using a binary choice assay, we determined that neither age and sex of emitters, nor the time of the day, affected the emission or avoidance of dSO. However, both sex and mating status affected the response to dSO. We also demonstrated that while D. melanogaster, D. simulans, and D. suzukii, have different dSO profiles, its avoidance was not species-specific. Thus, dSO should not be considered a pheromone but a general alarm signal for Drosophila. However, the response levels to both intra- and inter-specific cues differed between Drosophila species and possible reasons for these differences are discussed.

Lipophorin receptor regulates the cuticular hydrocarbon accumulation and adult fecundity of the pea aphid Acyrthosiphon pisum

Cuticular hydrocarbons form a barrier that protects terrestrial insects from water loss via the epicuticle. Lipophorin loads and transports lipids, including hydrocarbons, from one tissue to another. In some insects, the lipophorin receptor (LpR), which binds to lipophorin and accepts its lipid cargo, is essential for female fecundity because it mediates the incorporation of lipophorin by developing oocytes. However, it is unclear whether LpR is involved in the accumulation of cuticular hydrocarbons and its precise role in aphid reproduction remains unknown. We herein present the results of our molecular characterization, phylogenetic analysis, and functional annotation of the pea aphid (Acyrthosiphon pisum) LpR gene (ApLpR). This gene was transcribed throughout the A. pisum life cycle, but especially during the embryonic stage and in the abdominal cuticle. Furthermore, we optimized the RHA interference (RNAi) parameters by determining the ideal dose and duration for gene silencing in the pea aphid. We observed that the RNAi-based ApLpR suppression significantly decreased the internal and cuticular hydrocarbon contents as well as adult fecundity. Additionally, a deficiency in cuticular hydrocarbons increased the susceptibility of aphids to desiccation stress, with decreased survival rates under simulated drought conditions. Moreover, ApLpR expression levels significantly increased in response to the desiccation treatment. These results confirm that ApLpR is involved in transporting hydrocarbons and protecting aphids from desiccation stress. Furthermore, this gene is vital for aphid reproduction. Therefore, the ApLpR gene of A. pisum may be a novel RNAi target relevant for insect pest management.

Cuticular Hydrocarbon Plasticity in Three Rice Planthopper Species

Insect cuticular hydrocarbons (CHCs) are organic compounds of the surface lipid layer, which function as a barrier against water loss and xenobiotic penetration, while also serving as chemical signals. Plasticity of CHC profiles can vary depending upon numerous biological and environmental factors. Here, we investigated potential sources of variation in CHC profiles of Nilaparvata lugens, Laodelphax striatellus and Sogatella furcifera, which are considered to be the most important rice pests in Asia. CHC profiles were quantified by GC/MS, and factors associated with variations were explored by conducting principal component analysis (PCA). Transcriptomes were further compared under different environmental conditions. The results demonstrated that CHC profiles differ among three species and change with different developmental stages, sexes, temperature, humidity and host plants. Genes involved in cuticular lipid biosynthesis pathways are modulated, which might explain why CHC profiles vary among species under different environments. Our study illustrates some biological and ecological variations in modifying CHC profiles, and the underlying molecular regulation mechanisms of the planthoppers in coping with changes of environmental conditions, which is of great importance for identifying potential vulnerabilities relating to pest ecology and developing novel pest management strategies.

Host Plant Species of Bemisia tabaci Affect Orientational Behavior of the Ladybeetle Serangium japonicum and Their Implication for the Biological Control Strategy of Whiteflies

Serangium japonicum Chapin (Coleoptera: Coccinellidae) is a predominant predator with a preference for Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae). To date, the orientational behavior of S. japonicum toward B. tabaci-infested plants has seldom been reported. In this study, greenhouse cage experiments and bioassays with wind tunnels, a Y-tube olfactometer and B. tabaci-induced plant volatiles were executed to clarify this behavior. In greenhouse cage experiments, B. tabaci adults significantly preferred eggplant, cucumber and tobacco to cotton and tomato, whereas S. japonicum adults preferred B. tabaci-infested eggplant, cucumber and cotton to tobacco and tomato. In wind tunnel bioassays, B. tabaci showed a clear preference for eggplant, cucumber and tobacco. Compared with B. tabaci-infested eggplant, cucumber or cotton, B. tabaci-infested tobacco was rarely visited by S. japonicum. In Y-tube bioassays, S. japonicum did not distinguish between B. tabaci-infested and uninfested eggplant. Nine common plant volatiles were detected in different plant species, suggesting that these volatiles may play an important role in the process by which S. japonicum looks for prey. In light of the current results, we discuss the implications of our findings and put forward to a new strategy—i.e., an eggplant + B. tabaci + S. japonicum system—to control B. tabaci damage in the integrated management of whitefly.

Phenotypic Plasticity of Cuticular Hydrocarbon Profiles in Insects

The insect integument is covered by cuticular hydrocarbons (CHCs) which provide protection against environmental stresses, but are also used for communication. Here we review current knowledge on environmental and insect-internal factors which shape phenotypic plasticity of solitary living insects, especially herbivorous ones. We address the dynamics of changes which may occur within minutes, but may also last weeks, depending on the species and conditions. Two different modes of changes are suggested, i.e. stepwise and gradual. A switch between two distinct environments (e.g. host plant switch by phytophagous insects) results in stepwise formation of two distinct adaptive phenotypes, while a gradual environmental change (e.g. temperature gradients) induces a gradual change of numerous adaptive CHC phenotypes. We further discuss the ecological and evolutionary consequences of phenotypic plasticity of insect CHC profiles by addressing the question at which conditions is CHC phenotypic plasticity beneficial. The high plasticity of CHC profiles might be a trade-off for insects using CHCs for communication. We discuss how insects cope with the challenge to produce and "understand" a highly plastic, environmentally dependent CHC pattern that conveys reliable and comprehensible information. Finally, we outline how phenotypic plasticity of CHC profiles may promote speciation in insects that rely on CHCs for mate recognition.