Volatile organic compounds produced by human skin cells

Skin produces volatile organic compounds (VOCs) released to the environment with emission patterns characteristic of climatic conditions. It could be thought that these compounds are intermediaries in cell metabolism, since many intermediaries of metabolic pathways have a volatile potential. In this work, using gas chromatography, we answered the question of whether VOC profiles of primary cultures of human dermal fibroblasts were affected by the type of culture conditions. VOCs were determined for different types of culture, finding significant differences between skin cells grown in classical monolayer culture -2D- compared with 3D matrix immobilized cultures. This indicates that VOC profiles could provide information on the physiological state of skin cells or skin.     

Genetic variation in plant volatile emission does not result in differential attraction of natural enemies in the field

Volatile organic chemical (VOC) emission by plants may serve as an adaptive plant defense by attracting the natural enemies of herbivores. For plant VOC emission to evolve as an adaptive defense, plants must show genetic variability for the trait. To date, such variability has been investigated primarily in agricultural systems, yet relatively little is known about genetic variation in VOCs emitted by natural populations of native plants. Here, we investigate intraspecific variation in constitutive and herbivore-induced plant VOC emission using the native common milkweed plant (Asclepias syriaca) and its monarch caterpillar herbivore (Danaus plexippus) in complementary field and common garden greenhouse experiments. In addition, we used a common garden field experiment to gauge natural enemy attraction to milkweed VOCs induced by monarch damage. We found evidence of genetic variation in the total constitutive and induced concentrations of VOCs and the composition of VOC blends emitted by milkweed plants. However, all milkweed genotypes responded similarly to induction by monarchs in terms of their relative change in VOC concentration and blend. Natural enemies attacked decoy caterpillars more frequently on damaged than on undamaged milkweed, and natural enemy visitation was associated with higher total VOC concentrations and with VOC blend. Thus, we present evidence that induced VOCs emitted by milkweed may function as a defense against herbivores. However, plant genotypes were equally attractive to natural enemies. Although milkweed genotypes diverge phenotypically in their VOC concentrations and blends, they converge into similar phenotypes with regard to magnitude of induction and enemy attraction.     

In Vivo Volatile Organic Compound Signatures of Mycobacterium avium subsp. paratuberculosis

Mycobacterium avium ssp. paratuberculosis (MAP) is the causative agent of a chronic enteric disease of ruminants. Available diagnostic tests are complex and slow. In vitro, volatile organic compound (VOC) patterns emitted from MAP cultures mirrored bacterial growth and enabled distinction of different strains. This study was intended to determine VOCs in vivo in the controlled setting of an animal model. VOCs were pre-concentrated from breath and feces of 42 goats (16 controls and 26 MAP-inoculated animals) by means of needle trap microextraction (breath) and solid phase microextraction (feces) and analyzed by gas chromatography/ mass spectrometry. Analyses were performed 18, 29, 33, 41 and 48 weeks after inoculation. MAP-specific antibodies and MAP-specific interferon-γ-response were determined from blood. Identities of all marker-VOCs were confirmed through analysis of pure reference substances. Based on detection limits in the high pptV and linear ranges of two orders of magnitude more than 100 VOCs could be detected in breath and in headspace over feces. Twenty eight substances differed between inoculated and non-inoculated animals. Although patterns of most prominent substances such as furans, oxygenated substances and hydrocarbons changed in the course of infection, differences between inoculated and non-inoculated animals remained detectable at any time for 16 substances in feces and 3 VOCs in breath. Differences of VOC concentrations over feces reflected presence of MAP bacteria. Differences in VOC profiles from breath were linked to the host response in terms of interferon-γ-response. In a perspective in vivo analysis of VOCs may help to overcome limitations of established tests.     

Analyses of short-chain fatty acids and exhaled breath volatiles in dietary intervention trials for metabolic diseases

As an alternative to pharmacological treatment to diseases, lifestyle interventions, such as dietary changes and physical activities, can help maintain healthy metabolic conditions. Recently, the emerging analyses of volatile organic compounds (VOCs) from breath and short-chain fatty acids (SCFAs) from plasma/feces have been considered as useful tools for the diagnosis and mechanistic understanding of metabolic diseases. Furthermore, diet-induced changes of SCFAs in individuals with diagnosed metabolic abnormalities have been correlated with the composition changes of the gut microbiome. More interestingly, the analysis of exhaled breath (breathomics) has gained attention as a useful technique to measure the human VOC profile altered as a result of dietary interventions. In this mini-review, we examined recent clinical trials that performed promising dietary interventions, SCFAs analysis in plasma/feces, and VOC profile analysis in exhaling breath to understand the relationship between dietary intervention and metabolic health.     

Impacts of nitrogen fertilization on volatile organic compound emissions from decomposing plant litter

Nonmethane volatile organic compounds (VOCs) are reactive, low molecular weight gases that can have significant effects on soil and atmospheric processes. Research into biogenic VOC sources has primarily focused on plant emissions, with few studies on VOC emissions from decomposing plant litter, another potentially important source. Likewise, although there have been numerous studies examining how anthropogenic increases in nitrogen (N) availability can influence litter decomposition rates, we do not know how VOC emissions may be affected. In this study, we measured the relative contribution of VOCs to the total carbon (C) emitted from decomposing litter and how N amendments affected VOC emissions. We incubated decomposing litter from 12 plant species over 125 days, measuring both CO₂ and VOC emissions throughout the incubation. We found that VOCs represented a large portion of C emissions from a number of the litter types with C emissions as VOCs ranging from 0% to 88% of C emissions as CO₂. Methanol was the dominant VOC emitted, accounting for 28–99% of total VOC emissions over the incubation period. N additions increased CO₂ production in 7 of the 12 litter types by 5–180%. In contrast, N additions decreased VOC emissions in 8 of the 12 litter types, reducing net VOC emissions to near zero. The decrease in VOC emissions was occasionally large enough to account for the increased CO₂ emissions on a per unit C basis, suggesting that N additions may not necessarily accelerate C loss from decomposing litter but rather just switch the form of C emitted. Together these results suggest that, for certain litter types, failure to account for VOC emissions may lead to an underestimation of C losses from litter decomposition and an overestimation of the effects of N additions on rates of litter decomposition.     

Microbial consumption and production of volatile organic compounds at the soil-litter interface

Substantial amounts of volatile organic compounds (VOCs) can be released during decomposition and these compounds can affect atmospheric chemistry, belowground processes, and the structure of microbial communities in litter and soil. However, we have a limited understanding of the types, quantities and ecological impacts of VOCs emitted from litter. Here we used a closed flow-through system and proton transfer reaction mass spectrometry (PTR-MS) to characterize VOC emissions from soil and two litter types (Pinus taeda and Acer rubrum) over a 72-day incubation period. Microbial respiration rates were measured throughout the incubation, and the soils were harvested at the end of the incubation to determine how litter VOCs influenced soil C dynamics, N mineralization rates, and bacterial communities. Using the PTR-MS we identified over 100 VOCs, with 10 VOCs making up the majority of emissions. VOCs accounted for up to 2.5% of the C flux from litter. Soil was a net sink of litter VOCs, absorbing up to 80% of VOCs released by litter, and exposure of soil to litter VOCs increased microbial respiration rates in soil by up to 15%. However, we observed negligible impacts of litter VOCs on soil nutrient levels and bacterial community structure, suggesting that soils must be exposed to higher concentrations of VOCs than observed in our study, to cause effects on these soil characteristics. Overall, VOCs appear to have an important influence on C dynamics at the soil-litter interface and VOC emissions from decomposing litter may represent an understudied component of biosphere–atmosphere interactions.     

Chemodiversity in the Fingerprint Analysis of Volatile Organic Compounds (VOCs) of 35 Old and 7 Modern Apple Cultivars Determined by Proton‐Transfer‐Reaction Mass Spectrometry (PTR‐MS) in Two Different Seasons

Volatile organic compounds (VOCs) are chemical species that play an important role in determining the characteristic aroma and flavor of fruits. Apple (Malus × domestica Borkh .) cultivars differ in their aroma and composition of VOCs. To determine varietal differences in the aroma profiles, VOCs emitted by 7 modern and 35 old apple cultivars were analyzed using Proton Transfer Reaction Mass Spectrometry (PTR‐MS). PTR‐MS is a rapid, reproducible, and non‐destructive spectrometric technique for VOC analysis of single fruits, developed for direct injection analysis. In the present study, we analyzed the differences in the emission of VOCs from single fruits at harvest and after a storage period of 60±10 days, followed by 3 d of shelf life. Our results show that VOC profile differences among apple cultivars were more pronounced after storage than at harvest. Furthermore, chemodiversity was higher in old cultivars compared to modern cultivars, probably due to their greater genetic variability. Our data highlight the importance of storage and shelf life are crucial for the development of the typical aroma and flavor of several apple cultivars. The validity of the method is demonstrated by comparison of two different harvest years.     

Exhaled volatile organic compounds in patients with non-small cell lung cancer: cross sectional and nested short-term follow-up study

Background

Non-invasive diagnostic strategies aimed at identifying biomarkers of lung cancer are of great interest for early cancer detection. The aim of this study was to set up a new method for identifying and quantifying volatile organic compounds (VOCs) in exhaled air of patients with non-small cells lung cancer (NSCLC), by comparing the levels with those obtained from healthy smokers and non-smokers, and patients with chronic obstructive pulmonary disease. The VOC collection and analyses were repeated three weeks after the NSCLC patients underwent lung surgery.

Methods

The subjects'' breath was collected in a Teflon^® bulb that traps the last portion of single slow vital capacity. The 13 VOCs selected for this study were concentrated using a solid phase microextraction technique and subsequently analysed by means of gas cromatography/mass spectrometry.

Results

The levels of the selected VOCs ranged from 10^-12 M for styrene to 10^-9 M for isoprene. None of VOCs alone discriminated the study groups, and so it was not possible to identify one single chemical compound as a specific lung cancer biomarker. However, multinomial logistic regression analysis showed that VOC profile can correctly classify about 80 % of cases. Only isoprene and decane levels significantly decreased after surgery.

Conclusion

As the combination of the 13 VOCs allowed the correct classification of the cases into groups, together with conventional diagnostic approaches, VOC analysis could be used as a complementary test for the early diagnosis of lung cancer. Its possible use in the follow-up of operated patients cannot be recommended on the basis of the results of our short-term nested study.     

Translation of exhaled breath volatile analyses to sport and exercise applications

Background

Exhaled breath gases are becomingly increasingly investigated for use as non-invasive measurements for clinical diagnosis, prognosis and therapeutic monitoring. Exhaled volatile organic compounds (VOCs) in the breath, which make up the exhaled volatilome, offer a rich sample medium that provides both information to external exposures as well as endogenous metabolism. For these reasons, exhaled breath analyses can be extended further beyond disease-based investigations, and used for wider biomarker measurement purposes. The use of a rapid, non-invasive (and potentially non-physically demanding) test in an exercise and/or sporting situation may provide additional information for translation to performance sport, recreational exercise/fitness and clinical exercise health.

Aim of review

This review intends to provide an overview into the initial exploration of exhaled VOC measurements in sport and exercise science, and understand current limitations in knowledge and instrumentation that have restricted these methodologies in becoming common practice.

Key scientific concepts of review

Exhaled VOCs have been applied to sport/exercise investigations with a current emphasis on measurement of chemical exposure during and/or following exercise. This includes the measurement of disinfection by-products from chlorine-disinfected swimming pools, as well as exposure to petrochemicals from combustion engines (e.g. vehicle fumes). However, exhaled VOC measurements have been less employed in the context of performance sport. For example, the application of exhaled VOCs to map biochemical/physiological processes of intense exercise is currently under explored and warrants further study. Nevertheless, there is promise for exhaled VOC testing in the development of rapid/on-line anti-doping screens, with initial steps taken in this field.

     

Exploratory Cumulative Risk Assessment (CRA) Approaches Using Secondary Data

Cumulative risk assessments (CRAs) include the examination of risks posed by multiple stressors and include population-specific vulnerabilities and susceptibilities. In this case study, we assess potential hearing impairment hazard due to joint exposure from noise and volatile organic compounds (VOCs) in order to examine the strengths and limitations of using secondary data on exposure and health effects for a CRA. Block group-level noise categories were estimated using modeled street-level data. A quantile regression model of sociodemographic and personal predictors from the 1999–2000 U.S. National Health and Nutrition Examination Survey VOC dataset was used along with block group-level sociodemographic and personal variables to estimate VOC exposures. Hazard indices (HIs) for potential hearing impairment due to joint noise and VOC exposures were calculated. County-averaged HIs for hearing impairment ranged from 0.8 (10th total VOCs percentile and 45–60 dB) to 1.7 (90th total VOCs percentile and 71–75 dB). Limitations of the exposure and health effects data included issues combining heterogeneous data and a lack of established threshold levels for combined low-level exposures; yet, this case study illustrates that screening-level CRAs, including non-chemical stressors, can be accomplished with publicly available data and existing methods.     

Petrol exhaust pollution impairs honey bee learning and memory

Volatile organic compounds (VOCs) serve as important infochemicals, mediating several ecological interactions including herbivory and pollination. Atmospheric pollutants including traffic‐related air pollution may impair the detection of VOCs used by insects in insect–plant interactions. We investigated the indirect effect of petrol exhaust pollution on olfactory learning and memory (short and long term) in honey bees. Using appetitive olfactory conditioning, we trained bees to learn one of four floral VOC profiles; linalool, dipentene, myrcene and geranium. VOCs were unpolluted or polluted with exhaust collected from a petrol generator. Exhaust emissions included concentrations of CO (246.07 + 17 ppm), NO (20.50 + 6.90 ppb) and NO2 (20.93 + 0.05 ppb) consistent with those typically encountered in urban areas and near roads. Once bees had learnt the training VOC, we tested whether they could recognise that VOC 1 h, 24 h and 48 h post‐training. Bees took significantly longer to learn polluted VOCs and forgot them faster than unpolluted ones. We also tested the ‘masking’ potential of pollution on floral VOCs. Using gas chromatography mass spectroscopy we noted differences in the chemical profile of polluted versus unpolluted VOCs and tested whether bees could recognise polluted VOCs if trained using unpolluted ones. For several VOCs tested, bees could distinguish between polluted and unpolluted VOCs. Ultimately, our results show that air pollution changes the recognition and retention of floral VOCs by bees which may consequently impact foraging efficiency.     

Temporary vs. Permanent Sub-slab Ports: A Comparative Performance Study

Vapor intrusion (VI) is the migration of subsurface vapors, including radon and volatile organic compounds (VOCs), from the subsurface to indoor air. The VI exposure pathway extends from the contaminant source, which can be impacted soil or groundwater, to indoor air-exposure points. VOC contaminants of concern typically include halogenated solvents as well as petroleum hydrocarbons. Radon, a colorless radioactive gas that is released by radioactive decay of radionuclides in rock and soil, migrates into homes through VI in a similar fashion to VOCs. This project focused on the performance of permanent versus temporary sub-slab sampling ports for the determination of VI of halogenated VOCs and radon into an unoccupied house. VOC and radon concentrations measured simultaneously in soil gas using collocated temporary and permanent ports appeared to be independent of the type of port. The variability between collocated temporary and permanent ports was much less than the spatial variability between different locations within a single residential duplex. Post sampling leak test results suggested that the temporary SSP desiccation and cracking of the clay portion of the seal were not as detrimental to the port seal performance as would have been expected, this suggests that the Teflon tape portion of the seals served an important function. Pre and post sampling leak tests are advisable when temporary ports are used to collect a time-integrated sample. These results suggest that temporary sub-slab sampling ports can provide data equivalent to that collected from a permanent sub-slab sampling port.     

Immobilization materials mixed with activated sludge as column biofilters for the treatment of gaseous stream containing benzene and toluene

Activated sludge has been utilized for the treatment of volatile organic compounds (VOCs) which are emitted from industrial processes. Nevertheless, activated sludge systems often suffer from the problem caused by concentration gradients as well as pressure drops. Channeling is also a major problem in the treatment process. As the bed height of the packed activated sludge system increases, the pressure drop increases accordingly. To solve these problems, we proposed immobilized activated sludge column reactors for treating VOCs in air. The immobilization material used to mix with activated sludge was properly selected in this work. Elemental compositions of these materials were analyzed. In this study, we also proposed a VOC feed system so that more stable inlet concentrations could be achieved. Hence, the equipment and operating costs were reduced and the problem of VOCs leaking from peristaltic pumps was avoided. The moisture content of the system was well maintained and better VOC removal efficiency was achieved. With an operation condition of progressive VOC inlet concentrations, better removal efficiency of benzene and toluene was then obtained. In conclusion, by the utilization of immobilization materials selected from wastes as well as immobilized activated sludge column reactors, significant removal efficiency for both benzene and toluene was demonstrated.     

THE EVOLUTION OF FLORAL SCENT AND OLFACTORY PREFERENCES IN POLLINATORS: COEVOLUTION OR PRE‐EXISTING BIAS?

Coevolution is thought to be a major factor in shaping plant-pollinator interactions. Alternatively, plants may have evolved traits that fitted pre-existing preferences or morphologies in the pollinators. Here, we test these two scenarios in the plant family of Araceae and scarab beetles (Coleoptera, Scarabaeidae) as pollinators. We focused on floral volatile organic compounds (VOCs) and production/detection of VOCs by scarab beetles. We found phylogenetic structure in the production/detection of methoxylated aromatics in scarabs, but not plants. Within the plants, most of the compounds showed a well-supported pattern of correlated evolution with scarab-beetle pollination. In contrast, the scarabs showed no correlation between VOC production/detection and visitation to Araceae flowers, with the exception of the VOC skatole. Moreover, many VOCs were found in nonpollinating beetle groups (e.g., Melolonthinae) that are ancestors of pollinating scarabs. Importantly, none of the tested VOCs were found to have originated in pollinating taxa. Our analysis indicates a Jurassic origin of VOC production/detection in scarabs, but a Cretaceous/Paleocene origin of floral VOCs in plants. Therefore, we argue against coevolution, instead supporting the scenario of sequential evolution of floral VOCs in Araceae driven by pre-existing bias of pollinators.     

Investigation of volatile organic biomarkers derived from Plasmodium falciparum in vitro

ABSTRACT: BACKGROUND: There remains a need for techniques that improve the sensitive detection of viable Plasmodium falciparum as part of diagnosis and therapeutic monitoring in clinical studies and usual-care management of malaria infections. A non-invasive breath test based on P. falciparum-associated specific volatile organic compounds (VOCs) could fill this gap and provide insights into parasite metabolism and pathogenicity. The aim of this study was to determine whether VOCs are present in the headspace above in vitro P. falciparum cultures. METHODS: A novel, custom-designed apparatus was developed to enable efficient headspace sampling of infected and non-infected cultures. Conditions were optimized to support cultures of high parasitaemia (>20%) to improve the potential detection of parasite-specific VOCs. A number of techniques for VOC analysis were investigated including solid phase micro-extraction using two different polarity fibres, and purge and trap/thermal desorption, each coupled to gas chromatography-mass spectrometry. Each experiment and analysis method was performed at least on two occasions. VOCs were identified by comparing their mass spectra against commercial mass spectral libraries. RESULTS: No unique malarial-specific VOCs could be detected relative to those in the control red blood cell cultures. This could reflect sequestration of VOCs into cell membranes and/or culture media but solvent extractions of supernatants and cell lysates using hexane, dichloromethane and ethyl acetate also showed no obvious difference compared to control non-parasitized cultures. CONCLUSIONS: Future in vivo studies analysing the breath of patients with severe malaria who are harbouring a parasite biomass that is significantly greater than achievable in vitro may yet reveal specific clinically-useful volatile chemical biomarkers.     

Extension of the SAEM algorithm for nonlinear mixed models with 2 levels of random effects

This article focuses on parameter estimation of multilevel nonlinearmixed-effects models (MNLMEMs). These models are used to analyzedata presenting multiple hierarchical levels of grouping (clusterdata, clinical trials with several observation periods, ...).The variability of the individual parameters of the regressionfunction is thus decomposed as a between-subject variabilityand higher levels of variability (e.g. within-subject variability).We propose maximum likelihood estimates of parameters of thoseMNLMEMs with 2 levels of random effects, using an extensionof the stochastic approximation version of expectation–maximization(SAEM)–Monte Carlo Markov chain algorithm. The extendedSAEM algorithm is split into an explicit direct expectation–maximization(EM) algorithm and a stochastic EM part. Compared to the originalalgorithm, additional sufficient statistics have to be approximatedby relying on the conditional distribution of the second levelof random effects. This estimation method is evaluated on pharmacokineticcrossover simulated trials, mimicking theophylline concentrationdata. Results obtained on those data sets with either the SAEMalgorithm or the first-order conditional estimates (FOCE) algorithm(implemented in the nlme function of R software) are compared:biases and root mean square errors of almost all the SAEM estimatesare smaller than the FOCE ones. Finally, we apply the extendedSAEM algorithm to analyze the pharmacokinetic interaction oftenofovir on atazanavir, a novel protease inhibitor, from theAgence Nationale de Recherche sur le Sida 107-Puzzle 2 study.A significant decrease of the area under the curve of atazanaviris found in patients receiving both treatments.     

Quantification of 31 volatile organic compounds in whole blood using solid-phase microextraction and gas chromatography-mass spectrometry

The prevalence of exposure to volatile organic compounds (VOCs) has raised concern about possible health effects resulting from chronic human exposure. To support studies exploring the relation between VOC exposure and health effects, we developed an automated analytical method using solid-phase microextraction (SPME), capillary gas chromatography (GC), and quadrupole mass spectrometry (MS). This method quantifies trace levels (low parts per trillion) of 14 halogenated alkanes, 5 halogenated alkenes, 10 aromatic compounds, and 2 other VOCs in human blood. Detection limits for the SPME-GC-MS method range from 0.005 to 0.12 microg/L, with linear calibration curves spanning three orders of magnitude. The improved throughput of this method will enable us to expand biomonitoring efforts to assess nonoccupational VOC exposure in large epidemiological studies.     

Differential accumulation of volatile terpene and terpene synthase mRNAs during lavender (Lavandula angustifolia and L. x intermedia) inflorescence development

Despite the commercial importance of Lavandula angustifolia Mill. and L. x intermedia Emeric ex Loisel floral essential oils (EOs), no information is currently available on potential changes in individual volatile organic compound (VOC) content during inflorescence development. Calyces were found to be the main sites of VOC accumulation. The 20 most abundant VOCs could be separated into three sub‐groups according to their patterns of change in concentration The three groups of VOCs sequentially dominated the global scent bouquet of inflorescences, the transition between the first and second groups occurring around the opening of the first flower of the inflorescence and the one between the second and third groups at the start of seed set. Changes in calyx VOC accumulation were linked to the developmental stage of individual flowers. Leaves accumulated a smaller number of VOCs which were a subset of those seen in preflowering inflorescences. Their nature and content remained constant during the growing season. Quantitative real time polymerase chain reaction assessments of the expression of two terpene synthase (TPS) genes, LaLIMS and LaLINS, revealed similar trends between their patterns of expression and those of their VOC products. Molecular and chemical analyses suggest that changes in TPS expression occur during lavender inflorescence development and lead to changes in EO composition. Both molecular data and terpene analysis support the findings that changes in biosynthesis of terpene occurred during inflorescence development.     

(Pre)analytical imprecision, between-subject variability, and daily variations in serum and urine hepcidin: Implications for clinical studies

The utility of urine and serum hepcidin measurements in the clinic depends on their reproducibility. We sought to expand our previous work on the within-subject variability and between-subject variability of this novel iron parameter in the serum and urine of 24 healthy controls by time-of-flight mass spectrometry at four different time points during the day. A linear mixed model for repeated data was used to distinguish three components of the total variability in the measurements: within-day/within-subject variability, between-subject variability, and additional residual or (pre)analytical variability. Differences in diurnal hepcidin patterns were observed between urine and serum. Urine levels remained similar during the course of the morning and increased significantly during the afternoon, whereas serum levels increased significantly throughout both the morning and afternoon. Furthermore, in serum the (pre)analytical variability (28.6%) was smaller than the between-subject (48.1%) and within-day/within-subject variability (30.3%) compared with urine variability (97.2% vs. 67.7 and 77.3%, respectively). High serum ferritin levels were associated with higher serum hepcidin levels but not with urine levels. Transferrin saturation did not correlate with hepcidin levels. To minimize variability, we recommend (i) standardizing for sampling time and (ii) measuring serum hepcidin levels.     

Emission of volatile organic compounds by apple trees under spider mite attack and attraction of predatory mites

Emission rates of volatile organic compounds (VOCs) from Pirus malus L. subsp. mitis (Wallr.) var. Golden Delicious and var. Starking attacked by the phytophagous mite Panonychus ulmi Koch, and their attractiveness to the predatory mites Amblyseius andersoni Chant and Amblyseius californicus McGregor, were studied during three years. A large variability was found in the emission of individual VOCs depending on the infestation, the apple tree variety and the date. There were larger total VOC emission rates and larger total VOC leaf concentrations in apple trees attacked by phytophagous mites, especially in the var. Starking. In infested trees of this variety, there were also more predatory mites. An olfactometer assay showed that predatory mites preferentially chose branches infested by Panonychus ulmi (85% went to infested branches vs 15% to uninfested control branches) indicating that volatiles may be used as cues to find their prey.