Spread and potential host range of the invasive oak lace bug [Corythucha arcuata (Say, 1832) – Heteroptera: Tingidae] in Eurasia

1. The North American oak lace bug feeds on leaves of ‘white oaks” in its native range. In Europe, it was first discovered in northern Italy in 2000. In recent years, it has subsequently spread rapidly and population outbreaks have been observed in several European countries. In the present study, we summarize the steps of its expansion.
2. To predict its potential host range, we checked 48 oak species in 20 sentinel gardens in seven countries between 2013 and 2018.
3. In total, 27 oak species were recorded as suitable hosts; 13 of them are globally new ones, 23 out of the 29 in section Quercus (～ white oaks, an intrageneric taxonomic unit within genus Quercus), including Asian oaks, native to Japan, Korea and China, and four out of five in section Cerris (another intrageneric unit of the same genus), were accepted as hosts. None of the species in section Lobatae (red oaks) or in the Ilex group was accepted.
4. Host records were also collected in forest stands of 10 countries. We found 11 oak species that were infested. Outbreak populations were most commonly found on Quercus robur, Quercus frainetto, Quercus petraea and Quercus cerris, comprising widespread and outstandingly important oaks species in Europe.
5. Based on our findings, we conclude that suitable hosts for oak lace bug are present in most of Europe and Asia. This means that a lack of hosts will likely not restrict further range expansion.

     

Tree species identity and forest composition affect the number of oak processionary moth captured in pheromone traps and the intensity of larval defoliation

1. Pure forests are often seen as being more prone to damage by specialist pest insects than mixed forests, and particularly mixed forests associating host and nonhost species. We addressed the effect of tree diversity on oak colonization and defoliation by a major specialist pest, the oak processionary moth (OPM)
2. We quantified the number of male OPM moths captured and larval defoliation in pure stands of two oak host species (Quercus robur and Quercus petraea) and in mixed stands associating the two oak species or each oak species with another nonhost broadleaved species. We conducted two complementary studies to test the effect of host species and stand composition: (i) we used pheromone trapping to compare the number of males OPM captured throughout the distribution of oak hosts in France and (ii) we noted the presence of OPM nests and estimated defoliation in mature forests of north‐eastern France.
3. Oak species and stand composition significantly influenced the number of male OPM captured and defoliation by OPM larvae. Quercus petraea was consistently more attractive to and more defoliated by OPM than Q. robur. Both oak trees were attacked more in pure stands than in mixed stands, in particular mixed stands associating oaks with another (nonhost) broadleaved species.
4. The results of the present study support the view that mixed forests are more resistant to specialist pest insects than pure stands, and also indicate that this trend depends on forest composition. Our study provides new insights into OPM ecology and has potential implications for forest management, including the management of urban forests where OPM causes serious human health issues.

     

Suitability of native and ornamental oak species in California for Agrilus auroguttatus

Goldspotted oak borer, Agrilus auroguttatus Schaeffer (Coleoptera: Buprestidae), is a new invasive species in southern California, USA. The extent of the host range of this insect is not known, but this knowledge will have a major impact on assessment of the risks that this pest poses to oaks [Quercus spp. (Fagaceae)]. We conducted laboratory tests to determine the potential suitability of native and ornamental oak species for larvae and adults of A. auroguttatus. We infested 179 cut logs (from 163 different trees) with eggs or larvae, measured neonate survival and, after 5 months, counted feeding galleries, and noted the proportion of galleries with late instars. Initial larval survival was generally high when larvae penetrated the phloem (range: 62–98% among oak species), and low by the time larvae began to feed at the phloem/xylem interface (range: 0–25% among oak species). The majority of larvae that established a visible feeding gallery survived to the fourth instar (total of 73% for all oak species). Larval galleries were established with greater frequency in red oaks (Section Lobatae) compared with other oaks (19 vs. 7 or 4%). All red oaks tested (Q. agrifolia Née, Q. kelloggii Newberry, and Q. wislizeni A. DC.) were likely suitable hosts for larvae. Larvae were apparently able to feed on some of the other oaks (Q. chrysolepis Leibmann, Q. suber L., Q. lobata Née, and Q. douglasii Hook & Arn), although it remains unclear whether these species would be preferred hosts under natural conditions. Adult longevity and fecundity varied little by species of oak foliage fed to adults. The host range of A. auroguttatus is likely limited by suitability of oak species for the larval rather than the adult life stage. Results support published field observations that red oaks are more suitable hosts than white oaks.     

Risk posed by the invasive defoliator Uraba lugens to New Zealand native flora

• 1 The risk posed to New Zealand native flora by the recently‐established pest of Australian origin Uraba lugens Walker (Lepidoptera: Nolidae) (gum leaf skeletonizer) was assessed. Weed biological control host range testing methods were applied to identify those New Zealand plant species potentially at risk. Native plants tested were primarily in the Myrtaceae, the family that contains all the Australian hosts of U. lugens.
• 2 Experimental methods included no‐choice larval feeding assays and field cage and laboratory oviposition trials. Difficulty in ascertaining reliable oviposition preference data from cage oviposition trials limited the confidence with which the invader's field host range could be predicted.
• 3 Field surveys of plants attacked by U. lugens in the infested area supported the initial predictions but only some of the at‐risk native Myrtaceae were present in the area.
• 4 The risk to native New Zealand plants is presented in terms of two mechanisms: development of self‐sustaining populations and temporary spill‐over of solitary larvae. Development of self‐sustaining populations of the pest within native forests is deemed to be highly improbable.
• 5 Temporary spill‐over impacts are most likely in urban areas within mixed species plantings or boundaries between native/exotic forests and coinciding with the mobile larval life stages. Spill‐over impacts from U. lugens have only been recorded to date on Metrosideros excelsa.

     

A glimpse at future forests: predicting the effects of <Emphasis Type="Italic">Phytophthora ramorum</Emphasis> on oak forests of southern Appalachia

The highly pathogenic Phytophthora ramorum, causal organism of sudden oak death (SOD), is established in forests of the Pacific Northwest (USA) and is threatening invasion of other regions. Given the breadth of its host range, with dozens of asymptomatic ornamental hosts and with oaks, Quercus spp., in the red oak (Erythrobalanus) subgenus particularly susceptible, we investigated the consequences of its invasion and establishment in oak-dominated deciduous forests of the eastern USA. We evaluated the nature and extent of pathogen invasion using vegetation assessments coupled with growth simulations. The woody plant community was assessed in three strata (upper, mid- and lower) and was used to characterize forest composition and structure. Using the Forest Vegetation Simulator (FVS), we then projected woody vegetation growth 50 years into the future with and without the effects of SOD. In forest simulations lacking pathogen invasion, little change in composition or structure is forecasted. Both red oaks and white oaks (subgenus Leucobalanus) increase slightly but significantly over the length of the simulation. In contrast, in SOD-affected forests our projections predict a significant loss of red oaks within 10 years of pathogen invasion. Basal area of white oaks and non-oaks is expected to increase more so in the absence of red oaks. The loss of red oaks to pathogen infection will result in greater increases in red maple, Acer rubrum, and yellow poplar, Liriodendron tulipifera, than in forests free of SOD. Loss of red oak represents a significant loss of hard mast, with potentially devastating consequences for wildlife. Red oak loss will also affect decomposition rates, nutrient cycling, forest structure, and timber values, with consequences for forest health and sustainability.     

Density-related foraging behaviour in Closterocerus tricinctus, a parasitoid of the leaf-mining moth, Cameraria hamadryadella

• 1 We examined the foraging behaviour of the parasitoid wasp, Closterocerus tricinctus (Ashmead) (Chalcidoidea: Eulophidae), as it visited larvae of the leaf-mining moth, Cameraria hamadryadella (Lepidoptera: Gracillariidae), in an outbreak population.
• 2 We tracked females of C.tricinctus, recording the time spent searching for mines and handling host larvae. The density of leaf-mines (host larvae) and their condition were recorded for each leaf visited. A subset of leaves visited by C.tricinctus was enclosed in fine mesh bags so that foraging success could be determined by rearing or dissection. The average density of mines and the average leaf-area mined was estimated for a random sample of leaves from each tree.
• 3 The selection of leaves upon which to forage appears to be density-dependent. C.tricinctus visits leaves with leaf-mine densities twice the average, and when switching leaves lands directly on leaf-mines 5 times more often than expected assuming random landings.
• 4 The total time spent foraging on a leaf, the average time spent handling hosts, and the total search time within leaves tend to decline on leaves with many hosts, but the observed declines are not statistically significant.
• 5 The proportion of leaf-mines visited within a leaf is strongly inversely density-dependent. 30% of visits to leaf-mines are re-visits and 29% of handling time is spent re-handling previously visited hosts. Furthermore, only 21% of visits to mines lead to successful parasitism. We suggest that self-interference and the avoidance behaviour of the host may reduce the number of visits of leaf-mines by C.tricinctus within a leaf.
• 6 The effect of the strongly inversely density-dependent foraging investment within leaves is to offset the observed density-dependent pattern of leaf visitation making the overall spatial pattern of visitation by C.tricinctus to mines of C.hamadryadella inversely density-dependent.
• 7 We suggest that the uncertainty of C.tricinctus surviving on multiply mined leaves because of density-dependent host mortality due to intraspecific competition in high-density host populations, the rarity of high-density host populations, and the rarity of multiply-mined leaves in low-density host populations combine to select against an aggregative response within leaves by C.tricinctus.

     

Interspecific and intraspecific diversity in oak powdery mildews in Europe: coevolution history and adaptation to their hosts

Quercus has been reported as the genus with the largest number of attacking powdery mildews. In Europe, oak powdery mildew was rarely reported before 1907, when severe outbreaks were observed. These epidemics were attributed to the newly described species Erysiphe alphitoides, presumed to be of exotic origin. After the burst of interest following the emergence of the disease, research on this topic remained very limited. Interest in research was recently reactivated in response to the availability of molecular tools. This review summarizes current knowledge on the diversity of oak powdery mildews in Europe and their possible evolutionary relationships with European oaks. The most striking results are the evidence of cryptic diversity (detection in France of a lineage closely related to Erysiphe quercicola, previously thought to only have an Asian distribution), large host range (similarity of E. alphitoides and E. quercicola with powdery mildews of tropical plants) but also local adaptation to Quercus robur. These recent findings highlight the complexity of the history of oak powdery mildew in Europe and point to the question of host specialization and host jumps in the evolution of powdery mildew fungi.     

Ten years after: Release cutting around old oaks still affects oak vitality and saproxylic beetles in a Norway spruce stand

1. It is often suggested to release oaks (Quercus robur) from competition, to ensure their survival and boost their conservational value. However, few studies have explored how long-lasting this effect is and how it affects saproxylic beetles.
2. Ten years after cutting, we investigated effects of different release cutting levels (high, medium, and no release) around 140-year-old retained oaks in a commercial forest with Norway spruce (Picea abies).
3. We evaluated oak vitality using crown and dead wood measurements. Saproxylic beetles were caught in window traps, identified to species level, and grouped according to their association with oak and/or Norway spruce.
4. Released oaks had more light, higher temperatures, greater crown growth, and less dead wood in the crown compared to the no-release control.
5. After 10 years we still found a higher abundance of oak-associated beetles and higher overall species richness of saproxylic beetles in the released oaks. Beetle species composition significantly differed between released oaks and control.
6. We suggest avoiding planting trees beneath retained oaks when regenerating conifer forests and to proceed with conservation management during subsequent thinning by removing regeneration under the oak crowns. We emphasize the benefits of monitoring the retained oaks to maintain tree vitality, habitat quality, and insolation.

     

Leaf morphological analyses in four European oak species (Quercus) and their hybrids: A comparison of traditional and geometric morphometric methods

Abstract

In this study, leaf morphology was assessed in a mixed oak stand (western France) using two geometric morphometric (landmark and outline) datasets and one dataset of 19 leaf measures. Adult oaks (817 oaks), comprising four white oak species (Quercus petraea, Q. robur, Q. pubescens and Q. pyrenaica), were sampled for DNA extraction and genetic analysis (nuclear microsatellites). Leaf morphology was assessed on 336 oaks, comprising pure species and hybrids as determined by genetic assignment. This comparative study of oak leaf morphology, based on the use of two free size geometric morphometric methods and a set of leaf measurements, combined with the genetic assignment of individuals to pure species or hybrids, provided information about the differences among species and the intermediate leaf morphology of their hybrids.     

Does the periodical cicada,Magicicada septendecim,prefer to oviposit on native or exotic plant species?

Abstract 1. In 2004, Brood X of the periodical cicada (Magicicada spp.) emerged in Delaware. Extensive suburban development and concomitant planting of exotic species has occurred since the previous emergence of Brood X in 1987. 2. Exotic species could suffer extensive damage during years of cicada emergences if they are preferred for oviposition. Alternately, a shortage of suitable plant hosts may negatively affect remaining cicada populations. 3. We determined if the periodical cicada, Magicicada septendecim, preferred to oviposit on native or exotic woody plant species. Potential hosts were divided into three groups and planted in a randomised design near a likely source of cicadas. The first group, Natives, included 15 species native to Delaware. The second group, Non‐natives, included 15 exotic species that had a native congener represented in the Native group. The final group, Aliens, included 13 exotic species that did not have a native congener. 4. Based on observations of 428 plants, cicadas were more likely to oviposit on Natives or Non‐natives, which did not differ from each other in this measure, than on Aliens. Non‐natives had more oviposition holes per metre than Natives, which had more holes per metre than Aliens. The likelihood of stem flagging was lowest on Alien species; the percentage of the total plant that flagged was greatest for Natives. Plant morphology also influenced host preference and likelihood of flagging. 5. Taking plant morphology into consideration, we speculate that the evolutionary history between periodical cicadas and potential hosts is an important component of host preference.     

Responses of a native beetle to novel exotic plant species with varying invasion history

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The effects of host plant phenology on the demography and population dynamics of the leaf-mining moth, Cameraria hamadryadella (Lepidoptera: Gracillariidae)

Abstract.

• 1 We examined the effects of variation in the timing of spring leaf production and autumn leaf fall on the survival, mortality and abundance of Cameraria hamadryadella on Quercus alba and Q.macrocarpa.
• 2 We monitored and manipulated the timing of foliation on field and potted Q.alba trees and observed the abundance of C.hamadryadella on those trees. We also monitored and manipulated the timing of leaf fall on Q.alba and Q.macrocarpa trees in the field and observed its effects on survival, mortality and abundance of C.hamadryadella.
• 3 Variation in the timing of spring leaf production has no effect on C. hamudryadella abundance. However, a warm winter and spring in 1991 led to accelerated development and the imposition of a facultative third generation in one out of ten years of observation.
• 4 In 1989, leaves fell relatively early and leaf fall in the autumn accounted for more than 50% of the mortality of C.hamudryadella. in 1990 and 1991 leaves fell relatively late and leaf fall induced mortality was substantially reduced and overwinter survival was markedly increased.
• 5 The abundance of C.hamadryadella remained constant in the spring and summer of 1990 following the previous autumn's relatively early leaf fall, but increased by 10-fold in the spring of 1991 following the relatively late leaf fall of autumn 1990. The abundance of C.hamadryadella also increased 4-fold between the summer of 1991 and the spring of 1992 after another autumn of relatively late leaf fall. We attribute these increases in abundance in part to reduced mortality because of later leaf fall.
• 6 Variation in the timing of autumn leaf fall may be responsible for initiating outbreaks of C.hamadryadella.

     

Restoration of Florida scrub vegetation in an old field through 23 years after planting

Florida scrub, a fire‐maintained, xeromorphic shrubland, hosts many rare and declining species, but most scrub habitat has been lost to development and agriculture. Formerly cultivated sites offer an opportunity to restore Florida scrub. In this 23‐year study, we test whether scrub species could be restored to an old field and whether scrub vegetation composition and structure could be reestablished over time. Scrub oaks were planted in a section of an old field in 1992 after the site was cleared and treated with herbicide. Additional oaks and other scrub species were planted in 1993. We determined survival and growth annually of a marked sample of scrub species. We sampled vegetation cover in two height strata annually on 10 permanent line‐intercept transects in the old grove and on 20 transects in adjacent, intact scrub. Initial survival of Quercus geminata exceeded that of Q. chapmanii or Q. myrtifolia. Cover of scrub oaks greater than 0.5 m increased from 1.3% in 1992 to 65.2% in 2010. However, the vegetation structure of large Q. geminata did not resemble that of native scrub. Adaptive management led to cutting large oaks in the fall of 2010, and burning the site in early 2011. Cutting and burning appeared to stimulate sprouting and clonal spread of scrub oaks. Ordination analysis indicated directional change related to increasing scrub oak cover and time since planting but the old field still differed from intact scrub. Vegetation has developed toward scrub composition and structure but exotic grasses persist.     

Factors that affect compartmentalization and wound closure of Quercus rubra infested with Enaphalodes rufulus

• 1 The mechanisms by which hardwood trees resist invasion by native wood borers are still poorly understood.
• 2 We examined the importance of several host, herbivore and environmental variables in relation to Quercus rubra L., northern red oak, resistance to a native cerambycid, Enaphalodes rufulus (Haldeman) (Coleoptera: Cerambycidae), the red oak borer.
• 3 We employed tree‐ring techniques to date and measure two indicators of host tree physical resistance, vertical lesion size and wound closure of scars left by larval galleries, within 107 Q. rubra hosts from eight sites within the Ozark and Ouachita National Forests of Arkansas.
• 4 Host Q. rubra decline status proved a valuable predictor of resistance, wherein healthy trees exhibited better compartmentalization capacity, as indicated by shorter lesions and faster wound closure, than declining trees and those that died during a recent E. rufulus outbreak.
• 5 Only healthy Q. rubra exhibited a significant declining lesion length trend from 1988 to 2006 and were likely able to re‐allocate resources in favour of maintaining resistance as the feeding pressure from borers increased and then relaxed post‐outbreak.
• 6 Variables related to borer feeding pressure consistently explained more variation in lesion length than host or environmental variables. Hosts were better able to compartmentalize injury from small, unsuccessful E. rufulus galleries than large, successful galleries from which larvae survived to adulthood.
• 7 Healthy Q. rubra, as survivors of the recent outbreak, were able to effectively tolerate feeding pressure from borers at the same time as sustaining growth and maintenance. Tolerance may be an important resistance mechanism for hardwood hosts against the native wood borers that attack them.

     

Attachment may be a basis for specialization in oak aphids

ABSTRACT.

• 1 The potential role of tarsal attachment in host selection was studied in the specialist aphids Tuberculoides annulatus Hartig and Myzocallis schreiberi Hille Ris Lambers and Stroyan.
• 2 M.schreiberi could walk freely on its host, Quercus ilex and on Q.robur, whereas T.annulatus could walk freely on its host, Q.robur but had difficulty on Q. ilex.
• 3 Attachment to the rough-textured leaves of Q.robur was achieved by means of the pretarsal claws, and to the smooth upper surface of individual trichomes on the lower surface of Q.ilex using flexible pretarsal empodia.
• 4 Both behavioural and allometric differences can account for the inability of T.annulatus to grip onto Q.ilex.
• 5 The role of attachment by phytophagous insects in host plant resistance and selection is discussed.

     

Fungal Endophytic Communities in Healthy and Declining Quercus robur L. and Q. cerris L. Trees in Northern Italy

Healthy and declining English oak (Quercus robur) and Turkey oak (Q. cerris) in north‐western Italy, in a plain oak forest showing decline for oak puzzle disease, were tested to assess possible variations in the composition of their fungal endophytic communities and their relation to the health status of trees. Samples collected in spring (buds) and in autumn (leaves, annual shoots and twigs) were surface‐sterilized, cut into fragments and placed on potato dextrose agar for a month; 26 fungal species were isolated, the most frequent being Tubakia dryina, Dendrodochium sp., Eutypella sp. and a sterile mycelium. Correspondence analysis showed significant qualitative differences between assemblages inhabiting twigs and herbaceous tissues that were due to the low frequency of Tubakia dryina in twigs and its higher frequency in buds, leaves and shoots. Tubakia dryina was isolated more frequently from leaves of declining oaks and from buds of healthy oaks; Monochaetia monochaeta showed a preference for healthy trees, especially leaves and buds. According to the Shannon–Wiener index, endophytic fungal communities of leaves, twigs and buds of declining English oak were poorer than those of declining Turkey oak, but there were no significant differences between healthy hosts.     

Assemblages of native bryophytes in secondary forests with introduced Quercus rubra

We have studied the diversity of bryophytes in planted Polish post‐agricultural forests dominated by the native Scots pine Pinus sylvestris and the alien (North American) red oak Quercus rubra. The planted sites would be suitable for a mesic coniferous forest (abbreviation: CFS) or mesic broadleaved forest (abbreviation: BFS). We analysed the structure and composition of the bryophyte assemblages in relation to forest site and substrate availability. Special attention was paid to the introduced Q. rubra as a host species for native bryophytes. A total of 54 bryophyte species (9 liverworts and 45 mosses) were found in the 90 plots ( = phytosociological relevés, 10 × 10 m in area; 45 at each forest site) studied. DCA analysis showed that the bryophyte assemblages of the P. sylvestris–Q. rubra secondary forest community differed significantly between CFS and BFS sites; the similarity of the composition of bryophyte species was 36.8%. The substrate preferences (epigeic, epixylic, epiphytic), as well as the growth form and life form of the recorded bryophytes, also differed between CFS and BFS, while the proportion of bryophytes that had a particular life strategy was very similar. The introduced Q. rubra was inhabited by 28 bryophyte species, including two liverworts. This tree hosted 64% of the CFS and 47% of BFS bryophyte flora and as a host for epiphytes the species successfully fulfilled the functional role of the native oaks (Q. robur and Q. petraea). Thus, the introduction of Q. rubra may contribute to the restoration of post‐agricultural forests and to the conservation of epiphytic bryophyte species. On the other hand, the negative impact of Q. rubra observed on the ground flora (including bryophytes) puts the benefits of Q. rubra for the conservation of native biodiversity in general in question.     

Leaf buds, a factor in host selection by Battus philenor butterflies

ABSTRACT.

• 1 Field and laboratory experiments identified a character intrinsic to Aristolochia reticulata Nutt. host plants, the terminal leaf bud, that is involved in host-selection behaviour by female pipevine swallowtail butterflies (Battus philenor L.) searching for oviposition sites.
• 2 In the field, the frequency with which females landed on non-host buds declined seasonally as the proportion of host foliage that consisted of buds decreased. Female butterflies did not land on non-host species in proportion to their abundance; rather, females landed on those non-host species whose buds resembled those of A.reticulata.
• 3 A.reticulata plants whose terminal leaf bud was concealed by plastic tape were less susceptible to oviposition in the field than were control plants.
• 4 Female butterflies released in a large, outdoor enclosure were conditioned to search for leaf buds only when exposed to a host species bearing a prominent terminal leaf bud.
• 5 The significance of conditioning of leaf-bud searching behaviour is discussed with respect to discrimination between hosts and non-hosts, between host species, and among plants within a host species.

     

Flooding effects on starch partitioning during early growth of two oak species

Pedunculate (Quercus robur L.) and sessile (Q. petraea [Matt.] Liebl.) oaks are the most common oak species in Western Europe. They are known to display different ecological requirements, particularly relative to root hypoxia induced by flooding: In a glasshouse study of seedlings, we quantified the effects of flooding on starch mobilization from cotyledons and starch partitioning. Growth and distribution of lateral roots were also measured. The above-ground growth of Q. robur was less affected by flooding than that of Q. petraea which failed to develop a second flush. Root growth was also severely inhibited, particularly in Q. petraea. In Q. robur, lateral root initiation as well as elongation was restricted to the soil surface layer. Flooding markedly reduced total growth and concentrations of in all components except stems. Starch mobilization from cotyledons was delayed by flooding, especially in Q. robur seedlings. Under flooding, the decrease of cotyledons dry mass and starch content in Q. robur was lower than in Q. petraea, whereas Q. robur displayed larger growth than Q. petraea. The features of carbohydrate management may be crucial in the observed differences in flooding tolerance of these species.     

Hybridization of European oaks (Quercus ilex × Q. robur) results in a mixed isoprenoid emitter type

European oaks have been reported to emit isoprene or monoterpenes derived from recently fixed photosynthetic carbon. The emission type is plant species specific and can be used as chemo‐taxonomic marker. In the present article the isoprenoid biochemical properties of mature Quercus × turneri‘Pseudoturneri’ hybrids resulting from a crossing of a Mediterranean evergreen monoterpene‐emitting species (subgenus Sclerophyllodrys; Quercus ilex L.) and an isoprene‐emitting deciduous oak species (subgenus Lepidobalanus; Quercus robur L.) are described. Both species are compared with respect to the capacity for isoprenoid synthesis and the actual isoprenoid emission pattern of different tree‐types. The analysis showed that the oak hybrid combines properties of both parental species. Furthermore, it could be shown that the enzyme activities of isoprene synthase and monoterpene synthases are reflected in the isoprenoid emission pattern of the hybrids as well as in the observed emission rates.