Antennal responses of four species of tree-killing bark beetles (Coleoptera: Scolytidae) to volatiles collected from beetles,and their host and nonhost conifers

Summary. Host selection in tree-killing bark beetles (Coleoptera: Scolytidae) is mediated by a complex of semiochemical cues. Using gas chromatographic-electroantennographic detection (GC-EAD) and GC-mass spectrometric analyses, we conducted a comparative study of the electrophysiological responses of four species of tree-killing bark beetles, the Douglas-fir beetle, Dendroctonus pseudotsugae, Hopkins, the mountain pine beetle, D. ponderosae Hopkins, the spruce beetle, D. rufipennis Kirby, and the western balsam bark beetle, Dryocoetes confusus Swaine, to volatiles captured by aeration of 1) bole and foliage of four sympatric species of conifers, Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco, lodgepole pine, Pinus contorta var. latifolia Engelm., interior spruce, Picea engelmannii x glauca, and interior fir, Abies lasiocarpa x bifolia, and 2) con- and heterospecific beetles at three stages of attack. We identified 13 monoterpenes in the conifers and nine compounds in the volatiles of beetles that elicited antennal responses. There was no qualitative difference in the terpene constitution of the four species of conifers and very little difference across beetle species in their antennal response to compounds from conifers or beetles. The lack of species-specific major or minor components in conifers suggests that beetles would need to detect differences in the ratios of different compounds in conifers to discriminate among them. Attraction to hosts and avoidance of nonhost conifers may be accentuated by perception of compounds emitted by con- and heterospecific beetles, respectively. The 22 compounds identified are candidate semiochemicals with potential behavioural roles in host location and discrimination.     

New repellent semiochemicals for three species of <Emphasis Type="Italic">Dendroctonus</Emphasis> (Coleoptera: Scolytidae)

Summary. Nine compounds identified from captured volatiles of the Douglas-fir beetle, Dendroctonus pseudotsugae, the mountain pine beetle, D. ponderosae, and the spruce beetle, D. rufipennis, that elicited antennal responses in males and females of one or more of these species were tested in the field to determine behavioural activity. 1-Octen-3-ol, found in the volatiles of females of all three species decreased the response of male and female coastal and male interior D. pseudotsugae and both sexes of D. ponderosae to their aggregation pheromones. Acetophenone, identified in the volatiles of females of all three species, significantly decreased the response of interior female D. pseudotsugae. trans-Verbenol, a potent aggregation pheromone of D. ponderosae, decreased the response of both sexes of D. pseudotsugae, while 3-methyl-2-cyclohexen-1- one (MCH), the antiaggregation pheromone of D. pseudotsugae and D. rufipennis decreased the response of both sexes of D. ponderosae. While it has been demonstrated that semiochemical mediated interspecific communication occurs among bark beetles infesting the same host, this study demonstrates that beetles can perceive signals emitted by heterospecifics attacking nonhosts and can potentially use them to avoid attacking the wrong species of conifer.     

A survey of antennal responses by five species of coniferophagous bark beetles (Coleoptera: Scolytidae) to bark volatiles of six species of angiosperm trees

Summary. Using Porapak Q traps, we collected the bark volatiles of six angiosperm trees native to British Columbia: black cottonwood, Populus trichocarpa Torr. & A. Gray (Salicaceae), trembling aspen, P. tremuloides Michx. (Salicaceae), paper birch, Betula papyrifera Marsh. (Betulaceae), bigleaf maple, Acer macrophyllum Pursh (Aceraceae), red alder, Alnus rubra Bong. (Betulaceae), and Sitka alder, A. viridis ssp. sinuata (Regel) á. L?ve & D. L?ve (Betulaceae). Utilising coupled gas chromatographic-electroantennographic detection analysis, the captured volatiles were assayed for antennal responses in five species of coniferophagous bark beetles (Coleoptera: Scolytidae), sympatric with most or all of the angiosperm trees: the Douglas-fir beetle, Dendroctonus pseudotsugae Hopkins, the mountain pine beetle, D. ponderosae Hopkins, the spruce beetle, D. rufipennis (Kirby), the western balsam bark beetle, Dryocoetes confusus Swaine, and the pine engraver, Ips pini (Say). The identities of 25 antennally-active compounds were determined by coupled gas chromatographic-mass spectroscopic analysis, and co-chromatographic comparisons with authentic chemicals. The compounds identified were: hexanal, (E)-2-hexenal, (Z)-3-hexen-1-ol, 1-hexanol, heptanal, α-pinene, frontalin, benzaldehyde, β-pinene, 2-hydroxycyclohexanone, 3-carene, limonene, β-phellandrene, benzyl alcohol, (E)-ocimene, salicylaldehyde, conophthorin, guaiacol, nonanal, methyl salicylate, 4-allylanisole, decanal, thymol methyl ether, (E)-nerolidol, and dendrolasin. A number of these compounds are known semiochemicals that are active in the behaviour of other organisms, including bark beetles, suggesting a high degree of semiochemical parsimony. Antennally-active compounds ranged from seven in A. viridis to 17 in P. trichocarpa. The fewest number of compounds (9) were detected by I. pini and the largest number (24) were detected by D. pseudotsugae. Six compounds excited the antennae of all five species of bark beetles. The large number of antennally-active compounds detected in common by numerous bark beetles and present in common in numerous nonhost trees supports the hypothesis of olfaction-based recognition and avoidance of nonhost angiosperm trees during the process of host selection by coniferophagous bark beetles. Received 13 December 1999; accepted 14 March 2000     

Volatile attractant phytochemicals for a population of white-spotted longicorn beetles Anoplophora malasiaca (Thomson) (Coleoptera: Cerambycidae) fed on willow differ from attractants for a population fed on citrus

Male white-spotted longicorn beetles Anoplophora malasiaca raised on willow Salix schwerinii were preferentially attracted to the odour of wounded S. schwerinii branches when released near a female model in the laboratory. This attractiveness rapidly decreased within 2 h after wounding. Solid phase microextraction and subsequent gas chromatography analyses of volatiles from the wounded branches detected three highly volatile monoterpenes: 1,8-cineole, nerol and geraniol. Among the monoterpene compounds, nerol showed a significant attractiveness to the male beetles raised on Salix. Sesquiterpene hydrocarbons, which have been identified as attractants for a beetle population raised on citrus, were detected only in trace amounts in S. schwerinii branches. These results suggest that beetles raised on these two hosts use different odour cues for mate location.     

Weak defence in a tritrophic system: olfactory response to salicylaldehyde reflects prey specialization of potter wasps

Predatory arthropods are attracted to infochemicals emitted by their herbivore prey or by the prey’s host plants. We studied such a tritrophic system measuring the olfactory responses of three potter wasp species (Symmorphus murarius, Symmorphus gracilis, Discoelius zonalis, Hymenoptera: Eumeninae) to salicylaldehyde, sequestered as a defence compound by Chrysomela leaf beetle larvae when feeding on Salicaceae, and volatile organic compounds (VOCs) emitted by aspen (Populus tremula, Salicaceae). In electroantennographic recordings (EAG), the highly specialized S. murarius that almost exclusively feeds on larvae of Salicaceae-feeding Chrysomela species was more sensitive to salicylaldehyde than the less specialized S. gracilis, feeding on such Chrysomela species but also weevil larvae. In contrast the related D. zonalis, foraging for microlepidoptera caterpillars on various host plants, did not respond at all. Furthermore, the three wasp species responded differently to aspen VOCs in GC–MS/EAD measurements. These results indicate that the sense of smell of predatory potter wasps differs for prey and plant volatiles among related wasp species according to their degree of host specialization. The considerable differences in salicylaldehyde perception suggest that its originally defensive function has backfired as it is used by specialist potter wasps for prey location. This is an important clue on adaptive mechanisms of the highest trophic level of the well-studied evolutionary arms race among Chrysomela leaf beetles, their host plants and their enemies.     

Following a scented beetle: larval faeces as a key olfactory cue in host location of Stegobium paniceum (Coleoptera: Anobiidae) by Lariophagus distinguendus (Hymenoptera: Pteromalidae)

Lariophagus distinguendus (Hymenoptera: Pteromalidae) is a generalist solitary ectoparasitoid parasitising immature stages of at least 11 foodstuff beetles from five families, including species developing in seeds of Poaceae and Fabaceae. In this study, we tested the role of visual and olfactory stimuli affecting L. distinguendus host location in the trophic context of Stegobium paniceum (Coleoptera: Anobiidae) infesting chickpeas, Cicer arietinum (Fabales: Fabaceae). When either visual plus olfactory cues or olfactory cues alone were provided, S. paniceum-infested chickpeas were more attractive than uninfested chickpeas to naïve L. distinguendus females. Larval faeces of S. paniceum also evoked strong attraction to female wasps. Uninfested chickpeas covered with host faeces were preferred over uninfested ones without faeces, while infested chickpeas were as attractive as healthy chickpeas plus host faeces. Overall, results demonstrated that L. distinguendus females orient their host-seeking decisions mainly on the basis of faeces-borne olfactory stimuli. Further research is on-going to characterise the volatiles from S. paniceum larval faeces to evaluate their attractiveness towards L. distinguendus females.     

Influence of dung volatiles on the process of resource selection by coprophagous beetles

Summary. Most dung beetles colonize the faeces of several vertebrate species without much discrimination, and are thus often considered as polyphagous. Recent studies have provided evidence for clear feeding preferences in scarab beetles colonizing dung of herbivore species, but little is known about these insects’ abilities to discriminate among odours from faeces of various herbivores. In this study, trophic preferences were examined using blocks of pitfall traps baited with dung from four different herbivore species, i.e., sheep, cattle, horse, and red deer, in a mountainous area of south-central France. 4941 coprophagous scarabs, belonging to 27 species, were captured. Beetles were more attracted to dung of sheep (2257 individuals) than that of cattle (1294 individuals), followed by deer dung (768 individuals) and horse dung (622 individuals). Eleven of the 27 beetle species collected had significant feeding preferences for one of the four dung types. For each insect species, trophic habits did not vary between the two different sites of trapping, an open pasture and a wooded habitat. In laboratory olfactometer bioassays, scarab beetles orientated preferentially towards the dung volatiles from the dung type they preferred in the field. Trypocopris pyrenaeus, Anoplotrupes stercorosus, and Aphodius rufipes were more attracted to volatile compounds from sheep dung, Onthophagus fracticornis significantly preferred horse dung volatiles, and Aphodius haemorrhoidalis responded positively to deer dung odours. The role of dung olfactory cues in the process of resource selection by dung beetles is discussed.     

Electrophysiological responses of Dendroctonus armandi (Coleoptera: Curculionidae: Scolytinae) to volatiles of Chinese white pine as well as to pure enantiomers and racemates of some monoterpenes

To better understand the attractiveness of host tree, Chinese white pines (Pinus armandi Fr.) to Chinese white pine beetle (Dendroctonus armandi Tsai and Li), the antennal responses of D. armandi to the host volatile, as well as the pure enantiomers and racemates of some monoterpenes, were examined using an electroantennogram (EAG). EAG responses of male and female D. armandi to blended volatiles extracted from the host and some synthetic terpenes (α-phellandrene, (−)-β-pinene, (+)-α-pinene, (−)-α-pinene, (−)-camphene, β-myrcene, (S)-(−)-limonene, (+)-camphene and (R)-(+)-limonene) showed significant variation due to different compound concentrations and sex of the beetles. EAG responses to extracted blended volatiles were significantly greater in females than in males, but the EAG response was not always proportional to the volatile concentration. At lower concentrations, females responded strongly to α-phellandrene and males to (−)-β-pinene, while at higher concentrations, females responded most strongly to α-phellandrene and males to (+)-α-pinene. Females were significantly responsive to (−)-α-pinene, α-phellandrene and (−)-camphene, while males were more responsive to (S)-(−)-limonene, (+)-α-pinene and (R)-(+)-limonene. The EAG responses of the female D. armandi to the volatile oil were significantly higher than that of the males, and the infested pine volatiles could evoke higher EAG response. Most of the test compounds elicited similar responses, which suggested that several of the compounds may be used in combination by D. armandi in habitat and/or host community location at the Qinling forest ecosystem.     

Quantitative analysis of pyrrolizidine alkaloids sequestered from diverse host plants in Longitarsus flea beetles (Coleoptera, Chrysomelidae)

Summary. Several species of the flea beetles genus Longitarsus are able to sequester pyrrolizidine alkaloids (PAs) from their host plants. In five Longitarsus species we compare the concentration of PAs present in their host plants belonging to the Asteraceae or Boraginaceae with those found in the beetles. To get an estimate of the intrapopulation variability, three samples of five beetles each and five individual plants were analyzed for each comparison. A strong intrapopulation variability could be detected both among plant and beetle samples. The total concentration found in the beetles varied strongly between species. The local host plant and its phenology influence the concentrations present in the beetles as evidenced in comparisons of a single beetle species from two different hosts and of one beetle species collected at the same site at different times of the year. In addition, different beetle species apparently vary in their capacity to sequester the alkaloids, at the lowest extreme the mean PA concentration in the beetles (0.034 μg PA/mg dry weight) was 1/30 of the mean concentration found in the plant leaves (L. aeruginosus from Eupatorium cannabinum), at the highest extreme (2.098 μg PA/mg dw) the concentration in the beetles was a 1000 fold higher than in the plant leaves (L. nasturtii from Symphytum officinale). The highest mean concentration found in the beetles was 3.446 μg/mg dw (L. exoletus from Cynoglossum officinale). The absolute concentrations found in the beetles are comparable to other insects which have been shown to be effectively defended against their potential predators. Received 22 June 1999; accepted 25 August 1999     

Behavioral evidence for eavesdropping on prey song in two Palearctic sibling bat species

Eavesdropping on prey communication signals has never before been reported for a Palearctic bat species. In this study, we investigated whether lesser and greater mouse-eared bats, Myotis blythii oxygnathus and Myotis myotis, find tettigoniid bushcrickets (Tettigoniidae) by eavesdropping on their mate-attraction song. Tettigoniids are known to be the most important prey item for M. blythii oxygnathus, while carabid beetles and other epigaeic arthropods are the most important prey for its sibling species, M. myotis, in many places in Europe. M. myotis locates walking beetles by listening for their rustling sounds. We compared these two species’ response to four acoustic prey cues: calling song of two tettigoniid species, the rustling sound made by walking carabid beetles, and a control tone. Individuals of both bat species attacked the speaker playing tettigoniid song, which clearly indicates that both species eavesdrop on prey-generated advertisement signals. There were, however, species differences in response. M. blythii oxygnathus exhibited stronger predatory responses to the calling song of two species of tettigoniid than to the beetle rustling sound or the control. M. myotis, in contrast, exhibited stronger predatory responses to the beetle rustling and to one tettigoniid species but not the other tettigoniid or the control. Our study (1) for the first time demonstrates eavesdropping on prey communication signals for Palearctic bats and (2) gives preliminary evidence for sensory niche partitioning between these two sympatric sibling bat species.     

Making scents of defense: do fecal shields and herbivore-caused volatiles match host plant chemical profiles?

Many plant families have aromatic species that produce volatile compounds which they release when damaged, particularly after suffering herbivory. Monarda fistulosa (Lamiaceae) makes and stores volatile essential oils in peltate glandular trichomes on leaf and floral surfaces. This study examined the larvae of a specialist tortoise beetle, Physonota unipunctata, which feed on two M. fistulosa chemotypes and incorporate host compounds into fecal shields, structures related to defense. Comparisons of shield and host leaf chemistry showed differences between chemotypes and structures (leaves vs. shields). Thymol chemotype leaves and shields contained more of all compounds that differed than did carvacrol chemotypes, except for carvacrol. Shields had lower levels of most of the more volatile chemicals than leaves, but more than twice the amounts of the phenolic monoterpenes thymol and carvacrol and greater totals. Additional experiments measured the volatiles emitted from M. fistulosa in the absence and presence of P. unipunctata larvae and compared the flower and foliage chemistry of plants from these experiments. Flowers contained lower or equal amounts of most compounds and half the total amount, compared to leaves. Plants subjected to herbivory emitted higher levels of most volatiles and 12 times the total amount, versus controls with no larvae, including proportionally more of the low boiling point chemicals. Thus, chemical profiles of shields and volatile emissions are influenced by the amounts and volatilities of compounds present in the host plant. The implications of these results are explored for the chemical ecology of both the plant and the insect.     

Volatiles from the bark of trembling aspen, Populus tremuloides Michx. (Salicaceae) disrupt secondary attraction by the mountain pine beetle, Dendroctonus ponderosae Hopkins (Coleoptera: Scolytidae)

Summary. Coupled gas chromatographic-electroantennographic detection (GC-EAD) analysis of the Porapak Q-captured volatiles from the bark of trembling aspen, Populus tremuloides Michx., revealed four compounds that consistently elicited antennal responses by mountain pine beetles (MPBs), Dendroctonus ponderosae Hopkins. One of these, 1-hexanol, disrupted the capture of MPBs in multiple-funnel traps baited with the aggregation pheromones trans-verbenol and exo-brevicomin and the host kairomone myrcene, a blend of semiochemicals that mediates the secondary attraction response in which beetles mass attack and kill living pines. The other three EAD-active aspen bark volatiles, benzyl alcohol, benzaldehyde and nonanal, were inactive alone, but in binary and ternary combinations contributed to a disruptive effect in an additive and redundant manner when all four aspen bark volatiles were tested in all possible binary and ternary blends. The best ternary blend and the quarternary blend achieved ≥ 80% disruption. The quarternary blend enhanced the disruptive effect of the antiaggregation pheromone verbenone in traps, raising the disruptive effect to 98%, and also enhanced the inhibition of attack on attractant-baited lodgepole pines. This is the first demonstration of specific compounds from the bark of angiosperm trees that disrupt the secondary attraction response of sympatric coniferophagous bark beetles. The results support the hypothesis that such bark beetles are adapted to recognize and avoid non-host angiosperm trees by responding to a broad spectrum of volatiles that can act in various blends with equal effect. Received 27 October 1997; accepted 20 February 1998.     

Simulation and analysis of outbreaks of bark beetle infestations and their management at the stand level

Outbreaks of bark beetles in forests can result in substantial economic losses. Understanding the factors that influence the development and spread of bark beetle outbreaks is crucial for forest management and for predicting outbreak risks, especially with the expected global warming. Although much research has been done on the ecology and phenology of bark beetles, the complex interplay between beetles, host trees, beetle antagonists and forest management makes predicting beetle population development especially difficult. Using the recent infestations of the European Spruce Bark Beetle (Ips typographus L. Col. Scol.) in the Bavarian Forest National Park (Germany) as a case study, we developed a spatially explicit agent-based simulation model (SAMBIA) that takes into account individual trees and beetles. This model primarily provides a tool for analysing and understanding the spatial and temporal aspects of bark beetles outbreaks at the stand scale. Furthermore, the model should allow an estimation of the effectiveness of concurrent impacts of both antagonists and management to confine outbreak dynamics in practice. We also used the model to predict outbreak probabilities in various settings. The simulation results indicated a distinct threshold behaviour of the system in response to pressure by antagonists or management of the bark beetle population. Despite the different scenarios considered, we were able to extract from the simulations a simple rule of thumb for the successful control of an outbreak: if roughly 80% of individual beetles are killed by antagonists or foresters, outbreaks will rarely take place. Our model allows the core dynamics of this complex system to be reduced to this inherent common denominator.     

Electrophysiological responses of four fungivorous coleoptera to volatiles of Trametes versicolor: implications for host selection

Fungi of the genus Trametes are known as important wood decomposers and are colonized by various species of Coleoptera and other arthropods. The aim of the present study was to investigate the importance of volatile chemical compounds as key attraction factors in recognition and host selection by species of Erotylidae (Dacne bipustulata, Tritoma bipustulata) as well as Cisidae (Sulcacis affinis) and Tenebrionidae (Diaperis boleti). Volatiles from freshly collected Trametes versicolor were collected by headspace sampling technique and identified by combined gas chromatography–mass spectrometry (GC–MS). To evaluate the biological significance of the volatiles we performed behavioural tests and recorded antennal responses of the fungus-inhabiting species by gas chromatography with electroantennographic detection (GC-EAD). The scent of T. versicolor was found to be dominated by sesquiterpenes; in GC-EAD 6 of these compounds elicited reproducible antennal signals in the tested species. Highly significant attraction effects to the fungus, the obtained odour samples and previously described fungal C₈-compounds were observed in behavioural tests. The possibility to detect these chemical compounds as a key cue for host selection implicate that beetles are able to discriminate between fungi of different age as well as different stages of colonization.     

Behavioral and electrophysiological responses of the emerald ash borer, Agrilus planipennis, to induced volatiles of Manchurian ash, Fraxinus mandshurica

Summary. We investigated the volatile emissions of Manchurian ash seedlings, Fraxinus mandshurica, in response to feeding by the emerald ash borer, Agrilus planipennis, and to exogenous application of methyl jasmonate (MeJA). Feeding damage by adult A. planipennis and MeJA treatment increased volatile emissions compared to unexposed controls. Although the same compounds were emitted from plants damaged by beetles and treated with MeJA, quantitative differences were found in the amounts of emissions for individual compounds. Adult virgin female A. planipennis were similarly attracted to volatiles from plants damaged by beetles and those treated with MeJA in olfactometer bioassays; males did not respond significantly to the same volatiles. Coupled gas chromatographic-electroantennogram detection (GC-EAD) revealed at least 16 antennally-active compounds from F. mandshurica, including: hexanal, (E)-2-hexenal, (Z)-3-hexen-1-ol, 3-methyl-butylaldoxime, 2-methyl-butylaldoxime, (Z)-3-hexen-1-yl acetate, hexyl acetate, (E)-β-ocimene, linalool, 4,8-dimethyl-1,3,7-nonatriene, and E,E-α-farnesene. Electroantennogram (EAG) dose–response curves using synthetic compounds revealed that females had a stronger EAG response to linalool than males; and male responses were greater to: hexanal, (E)-2-hexenal, (Z)-3-hexen-1-ol, 3-methyl-butylaldoxime, 2-methyl-butylaldoxime, and hexyl acetate. These results suggest that females may use induced volatiles in long-range host finding, while their role for males is unclear. If attraction of females to these volatiles in an olfactometer is upheld by field experiments, host plant volatiles may find practical application in detection and monitoring of A. planipennis populations.     

Salience of multimodal flower cues manipulates initial responses and facilitates learning performance of bumblebees

Visual and olfactory cues are the first interface between flowers and their visitors and are adaptations to facilitate successful pollination. Initial responses to and associative learning of multimodal cues by flower visitors are based on the perception of colours and volatiles. In this study, we tested how visual, olfactory and multimodal stimuli affect the behaviour of bumblebees (Bombus terrestris) and correlated the properties of 28 stimuli in relation to the insects’ sensory equipment to these behaviours. Initial responses and associative learning were recorded using a radio frequency identification system, tracking the visitation sequences of individual bumblebees to artificial flowers treated with naturally occurring pigments and/or volatiles. The salience of the stimuli was evaluated as the colour contrast to the background and as electroantennogram responses. The main finding was that both initial responses and learning performance were positively correlated to the salience of the stimuli, suggesting that salience is a key feature of flower cues in the interactions with insects. The salience of compound stimuli consisting of two or more pigments and/or volatiles was largely additively determined by the saliences of individual compounds. Potentially, the valence of the stimuli may interfere with the positive relationship between salience and behaviour, which is indicated by our results, too. The salience of multimodal cues depends on the species-specific equipment of visual and olfactory receptors and thus enables flowers to be advertising for some but rather inconspicuous for other flower visitors.     

A dynamic host selection model for mountain pine beetle,Dendroctonus ponderosae Hopkins

The link between individual habitat selection decisions (i.e., mechanism) and the resulting population distributions of dispersing organisms (i.e., outcome) has been little-studied in behavioural ecology. Here we consider density-dependent habitat (i.e., host) selection for an energy- and time-limited forager: the mountain pine beetle (Dendroctonus ponderosae Hopkins). We present a dynamic state variable model of individual beetle host selection behaviour, based on an individual’s energy state. Field data are incorporated into model parameterization which allows us to determine the effects of host availability (with respect to host size, quality, and vigour) on individuals’ decisions. Beetles choose larger trees with thicker phloem across a larger proportion of the state-space than smaller trees with thinner phloem, but accept lower quality trees more readily at low energy- and time-states. In addition, beetles make habitat selection decisions based on host availability, conspecific attack densities, and beetle distributions within a forest stand. This model provides a framework for the development of a spatial game model to examine the implications of these results for attack dynamics of beetle populations.     

Multiple feeding stimulants in <Emphasis Type="Italic">Sinapis alba</Emphasis> for the oligophagous leaf beetle <Emphasis Type="Italic">Phaedon cochleariae</Emphasis>

Summary. In earlier investigations on host plant discrimination of leaf beetles glucosinolates were described as feeding stimulants for the Brassicaceae specialist Phaedon cochleariae F. (Coleoptera: Chrysomelidae). However, since these findings could not be confirmed in later studies offering 2-propenylglucosinolate in concentrations corresponding to those detected in host plant leaf material, the identification of feeding stimulants of this leaf beetle species remained unclear. In order to investigate which compounds of the host plant Sinapis alba (Brassicaceae) are involved in feeding stimulation, leaf extracts of different polarities were tested in bioassays with adults of P. cochleariae. Number of feeding beetles and net consumption rates were highest on pea leaves painted with methanol extracts of S. alba, whereas weak feeding responses were also detectable for hexane extracts. In subsequent bioassay-guided fractionations of methanol extracts with semi-preparative high performance liquid chromatography, two distinct fractions, one containing glucosinolates and another containing flavonoids, were found to stimulate beetles to feed to variable degrees. Other collected fractions had zero activity. The combination of both active fractions evoked significantly higher consumption rates and stimulated more beetles to feed than fractions tested individually. At least one compound of each fraction, among these the main glucosinolate of S. alba, 4-hydroxybenzylglucosinolate, act additively. Effects of two different naturally-occurring ratios of glucosinolates and flavonoids on the strength of feeding responses were investigated by use of extracts of two sets of host plants differently exposed to radiation. One set was outdoors-exposed, whereas the second set was kept in the greenhouse. However, the feeding behaviour of P. cochleariae was not affected by the significantly different relative compositions of both compound classes in the host material. In conclusion, mustard leaf beetles need a combination of distinct plant metabolites acting in concert for feeding stimulation, whereby the mere presence of these stimulants, but probably not the ratio of involved compounds, determines their feeding response.     

Maize field odorscape during the oviposition flight of the European corn borer

Most crop pests find a suitable host through chemical cues released from plants, but little is known about the odorscape encountered by host-seeking gravid females under natural, outdoor conditions. In this field study, the volatile organic compound (VOC) composition of maize (Zea mays, L.), a host for the European corn borer (ECB) (Ostrinia nubilalis Hüb.) was characterized during the oviposition flight and compared with a forest odorscape. VOCs from maize fields and the forest atmosphere were collected by solid phase microextraction and characterized by gas chromatography-mass spectrometry. The electroantennographic (EAG) response of female ECB antennae to candidate VOCs was tested. Analyses revealed clear differences between the maize field and the forest odorscapes, mainly composed of ubiquitous VOCs but in specific ratios. The maize field odorscape is more complex than the forest odorscape for maize found 18 VOCs but only eight in the forest. Both biotopes shared seven VOCs—green leaf volatiles (GLV), monoterpènes (MT) and homoterpenes. In addition, we found in the forest a distinctive sesquiterpene (SQT) identified as isoledene. The highest EAG responses were elicited by two GLVs and a MT shared by the two biotopes. SQT elicited weak EAG responses, except β-farnesene, only found in the maize field odorscape. Our results suggest that the two biotopes produce specific chemical signatures that insects may use as host cues. To the best of our knowledge this paper is the first report on the maize odorscapes under field conditions. The putative role of the VOCs in host plant detection and selection is discussed.     

Blends of (R)-3-hydroxyhexan-2-one and alkan-2-ones identified as potential pheromones produced by three species of cerambycid beetles

We present data indicating that three species of cerambycid beetles (subfamily Cerambycinae) produce the common cerambycine pheromone component (R)-3-hydroxyhexan-2-one as well as an alkan-2-one component, a possible new motif for cerambycid pheromone components. GC/MS analyses of headspace volatiles produced by male beetles indicated that Cyrtophorus verrucosus (Olivier) produced (R)-3-hydroxyhexan-2-one but also nonan-2-one at ~18 % of the hydroxyketone component, whereas Orwellion gibbulum arizonense (Casey) and Parelaphidion aspersum (Haldeman) produced decan-2-one at ~40 and 7 % of the amount of the hydroxyketone, respectively. In field bioassays, adult C. verrucosus were attracted by (R)-3-hydroxyhexan-2-one alone, but attraction was significantly enhanced by nonan-2-one. This effect was lost if the quantity of nonan-2-one exceeded 100 % of the hydroxyketone, suggesting that beetles could discern ratios of the two chemicals and were most strongly attracted to those approximating the blend produced by males. We suggest that nonan-2-one plays a role in the species specificity of the pheromone signal of C. verrucosus, and that decan-2-one plays a similar role in the semiochemical communication of O. g. arizonense and P. aspersum.