Emission of herbivore elicitor-induced sesquiterpenes is regulated by stomatal aperture in maize (Zea mays) seedlings

Maize seedlings emit sesquiterpenes during the day in response to insect herbivory. Parasitoids and predators use induced volatile blends to find their hosts or prey. To investigate the diurnal regulation of biosynthesis and emission of induced sesquiterpenes, we applied linolenoyl-L-glutamine (LG) to maize seedlings in the morning or evening using a cut-stem assay and tracked farnesene emission, in planta accumulation, as well as transcript levels of farnesyl pyrophosphate synthase 3 (ZmFPPS3) and terpene synthase10 (ZmTPS10) throughout the following day. Independent of time of day of LG treatment, maximum transcript levels of ZmFPPS3 and ZmTPS10 occurred within 3-4 h after elicitor application. The similarity between the patterns of farnesene emission and in planta accumulation in light-exposed seedlings in both time courses suggested unobstructed emission in the light. After evening induction, farnesene biosynthesis increased dramatically during early morning hours. Contrary to light-exposed seedlings dark-kept seedlings retained the majority of the synthesized farnesene. Two treatments to reduce stomatal aperture, dark exposure at midday, and abscisic acid treatment before daybreak, resulted in significantly reduced amounts of emitted and significantly increased amounts of in planta accumulating farnesene. Our results suggest that stomata not only play an important role in gas exchange for primary metabolism but also for indirect plant defences.

Identification of a sex pheromone produced by female velvetbean caterpillar moth

A sex pheromone produced by female velvetbean caterpillar moths,Anticarsia gemmatalis Hübner, that attracts conspecific males was isolated and identified as a blend of (Z,Z,Z)-3,6,9-eicosatriene and (Z,Z,Z)-3,6,9-heneicosatriene in a ratio of ca. 5∶3, respectively, when combined. The synthesized compounds elicited responses by velvetbean caterpillar moth males equivalent to those elicited by females in both laboratory wind tunnel bioassays and field trapping experiments.

Identification of volatile sex pheromone components released by the southern armyworm,Spodoptera eridania (Cramer)

Analysis of sex pheromone gland extracts and volatile pheromone components collected from the calling female southern armyworm,Spodoptera eridania (Cramer), by high-resolution capillary gas chromatography and mass spectroscopy indicated that a number of 14-carbon mono- and diunsaturated acetates and a monounsaturated 16-carbon acetate were produced. Gland extracts also indicated the presence of (Z)-9-tetradecen-1-ol. However, this compound was not found in collections of volatiles. Field trapping studies indicated that the volatile blend composed of (Z)-9-tetradecen-1-ol acetate (60%), (Z)-9-(E)-12-tetradecadien-1-ol acetate (17%), (Z)-9-(Z)-12-tetradecadien-1-ol acetate (15%), (Z)-9-(E)-11-tetradecadien-1-ol acetate (5%), and (Z)-11-hexadecen-1-ol acetate (3 %) was an effective trap bait for males of this species. The addition of (Z)-9-tetradecen-1-ol to the acetate blends tested resulted in the capture of beet armyworm,S. exigua (Hubner), males which provides further evidence that the alcohol is a pheromone component of this species.

Chemical and behavioral analyses of volatile sex pheromone components released by callingHeliothis virescens (F.) females (Lepidoptera: Noctuidae)

Gas chromatographic and mass spectral analyses were conducted on pheromone gland extracts, volatiles collected from excised pheromone glands, and volatiles collected from calling females. In addition to tetradecanal, (Z)-9-tetradecenal, hexadecanal, (Z)-7-hexadecenal, (Z)-9-hexadecenal, and (Z)-11-hexadecenal, four other compounds, tetradecanol, (Z)-9-tetradecenol, hexadecanol, and (Z)-11-hexadecenol, were also identified from gland extracts. Only the six aldehyde components were found in gland and female volatile collections. The mean percentage of components identified from volatiles collected from calling females was 13.0% tetradecanal, 18.1% (Z)-9-tetradecenal, 7.3% hexadecanal, 0.6% (Z)-7-hexadecenal, 1.0% (Z)-9-hexadecenal, and 60.0% (Z)-11-hexadecenal. Bioassays using rubber septa formulated to release the female volatile blend indicated that all six aldehyde components play major roles in close-range male reproductive behavior. Deletion of (Z)-9-hexadecenal from the six-component blend reduced the number of copulation attempts while (Z)-7-hexadecenal exerted subtle effects on all close range behaviors. Tetradecanal affected the number of times males reorient from close range. Deletion of hexadecanal from the six-component blend resulted in a significant reduction in the number of times males landed. Comparison of the six-component synthetic blend (released at somewhat less than 1 female equivalent per hour) with calling females indicated that the six-component blend was indistinguishable from the females in inducing all of the behaviors monitored.

Isolation, identification, and synthesis of male-produced sex pheromone of papaya fruit fly,Toxotrypana curvicauda Gerstaecker (Diptera: Tephritidae)

A male-produced sex pheromone of the papaya fruit fly,Toxotrypana curvicauda Gerstaecker, was isolated from volatiles collected from air passed over calling males and was identified as 2-methyl-6-vinylpyrazine by comparative gas-liquid chromatographic and spectroscopic evidence. Synthetic 2-methyl-6-vinylpyrazine elicited typical pheromonal responses from unmated mature female flies such as walking, running, and flying in an arena bioassay; flying upwind with a zigzag flight pattern; and hovering in the pheromone plume in a wind-tunnel bioassay. These responses were similar quantitatively and qualitatively to responses to naturally occurring pheromone from calling male papaya fruit flies.

Beneficial arthropod behavior mediated by airborne semiochemicals : IV. Influence of host diet on host-oriented flight chamber responses ofMicroplitis demolitor Wilkinson

Microplitis demolitor Wilkinson were reared fromHeliothis zea larvae fed either an artificial diet or a diet of cowpea seedlings. Responses of females from these two sources to semiochemicals from the insect hostcowpea complex were compared in a flight tunnel. Very limited responses were obtained from females reared from hosts fed artificial diet unless they first had preflight contact with frass from plant-fed hosts. Female parasite is reared from plant-fed hosts were generally responsive without preflight contact. Contact with cocoons containing frass of the plant fed host at the time of emergence was an important source of stimulation. It increased their subsequent responsiveness to the volatile semiochemicals of the insect host-plant complex. The cocoons of females reared from artificial diet-fed hosts were apparently lacking plant chemicals that are vital to their subsequent responsiveness. Imprinting of the adults at time of emergence from the cocoon is strongly indicated.

Properties of cuticular oxidases used for sex pheromone biosynthesis byHeliothis zea

Biosynthesis of the aldehydic sex pheromone components released by females ofHeliothis zea was found to be catalyzed by primary alcohol oxidases residing in the cuticle that covers the glands. Activity, as indicated by conversion of primary alcohol to aldehyde, was as high in cell-free cuticle as it was in intact pheromone glands. Studies indicated that some activity was associated with the surface of the epicuticle and could be removed, into buffer, by sonication. However, the majority of activity lies within the inner epicuticle and exo- and endocuticular layers. The oxidase was not functional in pharate pupae that did not have mature adult cuticle but became functional just prior to adult emergence. The enzyme in individual glands was saturated at alcohol concentrations above 100 n. moles. Nonionic detergents did not affect the activity of the oxidase in the cuticle but treatment with either 7 M urea or 1% SDS resulted in total loss of activity. Studies on the effect of pH indicated an optimum at 6.4; however, activity was high throughout the range of 5-9. The oxidase was functional in both dichloromethane and hexane, suggesting that this enzyme system may have applications for organic synthesis of aldehydes.

Isolation and identification of allelochemicals that attract the larval parasitoid,Cotesia marginiventris (Cresson), to the microhabitat of one of its hosts

Volatiles released from corn seedlings on which beet armyworm larvae were feeding were attractive to females of the parasitoid,Cotesia marginiventris (Cresson), in flight tunnel bioassays. Analyses of the collected volatiles revealed the consistent presence of 11 compounds in significant amounts. They were: (Z)-3-hexenal, (E)-2-hexenal, (Z)-3-hexen-1-ol, (Z)- 3-hexen-1-yl acetate, linalool, (3E)-4,8-dimethyl-1,3,7-nonatriene, indole, α-trans-bergamotene, (E)-β-farnesene, (E)-nerolidol, and (3E,7E)-4,8,12-trimethyl-1, 3,7,ll-tridecatetraene. A synthetic blend of all 11 compounds was slightly less attractive to parasitoid females than an equivalent natural blend. However, preflight experience with the synthetic blend instead of experience with a regular plant-host complex significantly improved the response to the synthetic blend. Our results suggest thatC. marginiventris females, in their search for hosts, use a blend of airborne semiochemicals emitted by plants on which their hosts feed. The response to a particular odor blend dramatically increases after a parasitoid experiences it in association with contacting host by-products.

Analysis, synthesis, formulation, and field testing of three major components of male mediterranean fruit fly pheromone

Three major components, ethyi-(E)-3-octenoate, geranyl acetate, and (E,E)-α-farnesene, emitted as volatiles by laboratory-reared and wild male medflies were collected and analyzed qualitatively and quantitatively. Peak emission of these compounds occurred during the third to fifth hours of the photophase and differences were observed in the ratios of the three components emitted by male laboratory-reared and wild flies. These three compounds were synthesized, and a method was developed to formulate a synthetic blend that released the compounds in a ratio similar to that emitted by wild male medflies. Attractiveness of the blend to female medflies was demonstrated under field conditions by comparing trap catches. Black spherical traps, baited with the synthetic blend to release 1.6 male equivalents, caught significantly more females than blank traps and traps from which the blend released was 0.3, 3.2 or 6.4 male equivalents.

Chemical mimicry: bolas spiders emit components of moth prey species sex pheromones

Field studies have indicated that bolas spiders attract male moth prey, apparently by mimicking the odor of female moth sex pheromones. Three moth sex pheromone compounds, (Z)-9-tetradecenyl acetate, (Z)-9-tetradecenal, and (Z)-11-hexadecenal, were identified in volatile substances emitted by hunting adult female Mastophora cornigera spiders. These compounds are components of pheromone blends that attract some of this spider's moth prey species.

Identification of the female Japanese beetle sex pheromone: inhibition of male response by an enantiomer

(Z)-5-(1-Decenyl)dihydro-2(3H)-furanone, isolated from virgin female Japanese beetles (Popillia japonica) attracted males of the species infield bioassays. However, the synthesized racemic mixture of this compound did not attract male Japanese beetles. The Z and E isomers and the saturated analog of both enantiomers of this compound were synthesized stereospecifically. Pure synthetic (R,Z)-5-(1-decenyl)dihydro-2(3H)-furanone was competitive with live females and with the pheromone isolated from live females in attracting males. Male response was strongly inhibited by small amounts of the S,Z isomer. Although the E isomer and the saturated analog of the pheromone are present in the material obtained from females, the role of these compounds in mediating the insect's behavior is unclear.

Sex pheromones produced by male boll weevil: isolation, identification, and synthesis

The response of female boll weevils to males, Anthonomus grandis Boheman, in laboratory bioassays can be reproduced by exposure to a mixture of compounds I, II, and either III or IV, all isolated from male weevils and their fecal material. The same response was elicited by mixtures of tile synthesized coimpounds. Compound I is (+)-cis-2-isopropenyl-l-methylcyclobtutaneethanol; II, cis-3,3-dimethiyl->(1,beta)cyclohexaneethanol; III, cis-3,3-dimethlyl->(1,alpha)-cyclohexaneacetaldehyde; and IV, trans-3,3-dimethyl->(1,alpha)-cyclohexanecetaldehyde.

Neural regulation of sex pheromone biosynthesis in Heliothis moths

Pheromone biosynthesis in females of Heliothis zea is regulated endogenously by a neuropeptide produced in the subesophageal ganglion. We have found that the ventral nerve cord must be intact for normal induction of pheromone biosynthesis and that pheromonotropic activity is associated with extracts of the abdominal nerve cord, but only during the period when pheromone is produced. We did not find evidence of pheromonotropic activity in hemolymph obtained from females that were producing pheromone. Extracts of the brain-subesophageal ganglion complex, which contain pheromone biosynthesis activating neuropeptide (PBAN), induced pheromone biosynthesis when applied to the terminal abdominal ganglion only if nerves from this ganglion to the pheromone gland were intact. Brain-subesophageal ganglion extracts did not induce biosynthesis when applied directly to the pheromone glands in vitro. From our results, we conclude that the target site of PBAN is not the pheromone gland but the terminal abdominal ganglion, and we hypothesize that the abdominal nerve cord transports PBAN to the terminal abdominal ganglion.

Parasitic wasps learn and report diverse chemicals with unique conditionable behaviors

Parasitoids exploit numerous chemical cues to locate hosts and food. Whether they detect and learn chemicals foreign to their natural history has not been explored. We show that the parasitoid Microplitis croceipes can associate, with food or hosts, widely different chemicals outside their natural foraging encounters. When learned chemicals are subsequently detected, this parasitoid manifests distinct behaviors characteristic with expectations of food or host, commensurate with prior training. This flexibility of parasitoids to rapidly link diverse chemicals to resource needs and subsequently report them with recognizable behaviors offers new insights into their foraging adaptability, and provides a model for further dissection of olfactory learning related processes.

Differential activity and degradation of plant volatile elicitors in regurgitant of tobacco hornworm (Manduca sexta) larvae

Plants respond to insect herbivory by emitting volatile compounds that attract natural enemies of the herbivores. Biosynthesis of many of these volatiles in plants is induced by herbivore-produced compounds. Components of tobacco hornworm (THW) regurgitant were investigated for their efficacy as elicitors of corn seedling volatiles. Two components that elicited the strongest release of volatiles were isolated and identified as N-linolenoyl-L-glutamine (18:3-GLN) and N-linolenoyl-L-glutamic acid (18:3-GLU). The approximately 10 times more active 18:3-GLN, which also is found in the regurgitant of several other Lepidopteran larvae, was rapidly degraded when THW regurgitant was left at room temperature, while 18:3-GLU degraded at a much slower rate. Different dietary sources of THW and tobacco bud worm larvae, including both host and nonhost plants, did not affect the amino acid composition of the fatty acid-amino acid conjugates in the regurgitant.

The influence of intact-plant and excised-leaf bioassay designs on volicitin- and jasmonic acid-induced sesquiterpene volatile release in Zea mays

Induced plant responses to insect attack include the release of volatile chemicals. These volatiles are used as host-location signals by foraging parasitoids, which are natural enemies of insect herbivores. A plant's response to herbivory can be influenced by factors present in insect oral secretions. Volicitin (N-(17-hydroxylinolenoyl)-L-glutamine), identified in beet armyworm (Spodoptera exigua) oral secretions, stimulates volatile release in corn (Zea mays L.) seedlings in a manner similar to beet armyworm herbivory. Volicitin is hypothesized to trigger release of induced volatiles, at least in part, by modulating levels of the wound hormone, jasmonic acid (JA). We compare the sesquiterpene volatile release of damaged leaves treated with aqueous buffer only or with the same buffer containing volicitin or JA. Leaves were damaged by scratching with a razor and test solutions were applied to the scratched area. The leaves were either excised from the plant or left intact shortly after this treatment. Plants were treated at three different times (designated as Evening, Midnight, and Morning) and volatiles were collected in the subsequent photoperiod. JA and volicitin treatments stimulated the release of volatile sesquiterpenes, namely beta-caryophyllene, (E)-alpha-bergamotene, and (E)-beta-farnesene. In all cases, JA stimulated significant sesquiterpene release above mechanical damage alone. Volicitin induced an increase in sesquiterpene volatiles for all excised-leaf bioassays and the Midnight intact plants. Volicitin treatments in the Evening and Morning intact plants produced more sesquiterpenes than the untreated controls, while mechanical damage alone produced an intermediate response that did not differ from either treatment group. Excised leaves produced a 2.5- to 8.0-fold greater volatile response than similarly treated intact plants. Excision also altered the ratio of JA-and volicitin-induced sesquiterpene release by preferentially increasing (E)-beta-farnesene levels relative to beta-caryophyllene. The inducibility of volatile release varied with time of treatment. On average, sesquiterpene release was highest in the Midnight excised leaves and lowest in the Morning intact plants. The duration of induced volatile release also differed between treatments. On average, JA produced a sustained release of sesquiterpenes over time, with over 20% of the combined sesquiterpenes released in the third and final volatile collection period. In contrast, less than 8% of the combined sesquiterpenes induced by volicitin were emitted during this period. The large quantitative differences between intact plants and detached leaves suggest that the results of assays using excised tissues should be cautiously interpreted when considering intact-plant models.

Caterpillar-induced nocturnal plant volatiles repel conspecific females

Plants respond to insect herbivory by synthesizing and releasing complex blends of volatile compounds, which provide important host-location cues for insects that are natural enemies of herbivores. The effects of these volatile blends on herbivore behaviour have been investigated to only a limited extent, in part because of the assumption that herbivore-induced volatile emissions occur mainly during the light phase of the photoperiod. Because many moths-whose larvae are some of the most important insect herbivores-are nocturnal, herbivore-induced plant volatiles have not hitherto been considered to be temporally available as host-location cues for ovipositing females. Here we present chemical and behavioural assays showing that tobacco plants (Nicotiana tabacum) release herbivore-induced volatiles during both night and day. Moreover, several volatile compounds are released exclusively at night and are highly repellent to female moths (Heliothis virescens). The demonstration that tobacco plants release temporally different volatile blends and that lepidopteran herbivores use induced plant signals released during the dark phase to choose sites for oviposition adds a new dimension to our understanding of the role of chemical cues in mediating tritrophic interactions.

An herbivore elicitor activates the gene for indole emission in maize

Maize and a variety of other plant species release volatile compounds in response to herbivore attack that serve as chemical cues to signal natural enemies of the feeding herbivore. N-(17-hydroxylinolenoyl)-l-glutamine is an elicitor component that has been isolated and chemically characterized from the regurgitant of the herbivore-pest beet armyworm. This fatty acid derivative, referred to as volicitin, triggers the synthesis and release of volatile components, including terpenoids and indole in maize. Here we report on a previously unidentified enzyme, indole-3-glycerol phosphate lyase (IGL), that catalyzes the formation of free indole and is selectively activated by volicitin. IGL's enzymatic properties are similar to BX1, a maize enzyme that serves as the entry point to the secondary defense metabolites DIBOA and DIMBOA. Gene-sequence analysis indicates that Igl and Bx1 are evolutionarily related to the tryptophan synthase alpha subunit.

Plant production of volatile semiochemicals in response to insect-derived elicitors

An increase in the release of volatile compounds by plants in response to insect feeding is triggered by interaction of elicitors in the oral secretions of insect herbivores with damaged plant tissues. This herbivore damage triggers de novo biosynthesis of volatile plant metabolites derived from several different biochemical pathways. Natural enemies of herbivores use these volatile semiochemicals to locate their hosts. Although some volatile compounds are released from storage in plants immediately whenever damage to cells or glands occurs, the induced compounds are only synthesized and released during the light period. This often results in a delay between feeding damage and release of volatiles. Plants release the induced compounds from undamaged as well as damaged leaves. Thus, damage to only a few leaves results in a systemic response and release of volatiles by the entire plant. We propose that plants respond differently to individual herbivore species at least in part due to the composition of insect elicitors that come in contact with the plant. Specialist parasitoids can differentiate the volatile blends released due to damage by hosts from those resulting from non-host damage as well as from mechanical damage, thereby facilitating host location for the parasitoid. Elicitors in the oral secretions of beet armyworm caterpillars have been identified and synthesized.

Concerted biosynthesis of an insect elicitor of plant volatiles

A variety of agricultural plant species, including corn, respond to insect herbivore damage by releasing large quantities of volatile compounds and, as a result, become highly attractive to parasitic wasps that attack the herbivores. An elicitor of plant volatiles, N-(17-hydroxylinolenoyl)-L-glutamine, named volicitin and isolated from beet armyworm caterpillars, is a key component in plant recognition of damage from insect herbivory. Chemical analysis of the oral secretion from beet armyworms that have fed on 13C-labeled corn seedlings established that the fatty acid portion of volicitin is plant derived whereas the 17-hydroxylation reaction and the conjugation with glutamine are carried out by the caterpillar by using glutamine of insect origin. Ironically, these insect-catalyzed chemical modifications to linolenic acid are critical for the biological activity that triggers the release of plant volatiles, which in turn attract natural enemies of the caterpillar.

A total system approach to sustainable pest management

A fundamental shift to a total system approach for crop protection is urgently needed to resolve escalating economic and environmental consequences of combating agricultural pests. Pest management strategies have long been dominated by quests for "silver bullet" products to control pest outbreaks. However, managing undesired variables in ecosystems is similar to that for other systems, including the human body and social orders. Experience in these fields substantiates the fact that therapeutic interventions into any system are effective only for short term relief because these externalities are soon "neutralized" by countermoves within the system. Long term resolutions can be achieved only by restructuring and managing these systems in ways that maximize the array of "built-in" preventive strengths, with therapeutic tactics serving strictly as backups to these natural regulators. To date, we have failed to incorporate this basic principle into the mainstream of pest management science and continue to regress into a foot race with nature. In this report, we establish why a total system approach is essential as the guiding premise of pest management and provide arguments as to how earlier attempts for change and current mainstream initiatives generally fail to follow this principle. We then draw on emerging knowledge about multitrophic level interactions and other specific findings about management of ecosystems to propose a pivotal redirection of pest management strategies that would honor this principle and, thus, be sustainable. Finally, we discuss the potential immense benefits of such a central shift in pest management philosophy.

De Novo Biosynthesis of Volatiles Induced by Insect Herbivory in Cotton Plants

In response to insect feeding on the leaves, cotton (Gossypium hirsutum L.) plants release elevated levels of volatiles, which can serve as a chemical signal that attracts natural enemies of the herbivore to the damaged plant. Pulse-labeling experiments with [13C]CO2 demonstrated that many of the volatiles released, including the acyclic terpenes (E,E)-[alpha]-farnesene, (E)-[beta]-farnesene, (E)-[beta]-ocimene, linalool, (E)-4,8-dimethyl-1,3,7-nonatriene, and (E/E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene, as well as the shikimate pathway product indole, are biosynthesized de novo following insect damage. However, other volatile constituents, including several cyclic terpenes, butyrates, and green leaf volatiles of the lipoxygenase pathway are released from storage or synthesized from stored intermediates. Analysis of volatiles from artificially damaged plants, with and without beet armyworm (Spodoptera exigua Hubner) oral secretions exogenously applied to the leaves, as well as volatiles from beet armyworm-damaged and -undamaged control plants, demonstrated that the application of caterpillar oral secretions increased both the production and release of several volatiles that are synthesized de novo in response to insect feeding. These results establish that the plant plays an active and dynamic role in mediating the interaction between herbivores and natural enemies of herbivores.

Volatile Semiochemicals Released from Undamaged Cotton Leaves (A Systemic Response of Living Plants to Caterpillar Damage)

Cotton plants (Gossypium hirsutum L.), attacked by herbivorous insects release volatile semiochemicals (chemical signals) that attract natural enemies of the herbivores to the damaged plants. We found chemical evidence that volatiles are released not only at the damaged site but from the entire cotton plant. The release of volatiles was detected from upper, undamaged leaves after 2 to 3 d of continuous larval damage on lower leaves of the same plant. Compounds released systemically were (Z)-3-hexenyl acetate, (E)-[beta]-ocimene, linalool, (E)-4,8-dimethyl-1,3,7-nonatriene, (E)-[beta]-farnesene, (E,E)-[alpha]-farnesene, and (E,E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene. All systemically released compounds are known to be induced by caterpillar damage and are not released in significant amounts by undamaged plants. Other compounds, specifically indole, isomeric hexenyl butyrates, and 2-methylbutyrates, known to be released by cotton in response to caterpillar damage, were not released systemically. However, when upper, undamaged leaves of a caterpillar-damaged plant were damaged with a razor blade, they released isomeric hexenyl butyrates, 2-methylbutyrates, and large amounts of constitutive compounds in addition to the previously detected induced compounds. Control plants, damaged with a razor blade in the same way, did not release isomeric hexenyl butyrates or 2-methylbutyrates and released significantly smaller amounts of constitutive compounds. Indole was not released systemically, even after artificial damage.

The integral role of triacyl glycerols in the biosynthesis of the aldehydic sex pheromones of Manduca sexta (L)

In a gland located near the tip of their abdomens, Manduca sexta females produce a pheromone blend comprised of hexadecanal, (Z)-9-hexadecenal, (Z)- and (E)-11-hexadecenal, (E,Z)- and (E,E)-10,12-hexadecadienal, and (E,E,Z)- and (E,E,E)-10,12,14-hexadecatrienal. These aldehydes are produced and released by evaporation from the surface of the gland only during a discrete period of the night. They are not stored in the gland and are found there only in very small amounts, if at all, during other times of the photoperiod. However, fatty acyl analogues of the pheromone aldehydes are present in the gland in relatively large amounts, primarily as components of triacyl glycerols, continuously from eclosion of the adults until death. The unsaturated components are produced from hexadecanoate, which is desaturated to the monoenes. Then, (Z)-11-hexadecenoate is desaturated and isomerized to form the conjugated dienes and ultimately the conjugated trienes. The fatty acyl precursors of the pheromones, stored as components of triacyl glycerols, are converted into aldehydes by a process triggered by a pheromone biosynthesis activating neuropeptide (PBAN), produced in the brain-subesophageal complex. It is not yet clear whether this conversion involves direct reduction of the acyl groups to aldehydes or reduction to alcohols followed by oxidation to aldehydes.

Pheromone biosynthesis activating neuropeptides: functions and chemistry

Sex pheromones are critical for reproductive success in most species of Lepidoptera and their production is regulated by the action of pheromone biosynthesis activating neuropeptides (PBAN). These peptides, composed of 33-34 amino acids, have approximately 80% sequence homology and share the C-terminal sequence FSPRL-NH2, which has been shown to be the minimum sequence required for pheromonotropic activity. This pentamer is structurally similar to the active core (FXPRL-NH2, X = V, T or G) of the insect myotropic pyrokinins. Structure-activity studies have shown that all of the pyrokinins have various degrees of pheromonotropic activity and that some have a superagonistic effect. Peptides that only have sequence homology with PBAN in the C-terminal pentapeptide region, but that are pheromonotropic, also have been identified from months. These findings suggest that induction of pheromone biosynthesis may be regulated by more than one peptide, that PBAN may have a number of physiological functions, and that these peptides regulate induction of pheromone production in a variety of ways.

How caterpillar-damaged plants protect themselves by attracting parasitic wasps

Parasitic and predatory arthropods often prevent plants from being severely damaged by killing herbivores as they feed on the plants. Recent studies show that a variety of plants, when injured by herbivores, emit chemical signals that guide natural enemies to the herbivores. It is unlikely that herbivore-damaged plants initiate the production of chemicals solely to attract parasitoids and predators. The signaling role probably evolved secondarily from plant responses that produce toxins and deterrents against herbivores and antibiotics against pathogens. To effectively function as signals for natural enemies, the emitted volatiles should be clearly distinguishable from background odors, specific for prey or host species that feed on the plant, and emitted at times when the natural enemies forage. Our studies on the phenomena of herbivore-induced emissions of volatiles in corn and cotton plants and studies conducted by others indicate that (i) the clarity of the volatile signals is high, as they are unique for herbivore damage, produced in relatively large amounts, and easily distinguishable from background odors; (ii) specificity is limited when different herbivores feed on the same plant species but high as far as odors emitted by different plant species and genotypes are concerned; (iii) the signals are timed so that they are mainly released during the daytime, when natural enemies tend to forage, and they wane slowly after herbivory stops.

The chemistry of eavesdropping, alarm, and deceit

Arthropods that prey on or parasitize other arthropods frequently employ those chemical cues that reliably indicate the presence of their prey or hosts. Eavesdropping on the sex pheromone signals emitted to attract mates allows many predators and parasitoids to find and attack adult insects. The sex pheromones are also useful signals for egg parasitoids since eggs are frequently deposited on nearby plants soon after mating. When the larval stages of insects or other arthropods are the targets, a different foraging strategy is employed. The larvae are often chemically inconspicuous, but when they feed on plants the injured plants respond by producing and releasing defensive chemicals. These plant chemicals may also serve as "alarm signals" that are exploited by predators and parasitoids to locate their victims. There is considerable evidence that the volatile "alarm signals" are induced by interactions of substances from the herbivore with the damaged plant tissue. A very different strategy is employed by several groups of spiders that remain stationary and send out chemical signals that attract prey. Some of these spiders prey exclusively on male moths. They attract the males by emitting chemicals identical to the sex pheromones emitted by female moths. These few examples indicate the diversity of foraging strategies of arthropod predators and parasitoids. It is likely that many other interesting chemically mediated interactions between arthropod hunters and their victims remain to be discovered. Increased understanding of these systems will enable us to capitalize on natural interactions to develop more ecologically sound, environmentally safe methods for biological control of insect pests of agriculture.

Pheromonotropic activity of naturally occurring pyrokinin insect neuropeptides (FXPRLamide) in Helicoverpa zea

Insect neuropeptides, having the common C-terminal sequence FXPRLamide X = V, T, S, or G), were tested for phyeromonotropic activity in the moth, Helicoverpa zea. Dose-response studies indicated that locustamyotropin-II or locustapyrokinin-II induced production of more pheromone than was stimulated by the pheromone biosynthesis activating neuropeptide of this moth. Other peptides showed various degrees of pheromonotropic activity. The data indicated that substitution of the variable amino acid in the C-terminal pentapeptide sequence resulted in significant differences in pheromonotropic activity. However, the overall structure of the peptide was also found to be of importance.

Diurnal cycle of emission of induced volatile terpenoids by herbivore-injured cotton plant

Cotton plants attacked by herbivorous insect pests emit relatively large amounts of characteristic volatile terpenoids that have been implicated in the attraction of natural enemies of the herbivores. However, the composition of the blend of volatile terpenes released by the plants varies remarkably throughout the photoperiod. Some components are emitted in at least 10-fold greater quantities during the photophase than during the scotophase, whereas others are released continuously, without conforming to a pattern, during the entire time that the plants are under herbivore attack. The diurnal pattern of emission of volatile terpenoids was determined by collecting and analyzing the volatile compounds emitted by cotton plants subjected to feeding damage by beet armyworm larvae in situ. The damage was allowed to proceed for 3 days, and volatile emission was monitored continuously. During early stages of damage high levels of lipoxygenase-derived volatile compounds [e.g., (Z)-3-hexenal, (Z)-3-hexenyl acetate] and several terpene hydrocarbons [e.g., alpha-pinene, caryophyllene] were emitted. As damage proceeded, high levels of other terpenes, all acyclic [e.g., (E)-beta-ocimene, (E)-beta-farnesene], were emitted in a pronounced diurnal fashion; maximal emissions occurred in the afternoon. These acyclic terpenes followed this diurnal pattern of emission, even after removal of the caterpillars, although emission was in somewhat smaller amounts. In contrast, the emission of cyclic terpenes almost ceased after the caterpillars were removed.

Systemic release of chemical signals by herbivore-injured corn

Corn seedlings respond to insect herbivore-inflicted injury by releasing relatively large amounts of several characteristic terpenoids and, as a result, become highly attractive to parasitic wasps that attack the herbivores. Chemical evidence showed that the induced emission of volatiles is not limited to the sites of damage but occurs throughout the plant. This evidence was obtained by comparing the release of volatiles from leaves of unharmed (control) seedlings with the release of volatiles from undamaged leaves of seedlings with two injured leaves treated with caterpillar regurgitant. Immediately after injury no differences were measured in the released volatiles, but several hours later the undamaged leaves of injured plants released the terpenoids linalool, (3E)-4,8-dimethyl-1,3,7-nonatriene, and (3E,7E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene in significantly larger amounts than leaves of unharmed plants. Other volatiles that are released by herbivore-injured leaves were detected occasionally only in trace amounts from the undamaged leaves of a damaged seedling. The systemic release of volatiles by injured corn coincided with attractiveness to the parasitoid Cotesia marginiventris; undamaged leaves of injured plants became significantly more attractive than leaves from control seedlings. These findings show conclusively that when a plant is injured by an insect herbivore the whole plant emits chemical signals.

Innervation and neural regulation of the sex pheromone gland in female Heliothis moths

Female Heliothis moths normally produce their species-specific male attractant (sex pheromone blend) during scotophase, and this production is stimulated by pheromone biosynthesis activating neuropeptide (PBAN), presumably carried in the hemolymph. Several lines of evidence indicate that the central nervous system plays another critical role in this regulation. Pheromone biosynthesis was induced during photophase by electrical stimulation of the ventral nerve cord or the peripheral nerves projecting from the terminal abdominal ganglion to the pheromone gland in the tip of the abdomen. Electron microscopy further revealed that axonal branches innervate the gland tissue. Nerve branches associated with pheromone gland cells are enwrapped in glia and contain dense-core vesicles, suggesting that the innervation of the gland might be neurosecretory. Finally, the biogenic monoamine octopamine was nearly as effective as purified Heliothis zea PBAN in stimulating pheromone biosynthesis when injected into intact females during mid-photophase. Furthermore, both octopamine and PBAN stimulated significant increases in the pheromone content of the glands in isolated abdomens lacking a ventral nerve cord but only when abdomens were treated at the onset of scotophase. These data suggest that the regulation of sex pheromone production in Heliothis is more complex than previously thought. Activation of the gland appears to be governed by both neural and hormonal mechanisms, and these control mechanisms depend on photoperiodic cues.

Exploitation of Herbivore-Induced Plant Odors by Host-Seeking Parasitic Wasps

Corn seedlings release large amounts of terpenoid volatiles after they have been fed upon by caterpillars. Artificially damaged seedlings do not release these volatiles in significant amounts unless oral secretions from the caterpillars are applied to the damaged sites. Undamaged leaves, whether or not they are treated with oral secretions, do not release detectable amounts of the terpenoids. Females of the parasitic wasp Cotesia marginiventris (Cresson) learn to take advantage of those plant-produced volatiles to locate hosts when exposed to these volatiles in association with hosts or host by-products. The terpenoids may be produced in defense against herbivores but may also serve a secondary function in attracting the natural enemies of these herbivores.

Trans-sexually grafted antennae alter pheromone-directed behaviour in a moth

When tobacco hornworm moths (Manduca sexta) are tested in a wind tunnel with a source of female pheromones upwind, males but not normal females show pheromone-modulated anemotaxis and a characteristic mate-seeking behavioural sequence. These behaviours are produced by stimulation of sensory neurones found only in male antennae. These neurones project axons only to dendrites of pheromone-specific interneurones in the macroglomerular complex, a region of neuropil in the antennal lobe characteristic of males but not present in normal females. Some interneurones in the antennal lobes of female moths that have received grafts of male antennae (gynandromorphs) respond postsynaptically to stimulation with bombykal, a major component of the pheromone. They branch into a region resembling the macroglomerular complex, like their counterparts in normal males. We show here that gynandromorphic females respond to pheromonal stimulation with anemotaxis. We also find that normal females display a similar sequence in response to the odour of their egg-laying site, the tobacco plant. It is likely that a common motor path is used either by pheromone-specific interneurones in the antennal lobes of males or by tobacco-specific interneurones in females. We assume that the interneurones in gynandromorphic females that branch into the macroglomerular complex induced by a grafted male antenna can activate this pathway.