Joseph C. Dickens

Enhancement of Insect Pheromone Responses by Green Leaf Volatiles

verfahren wird fiir jede Beaufort-St/irkestufe und jedes Jahrzehnt durchgefiihrt, so dab man als Funktion der Zeit far jede Beaufort-St/irke einen Betrag des Luftdruckgradienten erh/ilt. Diesen kann man, etwa mit der heute am besten bestimmten Skala [5], auf Windgeschwindigkeit umrechnen. Damit ist die Umrechnung der Beaufort-St/irkeSch/itzungen friiherer Jahrzehnte auf Windgeschwindigkeiten m6glich. Es sei noch darauf hingewiesen, dab unsere Methode unabh/ingig yon der H~iufigkeitsverteilung des Windes in einem Seegebiet ist, da sie getrennt ftir jede Beaufort-Stufe durchgeftihrt wird. Die sich in einem Gebiet nach der Rekonstruktion ergebenden Wind~inderungen kOnnen daher als lokale Klima/inderungen angesehen werden. Als Beispiel wird hier die rekonstruierte Zeitreihe des Windes aus dem Feld vor der brasilianischen Kiiste wiedergegeben (Fig. 3). W/ihrend man von 1880 bis 1940 eine Zunahme um 0,5 m/s findet, ist nach 1950 praktisch kein von Null verschiedener Trend festzustellen. Diese Aussage gilt speziell fiir das Eingegangen am 17. Oktober 1989

Green leaf volatiles enhance aggregation pheromone of boll weevil, Anthonomus grandis

Enhancement of an insect pheromone response by green leaf volatiles is reported for the first time in the boll weevil, Anthonomus grandis Boh. (Coleoptera: Curculionidae). Single cell recordings from antennal olfactory receptors revealed a class of cells selectively responsive to six carbon alcohols and aldehydes (i.e., ‘green leaf volatiles’). Field tests with released weevils demonstrated enhanced trap captures with trans‐2‐hexen‐1‐ol, cis‐3‐hexen‐1‐ol, or 1‐hexanol paired with grandlure, the boll weevil aggregation pheromone, when in competition with grandlure alone. Although dose‐response curves constructed from electroantennograms were indicative of similar populations of receptor cells for selected six carbon alcohols, one of the compounds tested, cis‐2‐hexen‐1‐ol, was inactive in field tests. Trans‐2‐hexenal was active in single cell recordings, but was also inactive in field tests. In tests in cotton fields with indigenous weevil populations, trans‐2‐hexen‐1‐ol not only enhanced pheromone trap captures, but also extended the longevity of attractiveness of pheromone‐baited traps. The combined electrophysiological and field data support ‘across‐fiber coding’ of green leaf volatiles by boll weevil olfactory receptors. The results are discussed with regard to the chemistry of the host plant of the boll weevil, cotton (Gossypium hirsutum L.), and potential economic significance for boll weevil eradication/suppression.

Electroantennogram responses of the mediterranean fruit fly,Ceratitis capitata, to a spectrum of plant volatiles

Electroantennograms (EAGs) were recorded from unmated, laboratory-reared, male and femaleCeratitis capitata (medfly) in response to a range of C1 and C2 to C12 carbon chain-length aliphatic alcohols, aldehydes, acetates, and acids, and lactones, some of which are known volatiles from leaves and fruits. A large degree of EAG response uniformity between the sexes was observed, with only eight of the 70 compounds tested eliciting significantly larger amplitude EAG responses from female than male antennae. In general, for the five functional-group series tested, aldehydes and alcohols elicited greater responses than acetates, lactones, and acids. The unsaturated alcohols, aldehydes, acetates, and acids elicited equal or larger amplitude EAG responses than their comparable saturated compounds. For four of the functional-group series tested, the EAG response amplitude was significantly greater for a particular carbon chain length, with responsiveness to primary alcohols and aldehydes peaking at C6, acids peaking at C5–6, and acetates peaking at both C5 and C8. The EAG responses to both the 2- and 3-position monoenic alcohols peaked at C6 and C8, while the secondary alcohols peaked at C7. The greatest EAG responses of all compounds tested were elicited by monoenic C6 alcohols and aldehydes that are constituents of the “general green-leaf odor” that emanates from most plants. The potential adaptive benefit of selective sensitivity to green-leaf volatiles is discussed in regard to foraging behavior of medflies.

Olfaction in the boll weevil,Anthonomus grandis Boh. (Coleoptera: Curculionidae): Electroantennogram studies

Electroantennogram (EAG) techniques were utilized to measure the antennal olfactory responsiveness of adult boll weevils,Anthonomus grandis Boh. (Coleoptera: Curculionidae), to 38 odorants, including both insect and host plant (Gossypium hirsutum L.) volatiles. EAGs of both sexes were indicative of at least two receptor populations: one receptor population primarily responsive to pheromone components and related compounds, the other receptor population primarily responsive to plant odors. Similar responses to male aggregation pheromone components (i.e., compounds I, II, and III + IV) were obtained from both sexes, but females were slightly more sensitive to I. Both sexes were highly responsive to components of the “green leaf volatile complex,” especially the six-carbon saturated and monounsaturated primary alcohols. Heptanal was the most active aldehyde tested. More acceptors responded to oxygenated monoterpenes than to monoterpene hydrocarbons. β-Bisabolol, the major volatile of cotton, was the most active sesquiterpene. In general, males, which are responsible for host selection and pheromone production, were more sensitive to plant odors than were females. In fact, males were as sensitive to β-bisabolol and heptanal as to aggregation pheromone components. Electrophysiological data are discussed with regard to the role of insect and host plant volatiles in host selection and aggregation behavior of the boll weevil.

Olfactory reception of potential pheromones and plant odors by tarnished plant bug,Lygus lineolaris (Hemiptera: Miridae)

Olfactory reception of potential pheromones and host-plant odors by male and female tarnished plant bugs (TPBs),Lygus lineolaris (Hemiptera: Miridae), was investigated by utilizing electroantennogram (EAG) techniques. In general, EAGs were similar between the sexes. Among 31 compounds of seven chemical groups tested, insect-produced butyrates and host-plant-containing green leaf volatiles (GLVs) were the most active. Hexyl butyrate and (E)-2-hexenyl butyrate elicited greater EAGs in males than in females. Females responded with significantly greater EAGs to alcohol and aldehyde GLVs than to their acetate derivatives. Among GLVs, sexual dimorphism was also observed in response to (E)-2-hexenol and (E)-2-hexenal. Females were more sensitive to the monoterpene geraniol than were males. While nonanal was the most stimulatory compound tested, no sexual differences in EAGs to this compound were observed. These studies reveal olfactory receptors on TPB antennae responsive to insect and host-plant volatiles that are likely to play a role in host finding and sexual attraction.

Antennal olfactory responsiveness ofMicroplitis croceipes (Hymenoptera: Braconidae) to cotton plant volatiles.

Antennal olfactory responses of the parasitoid,Microplitis croceipes (Cresson) (Hymenoptera: Braconidae), to 29 cotton volatile compounds were measured by electroantennogram (EAG) techniques. No significant sexual differences were found in EAGs of males and females to volatiles emanating from 100-μg stimulus loads of the 29 cotton compounds. Green leaf volatiles (saturated and monounsaturated six-carbon alcohols, aldehydes, and their acetate derivatives), heptanal, and the benzene derivatives, benzaldehyde and acetophenone, elicited the largest EAGs. Monoterpenes elicited moderate EAGs withβ-ocimene being the most effective monoterpene tested. Among the sesquiterpenes tested,β-bisabolol was the most effective stimulus at the 100-μg dose. Dose-response curves constructed from EAGs of females revealed a low threshold for (Z)-3-hexenyl acetate, a compound previously shown to be an effective attractant in wind-tunnel bioassays. Comparison of relative volatilities of the various odorants indicated differential selectivity and sensitivity ofM. croceipes antennal receptors to them. The roles of cotton plant volatiles in host habitat location ofM. croceipes are discussed.

Perfluorinated moth pheromones. Synthesis and electrophysiological activity.

Perfluoroalkyl analogs of pheromone constituents were synthesized and responses from male antennal olfactory receptor neurons from three moth species were recorded during stimulation by these analogs. In each analog, the hydrophobic terminus, either a butyl or hexyl substituent on the (Z)-alkenyl chain, was replaced with a perfluorobutyl (Pfb, C4F9) or perfluorohexyl (Pfh, C6F13) moiety. Perfluoroalkyl analogs were more volatile than their hydrocarbon analogs, showing a decrease in gas chromatographic retention time by two to four methylene equivalents (Kovàts retention indices). Specialist neurons of maleHeliothis zea responded to a 0.02-μg dose of (Z)-11-hexadecenal (Z11–16∶Al) and a dose of 200 μg of Pfb-Z11–16∶Al with similar spike discharge rates. The HS(a) neurons ofTrichoplusia ni responded to a dose of 0.02 μg ofZ7–14∶OAc and a dose of 10 μg of Pfb-Z7–12∶Ac with similar discharge rates. The same difference in sensitivity to Pfb-Z7–12∶OH andZ7–12∶OH was observed for the responses of the HS(b) neuron and for the responses of theNS(a) neurons to Pfb-Z9–14∶Ac andZ9–14∶Ac. Sensilla ofDiatraea grandiosella similarly showed 100- to 1000-fold greater sensitivity toZ9–16—Al andZ11–16∶Al than to Pfh-Z9–16∶Al and Pfb-Z11–16∶Al. Thus, replacement of terminal alkyl groups with perfluoroalkyl groups in pheromone components produced biologically active compounds with increased volatility and displaced electrophysiological response profiles. Because of the diminished receptor cell sensitivity, we suggest that the binding of the fluorinated analogs to a putative receptor is reduced as a result of less favorable interaction between the hydrophobic protein binding site and the more rigid and more polar perfluoroalkyl moiety.

Antennal olfactory responsiveness of three sympatricIps species [Ips avulsus (Eichhoff),Ips calligraphus (Germar),Ips grandicollis (Eichhoff)], to intra- and interspecific behavioral chemicals

Electroantennograms (EAGs) from male and femaleIps avulsus, I. calligraphus, andI. grandicollis to their pheromones and selected host odorants or kairomones verified the presence of antennal olfactory receptors in both sexes of each species capable of detecting ipsdienol, ipsenol,cis- andtrans-verbenol,endo-brevicomin α-pinene, frontalin, and verbenone. Each species possesses receptors with lower thresholds and in greater abundance for the compounds they produce and to which they are behaviorally most responsive. Detection of bothIps andDendroctonus pheromones by the three cohabiting species provides a sensory basis for olfactory interactions among the species. Differences in both threshold and saturation levels for EAGs for the various behavioral chemicals could denote differences in specific behavioral roles for each compound.

Electroantennogram responses of mediterranean fruit fly,Ceratitis capitata (Diptera: Tephritidae) to trimedlure and itstrans isomers

Electroantennograms (EAGs) of unmated laboratory-reared male and femaleCeratitis capitata (Wiedemann) were recorded in response to the attractant trimedlure [tert-butyl 4(and 5)-chloro-trans-2-methylcyclohexane-1-carboxylate] and its fourtrans isomers. For both sexes, the magnitude of the EAG response was relatively low as compared to other previously tested compounds (i.e., plant volatiles). Dosage-response curves generated for all TML isomers revealed that flies responded to increasing dosages over a relatively narrow range (two to three log steps). Responses for both sexes peaked at ca. 10 μg dose for all isomers. Antennal response in males was greatest to the C isomer followed by the B1, A, and B2 isomers, while responses of females were greatest for the A isomer followed by B1, C, and B2. Both sexes exhibited a long recovery period for the response potential to return to baseline at doses above 1 μg for all of the isomers tested, except for B2. The low EAG sensitivity to trimedlure and the apparent EAG selectivity to the C isomer in males are discussed in relation to the known field attractancy of males to the C, A, B1, and B2 isomers.

Behavioral and electrophysiological studies with live larvae and larval rinses of the red imported fire ant,Solenopsis invicta Buren (Hymenoptera: Formicidae)

Behavioral and electrophysiological studies with live intact larvae and larval rinses of the red imported fire ant,Solenopsis invicta Buren, give undeniable evidence of a volatile material associated with the larvae of the ant that is capable of eliciting a response from brood-tending workers. In a Y-tube bioassay, worker ants were attracted equally to an airstream blown over sibling larvae or heterocolonial larvae. Workers were also attracted to a rinse of the larvae in a spot bioassay, aggregated about a piece of surrogate brood in another bioassay, and retrieved surrogate brood treated with the rinse material. A dose-response curve constructed from electroantennograms of workers revealed a receptor response of 1–100 brood equivalents.