Ovipositional behavior ofbracon mellitor say (Hymenoptera: Braconidae), a parasitoid of boll weevil (anthonomus grandis Boh.)

FemaleBracon mellitor Say responded to the frass of dietreared boll weevil larvae by probing with the ovipositor. Similar responses were elicited by the hexane and chloroform-methanol fractions after differential extraction of boll weevil feces. The active component of the chloroform-methanol fraction was identified as methylp-hydroxybenzoate (methyl parasept). Because the methyl parasept is an artificial component of the diet, the results suggest that the parasitoid response may be associatively learned. The response to methyl parasept decreased with time in the absence of reinforcement. Results demonstrate that certain chemicals may release behavior normally evoked by kairomones, and may interfere with the isolation of natural kairomones.This paper was approved for publication as TA 12350 by the Director, Texas Agricultural Experiment Station. This study was conducted in cooperation with the USDA. It was supported, in part, by the National Science Foundation and the International Biological Program (NSF-SC-0030) of the University of California. The findings are those of the authors, and not necessarily those of the University of California or the National Science Foundation. Mention of a proprietary or commercial product in this paper does not constitute an endorsement of this product by the USDA.     

A sex pheromone for male boll weevils from females

Steam distillation of frass of the female boll weevil,Anthonomus grandis Boheman, yielded an extract that was more attractive to males than to females in the laboratory. Extracts purified by TLC were attractive to males only. The active components appear to be alcohols and hydrocarbons.Coleoptera: Curculionidae.In cooperation with the Mississippi Agricultural and Forestry Experiment Station, Mississippi State, Mississippi 39762.Mention of proprietary products does not constitute an endorsement by the USDA.     

Possible roles of cotton bud sugars and terpenoids in oviposition by the boll weevil

Several cottonGossypium spp. race stocks have been identified that possess resistance to the boll weevilAnthonomus grandis Boh. because oviposition is decreased. In this work, a number of known cotton constituents that influence stimulation of feeding and attractancy for this insect were found to have little or no influence on oviposition. These include gossypol, -bis-abolol, caryophyllene, some fatty acids and their methyl esters, some wax esters, flavonoids, condensed tannins, and chrysanthemin. Analysis of cotton bud surfaces showed that the content of volatile terpenoids was generally higher in resistant lines, but bioassays did not show decreased oviposition in the presence of the terpenoids. The sugars (glucose, fructose, and sucrose) found in anthers, uniformly stimulated oviposition in the bioassay, and their content was higher in susceptible lines. These results suggest that a major basis of resistance to boll weevils as related to oviposition may be the decreased content of sugars in resistant lines. The analysis of free sugars in the anthers, and perhaps also the analysis of bud surface terpenoids, may provide a basis for selection or genetic production of cotton lines resistant to the boll weevil.     

Identification and field evaluation of the compounds comprising the sex pheromone of the female boll weevil

The terpenoid compounds (+)-cis-2-isopropenyl-1-methylcyclo-butaneethanol (I), (Z)-3,3-dimethyl-A-cyclohexaneethanol (II), and -caryophyllene were isolated from frass of the female boll weevil (Anthonomus grandis Boheman). In laboratory bioassays, a mixture of these components attracted primarily males, whereas the male pheromone, grandlure, attracted primarily females. The addition to the cotton bud hydrocarbons, -pinene, myrcene, andl-lim-nene, improved the response by males so that the potency of the mixture was comparable to that of grandlure for females. In field tests, I + II + hydrocarbons attracted both sexes, but grandlure alone and grandlure + hydrocarbons were more effective.Coleoptera: Curculionidae.In cooperation with the Mississippi Agricultural and Forestry Experiment Station, Mississippi State, Mississippi 39762.Mention of a commercial or proprietary product in this paper does not constitute endorsement of this product by USDA.     

Role of host-produced stimuli and learning in host selection behavior ofCotesia (=Apanteles) marginiventris (Cresson)

The study was conducted to determine how various factors, including learning, influence the host-selection behavior of the larval parasitoid Cotesia (=Apanteles)marginiventris (Cresson). Frass, silk, and feeding damage fromSpodoptera frugiperda (J.E. Smith) larvae elicited visits and antennal examinations by females of the parasitoid. Learning played a role in the effective response to these stimuli in that previous contact experience with hosts and/or chewed leaves, exuviae, frass, or hemolymph significantly enhanced the response of the parasitoid. Previous contact with host plants alone did not improve the responses. Experienced parasitoids were more active and spent less time at rest than inexperienced parasitoids. Previous experience was also found to be of significant importance in the ability of the parasitoid to discriminate between parasitized and unparasitized host larvae.Hymenoptera: Braconidae.In cooperation with the University of Georgia College of Agriculture Experiment Stations, Coastal Plain Station, Tifton, Georgia 31793-0748.Mention of a proprietary product in this paper does not constitute endorsement of this product by the USDA.Visiting scientist with the Department of Entomology, Coastal Plain Station, Tifton, Georgia, during this study.     

Chemical cues from Murgantia histrionica eliciting host location and recognition in the egg parasitoid Trissolcus brochymenae

Host location and recognition by the egg parasitoid Trissolcus brochymenae were analyzed in terms of response to kairomones from several stages of its host, Murgantia histrionica. In a Y-tube olfactometer, parasitoid females responded by increasing residence time and/or reducing linear speed to chemical cues from gravid females, virgin females and males, fifth and third instars, and eggs. In an open arena, T. brochymenae females also responded to patches contaminated by chemicals from the host in the same stages, sexes, and/or physiological conditions as those tested in the olfactometer. The parasitoid displayed arrestment behavior, increased residence time, changed walking pattern, and intense substrate examination. When host egg clusters or glass dummies with a chemical egg extract were placed on the host-contaminated open arena, these elicited an orientation response in the parasitoid. In addition, the chemical egg extract without dummies elicited the same response, whereas dummies without extract did not influence parasitoid behavior. In a closed arena, the parasitoid females recognized and attempted to probe glass beads treated with chemical extracts of host eggs. There were no significant differences compared with their response to the host eggs, and they did not respond to untreated beads. Host recognition was elicited by chemicals from the follicular secretion used by the host to glue the eggs on the substrate. These results are discussed in relation to the level of the host selection sequence influenced by these cues.     

Ovipositional behavior ofBracon mellitor Say,a parasitoid of the boll weevil (Anthonomus grandis Boheman) III. Isolation and identification of natural releasers of ovipositor probing

The chemical cues in the frass of larvae of the boll weevil,Anthonomus grandis Boheman, that elicit an ovipositional response in females ofBracon mellitor Say were isolated by a combination of column, thin-layer, and gas-liquid chromatography. Derivatization, analytical tests, and mass spectrometry were used to identify the biologically active materials as long-chain fatty acid esters of cholesterol. Bioassays with natural and synthetic cholesteryl esters were used to confirm activity. The activity of the cholesteryl esters was also confirmed by using females that had not been previously exposed to hosts.Hymenoptera: Braconidae.Mention of a proprietary or commercial product in this paper does not constitute an endorsement of this product by the U.S. Department of Agriculture.Approved for publication in TA 12593 by the Director, Texas Agricultural Experiment Station.This study was supported, in part, by the National Science Foundation and the International Biological Program (NSF-SC-0030) to the University of California. The findings are those of the authors and not necessarily those of the University of California or NSF.     

Semiochemicals from Herbivory Induced Cotton Plants Enhance the Foraging Behavior of the Cotton Boll Weevil, Anthonomus grandis

The boll weevil, Anthonomus grandis, has been monitored through deployment of traps baited with aggregation pheromone components. However, field studies have shown that the number of insects caught in these traps is significantly reduced during cotton squaring, suggesting that volatiles produced by plants at this phenological stage may be involved in attraction. Here, we evaluated the chemical profile of volatile organic compounds (VOCs) emitted by undamaged or damaged cotton plants at different phenological stages, under different infestation conditions, and determined the attractiveness of these VOCs to adults of A. grandis. In addition, we investigated whether or not VOCs released by cotton plants enhanced the attractiveness of the aggregation pheromone emitted by male boll weevils. Behavioral responses of A. grandis to VOCs from conspecific-damaged, heterospecific-damaged (Spodoptera frugiperda and Euschistus heros) and undamaged cotton plants, at different phenological stages, were assessed in Y-tube olfactometers. The results showed that volatiles emitted from reproductive cotton plants damaged by conspecifics were attractive to adults boll weevils, whereas volatiles induced by heterospecific herbivores were not as attractive. Additionally, addition of boll weevil-induced volatiles from reproductive cotton plants to aggregation pheromone gave increased attraction, relative to the pheromone alone. The VOC profiles of undamaged and mechanically damaged cotton plants, in both phenological stages, were not different. Chemical analysis showed that cotton plants produced qualitatively similar volatile profiles regardless of damage type, but the quantities produced differed according to the plant’s phenological stage and the herbivore species. Notably, vegetative cotton plants released higher amounts of VOCs compared to reproductive plants. At both stages, the highest rate of VOC release was observed in A. grandis-damaged plants. Results show that A. grandis uses conspecific herbivore-induced volatiles in host location, and that homoterpene compounds, such as (E)-4,8-dimethylnona-1,3,7–triene and (E,E)-4,8,12-trimethyltrideca-1,3,7,11-tetraene and the monoterpene (E)-ocimene, may be involved in preference for host plants at the reproductive stage.     

Identification of Male Pecan Weevil Pheromone

The pecan weevil, Curculio caryae is a serious economic pest of pecans (Caryae illinoensis). In late summer, the weevil attacks maturing nuts and damages them when making feeding and/or oviposition punctures. The larvae leave the nut and burrow into the soil, remaining there for two to three years before emerging as adults to commence another cycle. This present work has resulted in the identification of the male pecan weevil pheromone as a mixture of four components; I as both the cis and trans isomers of 2-propenyl-1-methyl-cyclobutaneethanol [also identified as (1R,2S)?(+ and ?)-grandisol], II [(Z)-3,3-dimethylcyclohexane-Δ^1,β-ethanol], III [(Z)-3,3-dimethylcyclohexane-Δ^1,α-acetaldehyde], and IV [(E)-3,3-dimethylcyclohexane-Δ^1,α-acetaldehyde]. They are synthesized by the male pecan weevil, but not by the female, in the ratio 7:16:3:3 of I, II, III, and IV, respectively. These same compounds were earlier identified as the pheromone of the male boll weevil, Anthonomus grandis (Boh.), in which they were isolated from frass in the ratio 6:6:1.5:1.5. However, only the (+) isomer of grandisol was synthesized by male boll weevil. In laboratory tests, 80% of female pecan weevils were attracted to a synthetic formulation based on the ratio found in male pecan weevils, while only 28% of the females were attracted to a synthetic formulation based on the ratio found in boll weevil frass. The attraction of males to these synthetic formulations was minimal (14, 4, and 2%, respectively). Live males and their extracts were also attractive to females, but males did not respond to male or females. Preliminary field tests demonstrated that females were attracted to males and the synthetic pecan weevil formulation, but not to the synthetic boll weevil formulation.     

Collection of pheromone from atmosphere surrounding boll weevils,Anthonomus grandis

An effluvial method was developed to collect the pheromone, grandlure from actively calling male boll weevils,Anthonomus grandis Boheman. The adsorbant, Porapak Q (ethylvinylbenzene-divinylbenzene), was utilized to trap and concentrate the pheromone. Captured pheromone was desorbed from columns packed with Porapak Q by elution withn-pentane and quantified by capillary column gas-liquid chromatography. In recovery studies with known amounts of synthetic grandlure, we found that the amount of each pheromone component collected was a function of collection duration, elution volume, and initial concentration. This effluvial method was capable of recovering as much as 94.9% of a known quantity (80 μg) of grandlure. The chromatograms were free of extraneous peaks. In studies of insect-produced pheromone, the effluvial method was used to collect pheromone from the air space surrounding male boll weevils as they fed on flower buds from CAMD-E cotton. The quantity and quality of boll-weevil-produced pheromone was determined for days 6, 8, 10, 11, 12, 13, and 14 of boll weevil adulthood. The maximum quantity of natural pheromone was produced on day 13 (4.2 μg/weevil) with a pheromone component ratio of 2.41∶2.29∶0.95∶1 for components I, II, III, and IV, respectively. The effluvial method described in this report is an efficient method to collect and quantify boll weevil pheromone from the atmosphere surrounding actively calling insects. Other applications of this method are suggested.     

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.     

Feeding deterrent compounds to the boll weevil,Anthonomus grandis Boheman in Rose-of-Sharon,Hibiscus syriacus L.

The Rose-of-Sharon,Hibiscus syriacus (L.), can be a significant alternate host plant for the boll weevil,Anthonomus gradis (Boh.). Boll weevils are known to be deterred from feeding and ovipositing in the buds unless the calyx is removed. This investigation was initiated to identify calyx allelochemicals that deter feeding with the eventual strategy of breeding for cotton lines high in these allelochemicals in the appropriate tissues. The feeding deterrency of calyx tissue from the buds of Rose-of-Sharon for the boll weevil was confirmed. The most active deterrent fraction was found to contain mostly fatty acids and their methyl esters. Saturated fatty acids and their methyl esters were generally found to be stimulatory, while the unsaturated species were found to be deterrent. Higher quantities of the fatty acids, particularly the unsaturated species, were found in Rose-of-Sharon calyx tissue than in the buds without calyx. This supports the hypothesis developed through the isolational work and testing of standards that the unsaturated fatty acids are significant deterrents of boll weevil feeding.Coleoptera: Curculionidae.Malvales: Malvaceae.Mention of a commercial or proprietary product in this paper does not constitute endorsement of this product by Delta State University or USDA.     

Beneficial arthropod behavior mediated by airborne semiochemicals

Heliothis zea (Boddie) larvae fed cowpea seedlings produced volatile semiochemicals to whichMicroplitis demolitor Wilkinson responded in a wind tunnel. However, mostM. demolitor females reared fromH. zea larvae fed an artificial diet were not responsive at emergence to the same volatile semiochemicals. A preflight contact with frass fromH. zea fed cowpea was needed to stimulate a response of sustained flight in a wind tunnel. The most consistent flight response was 7–10 days postemergence. Response resulting from both antennal and ovipositor contact with host frass during preflight stimulation was no better than from antennal contact alone. Chilling the parasitoid pupae rendered most of the emerging females unresponsive to volatile semiochemicals.Hymenoptera: Braconidae.     

Plant-Derived Semiochemicals as Contact Host Location Stimuli for a Parasitoid of Leafminers

We elucidated the source of chemical cues in a system where the host is concealed and the parasitoid has no direct contact with the host larvae or its frass. Behavioral bioassays with Pholetesor bicolor, a larval parasitoid of the apple leafminer, Phyllonorycter pomonella, showed that the herbivore-damaged leaf epidermis (mine) elicited ovipositional probing of parasitoid females. Probing on larvae or frass was seldom observed. Hexane extracts of mines elicited the same ovipositional probing behavior while no response was observed with hexane extracts of larvae or frass or with methanol and diethyl ether extracts. In addition, gas chromatographic analyses showed qualitatively and quantitatively different profiles of these three components of the host-plant complex. By far the highest quantities and also the highest number of compounds was recovered from mine extracts. Identified compounds in the mine included six alkanes (n-C ₂₇ to n-C ₃₃) and squalene (C₃₀H₅₀). A synthetic blend of the seven compounds was slightly less active in biotests than the equivalent natural blend, as shown by a time delay in female response. We conclude that this leafminer parasitoid does not rely on host-derived kairomones but instead uses plant-derived semiochemicals for host location and ovipositional probing behavior.     

Feeding and oviposition deterrent activities of flower buds of globemallow,Sphaeralcea emoryi torrey,against boll weevil,Anthonomus grandis Boheman (Coleoptera: Curculionidae)

The globemallow,Sphaeralcea emoryi Torrey, a plant native to Arizona was evaluated as a source of feeding or oviposition deterrents to the boll weevil,Anthonomus grandis Boheman. Feeding and oviposition responses of reproductive weevils to the flower buds and artificial diets spiked with dry powder or extracts of the globemallow buds were determined. Boll weevils were deterred from feeding and ovipositing in the flower buds unless the calyxes were removed. Male and virgin female weevils were discouraged from feeding as much as gravid weevils. Secondary chemicals in the flower buds served primarily as feeding deterrents but also prevented oviposition. The concentration of these chemicals was highest in the calyxes of the buds, and potent deterrent activity could be extracted from the calyxes with methanol. Boll weevils were able to perceive the deterrents by contact chemosensory organs on the antennae, maxillary palps and labial palps.     

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.     

Components of male aggregation pheromone of strawberry blossom weevil, Anthonomus rubi herbst. (Coleoptera:Curculionidae)

The strawberry blossom weevil, Anthonomus rubi, is a major pest of strawberries in the United Kingdom and continental Europe. As part of a project to develop noninsecticidal control methods, the pheromone system of this species was investigated. Comparison of volatiles produced by field-collected, overwintering individuals of each sex led to identification of three male-specific compounds—(Z)-2-(3,3-dimethylcyclohexylidene)ethanol, (cis)-1-methyl-2-(1-methylethenyl)cyclobutaneethanol, and 2-(1-methylethenyl)5-methyl-4-hexen-1-ol (lavandulol)—in amounts of 6.1, 1.2, and 0.82 g/day/male. The first two compounds are components of the aggregation pheromone of the boll weevil, Anthonomus grandis, grandlure II and grandlure I, respectively. Grandlure I was the (1R,2S)-(+) enantiomer and lavandulol was a single enantiomer, although the absolute configuration was not determined. Trace amounts of the other two grandlure components (Z)-(3,3-dimethylcyclohexylidene)acetaldehyde (grandlure III) and (E)-(3,3-dimethylcyclohexylidene)acetaldehyde (grandlure IV) were also detected. (E,E)-1-(1-Methylethyl)-4-methylene8-methyl-2,7-cyclo-decadiene (germacrene-D), a known volatile from strawberry plants, Fragaria ananassa, was collected in increased amounts in the presence of pheromone-producing weevils. Male weevils only produced pheromone on F. ananassaand not on scented mayweed, Matracaria recutita, or cowparsley, Anthriscus sylvestris, although these are known food sources. In field trials using various combinations of synthetic grandlures I, II, III, and IV and lavandulol, significantly more weevils were caught in traps baited with blends containing grandlure I and II and lavandulol than in those baited with blends without lavandulol or unbaited controls. Addition of grandlure III and IV had no significant effect on attractiveness. Horizontal sticky traps were found to be more effective than vertical sticky traps or standard boll weevil traps. In mid-season females predominated in the catches, but later more males than females were trapped.     

Evidence for Additional Pheromonal Components Mediating Oviposition Aggregation in Schistocerca gregaria

Oviposition responses of gregarious adult gravid females of the desert locust Schistocerca gregaria to sand previously used for oviposition by gregarious females (contaminated sand, with eggs and froth removed) and to sterilized sand (noncontaminated sand) were compared in bioassays. Gregarious females preferentially oviposited into contaminated sand. The number of egg pods laid by the females was found to correlate with the frequency of use of the contaminated sand for oviposition. Charcoal-trapped volatiles from the contaminated sand also elicited similar responses from gregarious females. Gas chromatography–electroantennographic detection (GC-EAD) analysis of the trapped volatiles revealed the presence of three electrophysiologically active compounds that were identified by GC-MS as (Z)-6-octen-2-one, (E,E)-3,5-octadien-2-one and its geometric isomer (E,Z)-3,5-octadien-2-one. The relative amounts of the three EAG-active compounds in the volatiles increased with consecutive oviposition by the gregarious females. These results indicate mediation of additional pheromonal components in the oviposition behavior of gregarious desert locust that are distinct from those that were previously identified in the froth volatiles.     

Influence of Two Acyclic Homoterpenes (Tetranorterpenes) on the Foraging Behavior of <Emphasis Type="Italic">Anthonomus grandis</Emphasis> Boh

Previous studies have shown that the boll weevil, Anthonomus grandis, is attracted to constitutive and conspecific herbivore-induced cotton volatiles, preferring the blend emitted by cotton at the reproductive over the vegetative stage. Moreover, this preference was paralleled by the release of the acyclic homoterpenes (tetranorterpenes) (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT) and (E,E)-4,8,12-trimethyltrideca-1,3,7,11-tetraene (TMTT) in Delta Opal cotton being higher at the vegetative than at the reproductive stage. Here, we evaluated whether this difference in release of acyclic homoterpenes also occurred in other cotton varieties, and if boll weevils could recognize these compounds as indicators of a specific cotton phenological stage. Results showed that cotton genotypes CNPA TB-90, BRS-293 and Delta Opal all produced higher levels of DMNT and TMTT at the vegetative stage than at the reproductive stage and that these homoterpenes allowed for principal component analysis separation of volatiles produced by the two phenological stages. Electroantennograms confirmed boll weevil antennal responses to DMNT and TMTT. Behavioral assays, using Y-tube olfactometers, showed that adding synthetic homoterpenes to reproductive cotton volatiles (mimicking cotton at the vegetative stage in terms of homoterpene levels) resulted in reduced attraction to boll weevils compared to that to unmodified reproductive cotton. Weevils showed no preference when given a choice between plants at the vegetative stage and the vegetative stage-mimicked plant. Altogether, the results show that DMNT and TMTT are used by boll weevils to distinguish between cotton phenological stages.     

The Egg Parasitoid <Emphasis Type="Italic">Trissolcus basalis</Emphasis> uses <Emphasis Type="Italic">n</Emphasis>-nonadecane,a Cuticular Hydrocarbon from its Stink Bug Host <Emphasis Type="Italic">Nezara viridula</Emphasis>, to Discriminate Between Female and Male Hosts

Contact kairomones from adult southern green stink bugs, Nezara viridula (L.) (Heteroptera: Pentatomidae) that elicit foraging behavior of the egg parasitoid Trissolcus basalis (Wollaston) were investigated in laboratory experiments. Chemical residues from tarsi and scutella of N. viridula induced foraging by gravid female T. basalis. Residues from body parts of female N. viridula elicited stronger responses than those from the corresponding body parts of males. Deproteinized tarsi still elicited searching responses from wasps, indicating that the kairomone was not proteinaceous. Hexane extracts of host cuticular lipids induced searching responses from T. basalis, with a strong preference for extracts from female hosts. Extracts consisted primarily of linear alkanes from nC₁₉ to nC₃₄, with quantitative and qualitative differences between the sexes. Extracts of female N. viridula contained more nC₂₃, nC₂₄, and nC₂₅ than the corresponding extracts from males, whereas nC₁₉ was detected only in extracts from males. Direct-contact solid phase microextraction (DC-SPME) of N. viridula cuticle and of residues left by adult bugs walking on a glass plate confirmed gender-specific differences in nC₁₉. Trissolcus basalis females responded weakly to a reconstructed blend of the straight-chain hydrocarbons, suggesting that minor components other than linear alkanes must be part of the kairomone. Addition of nC₁₉ to hexane extracts of female N. viridula significantly reduced the wasps’ arrestment responses, similar to wasps’ responses to hexane extracts of male hosts. Overall, our results suggest that a contact kairomone that elicits foraging by T. basalis females is present in the cuticular lipids of N. viridula, and that the presence or absence of nC₁₉ allows T. basalis females to distinguish between residues left by male or female hosts. The ecological significance of these results in the host location behavior of scelionid egg parasitoids is discussed.