Change in Behavioral Response to Herbivore-induced Plant Volatiles in a Predatory Mite Population

Damage by herbivorous spider mites induces plants to produce volatiles that attract predatory mites that consume the spider mites. A clear attraction to volatiles from Lima bean plants infested with the spider mite Tetranychus urticae has been consistently reported during more than 15 years for the predatory mite Phytoseiulus persimilis. We have monitored the response to volatiles from spider-mite infested Lima bean plants for a laboratory population of the predatory mite from 1991 to 1995 on a regular basis. A reduction in the level of attraction in the laboratory population of P. persimilis was recorded in mid-1992. The attraction of the laboratory population was weaker than that of a commercial population in the latter part of 1992, but the responses of these two populations were similarly weak in 1994 and 1995. Therefore, a behavioral change has also occurred in this commercial population. Experiments were carried out to address the potential causes of this change in attraction. The attraction of predators from a commercial population with a strong response decreased after being reared in our laboratory. Within a predator population with a low degree of attraction, strongly responding predators were present and they could be isolated on the basis of their behavior: predators that stayed on spider-mite infested plants in the rearing set-up had a strong attraction, while predators that had dispersed from the rearing set-up were not attracted to prey-infested bean plants. From our laboratory population with a low degree of attraction, isofemale lines were initiated and maintained for more than 20 generations. All isofemale lines exhibited a consistently strong attraction to spider mite-induced plant volatiles, similar to the attraction recorded for several populations in the past 15 years. Neither in a population with a strong attraction nor in two with a weak attraction was the response of the predators affected by a starvation period of 1–3 hr. Based on these results, possible causes for the observed reduction in predator attraction to spider mite-induced bean volatiles are discussed. The predatory mite P. persimilis is a cornerstone of biological control in many crops worldwide. Therefore, the change in foraging behavior recorded in this predator may have serious consequences for biological control of spider mites.     

Impact of Herbivore-induced Plant Volatiles on Parasitoid Foraging Success: A Spatial Simulation of the <Emphasis Type="Italic">Cotesia rubecula,Pieris rapae,</Emphasis> and <Emphasis Type="Italic">Brassica oleracea</Emphasis> System

Many parasitoids are known to use herbivore-induced plant volatiles as cues to locate hosts. However, data are lacking on how much of an advantage a parasitoid can gain from following these plant cues and which factors can limit the value of these cues to the parasitoid. In this study, we simulate the Cotesia rubecula-Pieris rapae-Brassica oleracea system, and ask how many more hosts can a parasitoid attack in a single day of foraging by following plant signals versus randomly foraging. We vary herbivore density, plant response time, parasitoid flight distance, and available host stages to see under which conditions parasitoids benefit from herbivore-induced plant cues. In most of the parameter combinations studied, parasitoids that responded to cues attacked more hosts than those that foraged randomly. Parasitoids following plant cues attacked up to ten times more hosts when they were able to successfully attack herbivores older than first instar; however, if parasitoids were limited to first instar hosts, those following plant cues were at a disadvantage when plants took longer than a day to respond to herbivory. At low herbivore densities, only parasitoids with a larger foraging radius could take advantage of plant cues. Although preference for herbivore-induced volatiles was not always beneficial for a parasitoid, under the most likely natural conditions, the model predicts that C. rubecula gains fitness from following plant cues.     

Attraction to Herbivore-induced Plant Volatiles by the Host-foraging Parasitoid Fly <Emphasis Type="Italic">Exorista japonica</Emphasis>

Responses of the tachinid fly Exorista japonica Townsend to odors from corn plants infested with the fly’s host, the larvae of the noctuid moth Mythimna separata (Walker), were examined in a wind tunnel. Naïve female flies showed a higher rate of landing on M. separata-infested corn plants from which the host larvae had been removed than on artificially damaged or intact corn plants. When paper impregnated with a solution of headspace volatiles collected from host-infested plants was attached to intact plants, females landed on the plants at a high rate. Females also responded to intact plants to which had been attached with paper impregnated with a synthetic blend of nine chemicals identified previously in host-infested plants. There was an optimum concentration of the synthetic blend for the females’ landing. Of the nine chemicals identified previously, four [(E)-4,8-dimethyl-1,3,7-nonatriene, indole, 3-hydroxy-2-butanone, and 2-methyl-1-propanol] released only by host-infested plants were classified as a host-induced blend. The other five [(Z)-3-hexen-1-yl acetate, (E)-2-hexenal, hexanal, (Z)-3-hexen-1-ol, and linalool] were classified as a non-specific blend released not only by infested plants but also by artificially damaged or intact plants. In the wind tunnel, E. japonica females did not respond to intact plants to which paper containing a solution of non-specific blend or host-induced blend was attached. However, they showed a high level of response to a mixture of the non-specific and host-induced blends. These results indicate that naïve E. japonica use a combination of non-specific and host-induced blends as an olfactory cue for locating host-infested plants.     

Identification of Volatiles That Are Used in Discrimination Between Plants Infested with Prey or Nonprey Herbivores by a Predatory Mite

Carnivorous arthropods can use herbivore-induced plant volatiles to locate their herbivorous prey. In the field, carnivores are confronted with information from plants infested with herbivores that may differ in their suitability as prey. Discrimination by the predatory mite Phytoseiulus persimilis between volatiles from lima bean plants infested with the prey herbivore Tetranychus urticae, or plants infested with the nonprey caterpillar Spodoptera exigua, depends on spider mite density. In this article, we analyzed the chemical composition of the volatile blends from T. urticae-infested lima bean plants at different densities of spider mites, and from S. exigua-infested plants. Based on the behavioral preferences of P. persimilis and the volatile profiles, we selected compounds that potentially enable the mite to discriminate between T. urticae-induced and S. exigua-induced volatiles. Subsequently, we demonstrated in Y-tube olfactometer assays that the relatively large amounts of methyl salicylate and (3E, 7E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene emitted by T. urticae-infested bean plants compared to S. exigua-infested plants enable the predators to discriminate. Our data show that specific compounds from complex herbivore-induced volatile blends can play an important role in the selective foraging behavior of natural enemies of herbivorous arthropods.     

Essential Compounds in Herbivore-Induced Plant Volatiles that Attract the Predatory Mite <Emphasis Type="Italic">Neoseiulus womersleyi</Emphasis>

Carnivorous arthropods use volatile infochemicals emitted from prey-infested plants in their foraging behavior. Although several volatile components are common among plant species, the compositions differ among prey–plant complexes. Studies showed that the predatory mite Neoseiulus womersleyi is attracted only to previously experienced plant volatiles. In this study, we identified the attractant components in prey-induced plant volatiles of two prey–plant complexes. N. womersleyi reared on Tetranychus kanzawai-infested tea leaves showed significant preference for a mixture of three synthetic compounds [mimics of the T. kanzawai-induced tea leaves volatiles: (E)-β-ocimene, (E)-4,8-dimethyl-1,3,7-nonatriene (DMNT), and (E,E)-α-farnesene] at a level comparable to that for T. kanzawai-induced tea plant volatiles. However, mixtures lacking any of these compounds did not attract the predatory mites. Likewise, N. womersleyi reared on T. urticae-infested kidney bean plants showed a significant preference for a mixture of four synthetic compounds [mimics of the T. urticae-induced kidney bean volatiles: DMNT, methyl salicylate (MeSA), β-caryophyllene, and (E,E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene] at a level comparable to that for T. urticae-induced kidney bean volatiles. The absence of any of the four compounds resulted in no attraction. These results indicate that N. womersleyi can use at least four volatile components to identify prey-infested plants.     

Attraction and repulsion of the aphid,Cavariella aegopodii,by Plant Odors

Cavariella aegopodii is induced to land on traps by the monoterpene carvone, and the relevance of this to host-finding by the aphid is discussed. Catches are reduced by linalool. The interaction of plant chemicals in natural communities is discussed, and the possibility of using repellent chemicals for crop protection is suggested.     

Olfactory Reception of Conspecific Aggregation Pheromone and Plant Odors by Nymphs of the Predator, Podisus maculiventris

Olfactory reception of 23 odorants, including plant volatiles and male-produced aggregation pheromone, by third and fifth instars of the spined soldier bug (SSB) Podisus maculiventris was investigated by using electroantennograms (EAGs). Both nymphal stages were sensitive to male-produced aggregation pheromone components (E)-2-hexenal, benzyl alcohol, and -terpineol. The plant volatile, (E)-2-hexen-1-ol (a chemical known to be released by plants in response to prey feeding over the short-term), elicited the largest EAGs of all volatiles tested. While third instars were sensitive to nonanal, only fifth instars responded to both nonanal and (±)-linalool, both compounds released systemically by plants in response to feeding by potential prey. Antennal extirpation experiments showed that sensilla responsive to hexan-1-ol, (E)-2-hexenal, and -terpineol are situated mainly on the terminal antennal segment. The results support the hypothesis that P. maculiventris nymphs use both plant volatiles and pheromone components in locating potential prey and other behaviors.     

Herbivore-induced Plant Volatiles Trigger Sporulation in Entomopathogenic Fungi: The Case of <Emphasis Type="Italic">Neozygites tanajoae</Emphasis> Infecting the Cassava Green Mite

A large body of evidence shows that plants release volatile chemicals upon attack by herbivores. These volatiles influence the performance of natural enemies. Nearly all the evidence on the effect of plant volatiles on natural enemies of herbivores concerns predators, parasitoids, and entomophagous nematodes. However, other entomopathogens, such as fungi, have not been studied yet for the way they exploit the chemical information that the plant conveys on the presence of herbivores. We tested the hypothesis that volatiles emanating from cassava plants infested by green mites (Mononychellus tanajoa) trigger sporulation in three isolates of the acaropathogenic fungus Neozygites tanajoae. Tests were conducted under climatic conditions optimal to fungal conidiation, such that the influence of the plant volatiles could only alter the quantity of conidia produced. For two isolates (Altal.brz and Colal.brz), it was found that, compared with clean air, the presence of volatiles from clean, excised leaf discs suppressed conidia production. This suppressive effect disappeared in the presence of herbivore-damaged leaves for the isolate Colal.brz. For the third isolate, no significant effects were observed. Another experiment differing mainly in the amount of volatiles showed that two isolates produced more conidia when exposed to herbivore-damaged leaves compared with clean air. Taken together, the results show that volatiles from clean plants suppress conidiation, whereas herbivore-induced plant volatiles promote conidiation of N. tanajoae. These opposing effects suggest that the entomopathogenic fungus tunes the release of spores to herbivore-induced plant signals indicating the presence of hosts.     

Electrophysiological and Behavioral Responses of the Cabbage Moth to Plant Volatiles

Plant volatiles from cabbage and chrysanthemum were studied as to how they affect behavior of the cabbage moth, Mamestra brassicae (L.). Chemical, electrophysiological, and behavioral techniques were used. The electroantennographic (EAG) evaluation of selected compounds from Brassi-caceae showed that isothiocyanates (NCS) elicited weak responses, and some did not evoke significant EAG responses at all. Green leaf volatiles (GLVs) evoked the strongest responses in both male and female antennae. The capacity of NCS to stimulate upwind flight of mated females was not different at doses of 10^–7, 10^–6, or 10^–5 g when tested in a wind tunnel. At the higher doses, allyl NCS stimulated upwind flight in the females more than the other compounds. Allyl NCS was significantly better than the other compounds at stimulating females to land on targets. Mated females flew upwind and landed on the targets with allyl NCS more often than virgin females and males. With respect to the behavioral activity of GLVs, only (E)-2-hexenal and (Z)-3-hexenyl acetate elicited upwind flight and landing in females. Ten compounds were identified from a chrysanthemum extract by using coupled gas chromatography–electroantennography. Five of these, (Z)-3-hexenyl acetate, 1-8-cineole, -terpinene, chrysanthenone, and camphor, elicited upwind flight of mated females, but only three stimulated landing.     

Experience-Induced Habituation and Preference Towards Non-Host Plant Odors in Ovipositing Females of a Moth

In phytophagous insects, experience can increase positive responses towards non-host plant extracts or induce oviposition on non-host plants, but the underlying chemical and behavioral mechanisms are poorly understood. By using the diamondback moth, Plutella xylostella, its host plant Chinese cabbage, and a non-host plant Chrysanthemum morifolium, as a model system, we observed the experience-altered olfactory responses of ovipositing females towards volatiles of the non-host plant, volatiles of pure chemicals (p-cymene and α-terpinene) found in the non-host plant, and volatiles of host plants treated with these chemicals. We assessed the experience-altered oviposition preference towards host plants treated with p-cymene. Naive females showed aversion to the odors of the non-host plant, the pure chemicals, and the pure chemical-treated host plants. In contrast, experienced females either became attracted by these non-host odors or were no longer repelled by these odors. Similarly, naive females laid a significantly lower proportion of eggs on pure chemical-treated host plants than on untreated host plants, but experienced females laid a similar or higher proportion of eggs on pure chemical-treated host plants compared to untreated host plants. Chemical analysis indicated that application of the non-host pure chemicals on Chinese cabbage induced emissions of volatiles by this host plant. We conclude that induced preference for previously repellent compounds is a major mechanism that leads to behavioral changes of this moth towards non-host plants or their extracts.     

Volatile Stimuli Related to Feeding Activity of Nonprey Caterpillars, Spodoptera exigua, Affect Olfactory Response of the Predatory Mite Phytoseiulus persimilis

The effect of volatiles related to feeding activity of nonprey caterpillars, Spodoptera exigua, on the olfactory response of the predatory mites Phytoseiulus persimilis was examined in a Y-tube olfactometer. At a low caterpillar density (20 caterpillars on 10 Lima bean leaves), the predators were significantly more attracted to volatiles from infested leaves on which the caterpillars and their products were present or from infested leaves from which the caterpillars and their products had been removed when compared to volatiles from uninfested leaves. The predators, however, significantly avoided odors from 20 caterpillars and their products (mainly feces) removed from bean leaves. In contrast, at a higher caterpillar density (100 caterpillars on 10 Lima bean leaves), the predators avoided volatiles from caterpillar-infested bean leaves. Volatiles from infested leaves from which the caterpillars and their products had been removed were not preferred over volatiles from uninfested leaves. Volatiles from feces collected from 100 caterpillars were strongly avoided by the predators, while the behavior of the predatory mites was not affected by volatiles from 100 caterpillars removed from a plant. The data show that carnivorous arthropods may avoid nonprofitable herbivores. This avoidance seems to result from an interference of volatiles from herbivore products with the attraction to herbivore-induced plant volatiles.     

Correction for Differences in Volatility Among Olfactory Stimuli and Effect on EAG Responses of Dioryctria abietivorella to Plant Volatiles

We describe a method using paraffin oil solutions and gas chromatography to measure and correct for differences in volatility among test compounds applied to filter paper and to address the problem of minimizing solvent contributions to EAG responses. To examine the effect of the volatility bias, we evaluated the EAG responses of female fir coneworm, Dioryctria abietivorella, to five volatile plant compounds, using a new method to normalize EAG responses to account for the loss of antennal sensitivity that occurs over time. Stimuli were generated either from equimolar (uncorrected) solutions or from corrected solutions that were adjusted to yield equimolar airborne concentrations in the air puffed over antennae. When uncorrected solutions were tested, the two most volatile compounds, (E)-2-hexenal and (E)-3-hexenyl acetate, elicited significantly larger EAG responses than three terpenes. When corrected concentrations were tested, the ranking of these responses changed: (E)-2-hexenal elicited significantly smaller EAGs than (–)--pinene, (–)-limonene, and (E)-3-hexenyl acetate. On the other hand, there was no effect on the ranking of EAG responses to the two monoterpenes and a sesquiterpene, (–)-trans-caryophyllene, relative to each other. Normalization of EAG data did not affect the overall results (i.e., stimulus rankings) but did reduce their variance within preparations. The results show that when compounds with widely different volatilities are compared in olfactory bioassays, the concentrations of test solutions should be adjusted to produce emissions with equimolar airborne concentrations.     

Learning of Herbivore-Induced and Nonspecific Plant Volatiles by a Parasitoid,Cotesia kariyai

Learning of host-induced plant volatiles by Cotesia kariyai females was examined with synthetic chemicals in a wind tunnel. Wasps were preconditioned by exposure to volatiles and feces simultaneously. A blend of four chemicals, geranyl acetate, -caryophyllene, (E)--farnesene, and indole, which are known to be specifically released from plants infested by host larvae Mythimna separata (host-induced blend), elicited a response in naive C. kariyai, but did not enhance the response after conditioning. A blend of five chemicals, (E)-2-hexenal, (Z)-3-hexen-1-ol, (Z)-3-hexen-1-yl acetate, -myrcene, and linalool, which are known to be released not only from plants infested by the host larvae, but also from artificially damaged plants or undamaged ones (unspecific blend), elicited little response in naive wasps, but significantly enhanced the wasps' response after conditioning. With a blend of the above nine chemicals, wasps could learn the blend at lower concentrations than they did in the nonspecific blend. Hence, both the host-induced and nonspecific volatile compounds appear to be important for C. kariyai females to learn the chemical cues in host location.     

Induction of Direct and Indirect Plant Responses by Jasmonic Acid, Low Spider Mite Densities, or a Combination of Jasmonic Acid Treatment and Spider Mite Infestation

Jasmonic acid (JA) and the octadecanoid pathway are involved in both induced direct and induced indirect plant responses. In this study, the herbivorous mite, Tetranychus urticae, and its predator, Phytoseiulus persimilis, were given a choice between Lima bean plants induced by JA or spider mites and uninduced control plants. Infestation densities resulting in the induction of predator attractants were much lower than thus far assumed, i.e., predatory mites were significantly attracted to plants that were infested for 2 days with only one or four spider mites per plant. Phytoseiulus persimilis showed a density-dependent response to volatiles from plants that were infested with different numbers of spider mites. Similarly, treating plants with increasing concentrations of JA also led to increased attraction of P. persimilis. Moreover, the duration of spider mite infestation was positively correlated with the proportion of predators that were attracted to mite-infested plants. A pretreatment of the plants with JA followed by a spider mite infestation enhanced the attraction of P. persimilis to plant volatiles compared to attraction to volatiles from plants that were only infested with spider mites and did not receive a pretreatment with JA. The herbivore, T. urticae preferred leaf tissue that previously had been infested with conspecifics to uninfested leaf tissue. In the case of choice tests with JA-induced and control leaf tissue, spider mites slightly preferred control leaf tissue. When spider mites were given a choice between leaf discs induced by JA and leaf discs damaged by spider mite feeding, they preferred the latter. The presence of herbivore induced chemicals and/or spider mite products enhanced settlement of the mites, whereas treatment with JA seemed to impede settlement.     

Behavioral Response of Lygus hesperus to Conspecifics and Headspace Volatiles of Alfalfa in a Y-Tube Olfactometer

The western tarnished plant bug, Lygus hesperus Knight, feeds and develops on a variety of weeds in the spring, with later generations moving to alfalfa and cotton where severe damage to reproductive structures can occur. A synthetic attractant for monitoring or mass-trapping L. hesperus, or the identification of potential attractants for natural enemies, would be useful tools for integrated pest management programs. Studies investigated the response of naive and experienced fifth-instar and adult L. hesperus to odors associated with conspecifics and alfalfa, Medicago sativa L. Fifth-instar L. hesperus responded to all plant/insect combinations, whereas female L. hesperus only responded preferentially to vegetative and flowering alfalfa where conspecifics had fed for 24-72 hr, and to vegetative alfalfa where conspecifics were added approximately 30 min before the test began. Males were not attracted to headspace volatiles from any of the alfalfa treatments. Analysis of headspace volatiles showed that (E)-2-hexanal, (Z)-3-hexen-l-ol, alpha-pinene, (Z)-3-hexenyl acetate, (E)-2-hexenyl acetate, limonene, (Z)-ocimene, (E)-beta-ocimene, linalool, (3E)-4,8-dimethyl-1,3,7-nonatriene, and (E, E)-alpha-farnesene are emitted from both vegetative and flowering alfalfa. Indole and (3E, 7E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene were only detected in flowering alfalfa. Damage to alfalfa by L. hesperus increased emissions of (Z)-ocimene, (E)-beta-ocimene, (E)-beta-caryophyllene, and (E, E)-alpha-farnesene, while beta-pinene, myrcene, methyl salicylate, and (3E, 7E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene were only detected from damaged plants. Thus, individual or mixtures of these alfalfa volatiles may be useful as attractants for capturing nymphs and adult females of L. hesperus in the field.     

Electroantennogram Responses of a Predator, Perillus bioculatus, and its Prey, Leptinotarsa decemlineata, to Plant Volatiles

The two-spotted stinkbug, Perillus bioculatus, is a predator of the Colorado potato beetle (CPB), Leptinotarsa decemlineata. Behavioral tests revealed that P. bioculatus is attracted to potato plants, Solanum tuberosum L. (Solanaceae), infested by the CPB. Electroantennograms from the antennae of P. bioculatus were recorded in response to compounds present in the headspace of CPB-infested potato plants. (Z)-3-Hexen-1-ol and 2-phenylethanol elicited the highest EAG amplitudes. Linalool, 4,8-dimethyl-1,3(E),7-nonatriene, nonanal, decanal, and (R)-(+)-limonene evoked lower EAG amplitudes. The major headspace components -caryophyllene and -selinene produced only weak EAG responses. Antennal sensitivity of the CPB to (Z)-3-hexen-1-ol was higher than that of P. bioculatus, whereas the stinkburg was more sensitive to 2-phenylethanol, -caryophyllene, (R)-(+)-limonene, and decanal. Among these compounds, 2-phenylethanol is of special interest since it was observed to be emitted by potato foliage only after being damaged by CPBs.     

Female-Biased Attraction of Oriental Fruit Fly, Bactrocera dorsalis (Hendel), to a Blend of Host Fruit Volatiles From Terminalia catappa L.

Coupled gas chromatography-electroantennogram detection (GC-EAD) analysis of volatiles from tropical almond fruit, Terminalia catappa L., revealed 22 compounds that were detected by antennae of oriental fruit fly females, Bactrocera dorsalis (Hendel). Both solid-phase microextraction (SPME) and Porapak Q were used for sampling odors in fruit headspace, with SPME collections producing larger EAD responses from a greater number of compounds. Geranyl acetate and methyl eugenol elicited the largest EAD responses. A synthetic blend containing SPME collected, EAD stimulatory compounds showed female-biased attraction in laboratory wind tunnel bioassays, but heavily male-biased trap captures in a larger olfactometer arena. A nine-component subset of compounds eliciting relatively small EAD responses (EAD minor) and consisting of equal parts ethanol, ethyl acetate, ethyl hexanoate, hexyl acetate, linalyl acetate, ethyl nonanate, nonyl acetate, ethyl cinnamate, and (E)-β-farnesene, attracted mainly females. This EAD minor blend was as attractive to females and much less attractive to males when compared to torula yeast in field cage experiments using glass McPhail traps. Similar results were obtained with outdoor rotating olfactometer tests in which the EAD minor blend was almost completely inactive for males.     

Olfactory Responses of the Predatory Mites (N eoseiulus cucumeris) and Insects (O rius strigicollis ) to Two Different Plant Species Infested with Onion Thrips (T hrips tabaci)

Responses of Neoseiulus cucumeris (a predatory mite) and the predatory insect Orius strigicollis to volatiles associated with two different plant species infested with onion thrips, Thrips tabaci, were examined in a Y-tube olfactometer. Both predators species showed a significant preference for volatiles from infested cucumber leaves without T. tabaci over clean air. However, they were not attracted to volatiles from uninfested cucumber leaves, artificially damaged cucumber leaves, or volatiles from T. tabaci plus their visible products collected from cucumber leaves. These results suggest that both predator species are capable of exploiting herbivore-induced volatiles from T. tabaci-infested cucumber leaves as a foraging cue. Neither predator was attracted to volatiles from uninfested spring onion leaves, infested spring onion leaves without T. tabaci, or volatiles from T. tabaci plus their visible products collected from spring onion leaves. Interestingly, they avoided volatiles from artificially damaged spring onion leaves. A possible explanation for the non-significant olfactory responses of the predator species to spring onion plants with infestation damage of T. tabaci is discussed.     

Attraction of the Parasitoid <Emphasis Type="Italic">Anagrus nilaparvatae</Emphasis> to Rice Volatiles Induced by the Rice Brown Planthopper <Emphasis Type="Italic">Nilaparvata lugens</Emphasis>

Anagrus nilaparvatae, an egg parasitoid of the rice brown planthopper Nilaparvata lugens, was attracted to volatiles released from N. lugens-infested plants, whereas there was no attraction to volatiles from undamaged plants, artificially damaged plants, or volatiles from N. lugens nymphs, female adults, eggs, honeydew, and exuvia. There was no difference in attractiveness between plants infested by N. lugens nymphs or those infested by gravid females. Attraction was correlated with time after infestation and host density; attraction was only evident between 6 and 24 hr after infestation by 10 adult females per plant, but not before or after. Similarly, after 24 hr of infestation, wasps were attracted to plants with 10 to 20 female planthoppers, but not to plants with lower or higher numbers of female planthoppers. The attractive time periods and densities may be correlated with the survival chances of the wasps' offspring, which do not survive if the plants die before the wasps emerge. Wasps were also attracted to undamaged mature leaves of a rice plant when one of the other mature leaves had been infested by 10 N. lugens for 1 d, implying that the volatile cues involved in host location by the parasitoid are systemically released. Collection and analyses of volatiles revealed that 1 d of N. lugens infestation did not result in the emission of new compounds or an increase in the total amount of volatiles, but rather the proportions among the compounds in the blend were altered. The total amounts and proportions of the chemicals were also affected by infestation duration. These changes in volatile profiles might provide the wasps with specific information on host habitat quality and thus could explain the observed behavioral responses of the parasitoid.