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A study was conducted to determine the primary source of volatile cues within the plant-host complex used by hostseeking freeflying female Microplitis cro-ceipesCresson in flight tunnel bioassays. In single-source and two-choice tests, using wasps given an oviposition experience on either cotton (Gossypium hirsutum) or cowpea (Vigna unguiculata) seedlings damaged by corn earworm (CEW; Helicoverpa zea Boddie), the damaged seedlings were significantly more attractive than the CEW frass, which was in turn more attractive than the larvae themselves. In a series of two-choice wind-tunnel tests, the discriminatory ability of the wasps was examined, following various oviposition experiences. Significantly more wasps flew to plants with old damage than to plants with fresh damage, regardless of whether they had experience on fresh or old damage. In a comparison of plant species, wasps with only one experience on either hostdamaged cotton or host-damaged cowpea were unable to distinguish between them, and showed no preference for either plant, whereas wasps with multiple experiences on a particular plant preferentially flew to that plant in the choice test. In comparing hosts with nonhosts, wasps successfully learned to distinguish CEW from beet armyworm (BAW; Spodoptera exigua) on cotton but were unable to distinguish CEW from either BAW or cabbage looper (Trichoplusia ni) on cowpea. The results show the important role played by plant volatiles in the location of hosts by M. croceipesand indicate the wasps limitations in discriminating among the various odors. The ecological advantages and disadvantages of this behavior are discussed.

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 that C. 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.

Responses of individual females of the parasitoid Cotesia marginiventris to the odors of four different complexes of host larvae feeding on leaves were observed in a four-arm olfactometer. The plant-host complexes were composed of fall armyworm (FAW) larvae or cabbage looper (CL) larvae feeding on either corn or cotton seedlings. Prior to testing, each female was given a brief foraging experience on a plant-host complex and was then exposed to the odors of the same complex in the olfactometer. The experienced females responded to familiar odors in a dose-related manner, and these responses were virtually identical to all four complexes. Preferences for the odors of one of two plant-host complexes were tested in dual choice situations. Generally, FAW odors were preferred over CL odors and corn odors over cotton odors. A short foraging experience significantly affected the females' odor preferences in favor of the odors released by the experienced complex. Additional experiments revealed that neither longer bouts of experience nor bouts that included ovipositions resulted in a stronger change in preference. Experience affected preference in combinations where only the host species was varied as well as in combinations where only the plant species was varied. The results, therefore, strongly indicate that both the plants and the hosts somehow are involved in the production and/or release of the semiochemicals that attract C. marginiventris.

The demonstration that parasitoids are attracted to volatile compounds released by plants in response to herbivore feeding has generated a great deal of interest over the past ten years. The release of volatile signals by plants occurs not only in response to tissue damage but is also specifically initiated by exposure to herbivore salivary secretions. Although some volatile compounds are stored in plant tissues and immediately released when damage occurs, others are induced by herbivore feeding and released not only from damaged tissue but also from undamaged leaves. Thus, damage localized to only a few leaves results in a systemic response and the release of volatiles from the entire plant. New evidence suggests that, in addition to being highly detectable and reliable indicators of herbivore presence, herbivore-induced plant volatiles may convey herbivore-specific information that allows parasitoids to discriminate even closely-related herbivore species at long range. Here we give an overview of the recent developments in the investigation of plant-parasitoid interactions. A demonstração de que os parasitóides são atraídos pelos compostos voláteis liberados pelas plantas, em resposta ao ataque dos insetos herbívoros, gerou considerável interesse nos últimos dez anos. A liberação dos compostos voláteis pelas plantas ocorre não somente em resposta aos danos causados aos seus tecidos, mas ela é também, especificamente, iniciada pela exposição às secreções salivares dos herbívoros. Alguns compostos voláteis são armazenados nos tecidos vegetais e liberados no momento em que o dano ocorre, outros são induzidos pelo dano causado pelo herbívoro e são, geralmente, liberados, não apenas pelo tecido lesado, mas também pelas folhas não atacadas. Desse modo o dano causado em somente algumas folhas, resulta numa resposta sistêmica e na liberação de compostos voláteis por toda a planta. Novas evidências sugerem que os compostos voláteis induzidos pelos insetos herbívoros, além de facilmente detectáveis e de serem indicadores seguros da presença de herbívoros, podem ainda, transmitir informação específica, que permite aos parasitóides discriminarem a longa distância, espécies de herbívoros muito próximas. Daremos aqui uma visão geral dos desenvolvimentos mais recentes na investigação das interações plantas-parasitóides.