James H. Tumlinson

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.

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.

Learning of Host-Finding Cues by Hymenopterous Parasitoids

Interactions between insect parasitoids and their arthropod hosts characteristically result in the premature death of the hosts, and are obligatory for the development of the parasitic insects. This obviously places strong pressure on the hosts to avoid detection by parasitoids, and on the parasitoids themselves to improve encounter rates with suitable hosts. To confront the challenge of finding the often-inconspicuous, well-hidden hosts, parasitoids have developed various sophisticated searching strategies that depend on a vast array of environmental cues.

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.

Simultaneous analysis of phytohormones, phytotoxins, and volatile organic compounds in plants

Phytohormones regulate the protective responses of plants against both biotic and abiotic stresses by means of synergistic or antagonistic actions referred to as signaling crosstalk. A bottleneck in crosstalk research is the quantification of numerous interacting phytohormones and regulators. The chemical analysis of salicylic acid, jasmonic acid, indole-3-acetic acid, and abscisic acid is typically achieved by using separate and complex methodologies. Moreover, pathogen-produced phytohormone mimics, such as the phytotoxin coronatine (COR), have not been directly quantified in plant tissues. We address these problems by using a simple preparation and a GC-MS-based metabolic profiling approach. Plant tissue is extracted in aqueous 1-propanol and mixed with dichloromethane. Carboxylic acids present in the organic layer are methylated by using trimethylsilyldiazomethane; analytes are volatilized under heat, collected on a polymeric absorbent, and eluted with solvent into a sample vial. Analytes are separated by using gas chromatography and quantified by using chemical-ionization mass spectrometry that produces predominantly [M+H]+ parent ions. We use this technique to examine levels of COR, phytohormones, and volatile organic compounds in model systems, including Arabidopsis thaliana during infection with Pseudomonas syringae pv. tomato DC3000, corn (Zea mays) under herbivory by corn earworm (Helicoverpa zea), tobacco (Nicotiana tabacum) after mechanical damage, and tomato (Lycopersicon esculentum) during drought stress. Numerous complex changes induced by pathogen infection, including the accumulation of COR, salicylic acid, jasmonic acid, indole-3-acetic acid, and abscisic acid illustrate the potential and simplicity of this approach in quantifying signaling crosstalk interactions that occur at the level of synthesis and accumulation.

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.

An elicitor in caterpillar oral secretions that induces corn seedlings to emit chemical signals attractive to parasitic wasps

Regurgitate of corn-fed beet armyworm (BAW) caterpillars,Spodoptera exigua, when applied to damaged sites of corn (Zea mays) seedlings, causes the release of relatively large amounts of terpenes by the seedlings several hours later. This plant response could be induced by merely placing the cut stem of seedlings in a solution of BAW regurgitate for 12 hr, a response that could not be induced by placing seedlings in water only. Regurgitate of BAW fed various diets, including a minimal diet of filter paper, were all active. However, seedlings placed in corn leaf juice, BAW hemolymph, or BAW feces extract released significantly smaller amounts of terpenes than did seedlings placed in BAW regurgitate. These results indicate that the active components are present in relatively large concentrations in regurgitate and that they are not related to the food source. Furthermore, regurgitate from several other species of caterpillars (Spodoptera frugiperda, Helicoverpa zea,Trichoplusia ni, andAnticarsia gemmatalis) as well as from the grasshopperSchistocerca americana induced the release of significant amounts of terpenes in corn seedlings. The release of these volatiles, therefore, appears to be a general response to attack by phytophagous insects. The terpene-releasing corn seedlings were highly attractive to the generalist parasitoidCotesia marginiventris and to the specialized parasitoidMicroplitis croceipes. This study confirms a systemic herbivore-elicited release of terpenes in corn. It is proposed that such chemicals serve multifunctional purposes that directly and indirectly protect plants against herbivorous arthropods and pathogens.

Volatiles emitted by different cotton varieties damaged by feeding beet armyworm larvae.

Volatile compounds elicited by insect herbivore feeding damage in five cotton cultivars and one naturalized cotton variety were examined by allowing beet armyworm larvae to feed overnight on leaves and collecting volatiles from the plants in situ. Of 23 compounds identified from larval damaged leaves, terpenes and lipoxygenase-hydroperoxide lyase-derived volatiles predominated. No pronounced differences in the levels of volatile emission were noted from leaves of undamaged plants of the different varieties. However, average volatile emission from damaged leaves of the naturalized variety was almost sevenfold higher than from damaged leaves of the commercial cultivars. This was despite the fact that larvae preferred feeding on the leaves of commercial cultivars over those of the naturalized variety in choice tests.

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.

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,βcyclohexaneethanol; III, cis-3,3-dimethlyl->1,α-cyclohexaneacetaldehyde; and IV, trans-3,3-dimethyl->1,α-cyclohexanecetaldehyde.