Ted C. J. Turlings

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.

Recruitment of entomopathogenic nematodes by insect-damaged maize roots

Plants under attack by arthropod herbivores often emit volatile compounds from their leaves that attract natural enemies of the herbivores. Here we report the first identification of an insect-induced belowground plant signal, (E)-β-caryophyllene, which strongly attracts an entomopathogenic nematode. Maize roots release this sesquiterpene in response to feeding by larvae of the beetle Diabrotica virgifera virgifera, a maize pest that is currently invading Europe. Most North American maize lines do not release (E)-β-caryophyllene, whereas European lines and the wild maize ancestor, teosinte, readily do so in response to D. v. virgifera attack. This difference was consistent with striking differences in the attractiveness of representative lines in the laboratory. Field experiments showed a fivefold higher nematode infection rate of D. v. virgifera larvae on a maize variety that produces the signal than on a variety that does not, whereas spiking the soil near the latter variety with authentic (E)-β-caryophyllene decreased the emergence of adult D. v. virgifera to less than half. North American maize lines must have lost the signal during the breeding process. Development of new varieties that release the attractant in adequate amounts should help enhance the efficacy of nematodes as biological control agents against root pests like D. v. virgifera.

The Effects of Abiotic Factors on Induced Volatile Emissions in Corn Plants

Many plants respond to herbivory by releasing a specific blend of volatiles that is attractive to natural enemies of the herbivores. In corn (Zea mays), this induced odor blend is mainly composed of terpenoids and indole. The induced signal varies with plant species and genotype, but little is known about the variation due to abiotic factors. Here, we tested the effect of soil humidity, air humidity, temperature, light, and fertilization rate on the emission of induced volatiles in young corn plants. Each factor was tested separately under constant conditions for the other factors. Plants released more when standing in dry soil than in wet soil, whereas for air humidity, the optimal release was found at around 60% relative humidity. Temperatures between 22°C and 27°C led to a higher emission than lower or higher temperatures. Light intensity had a dramatic effect. The emission of volatiles did not occur in the dark and increased steadily with an increase in the light intensity. An experiment with an unnatural light-dark cycle showed that the release was fully photophase dependent. Fertilization also had a strong positive effect; the emission of volatiles was minimal when plants were grown under low nutrition, even when results were corrected for plant biomass. Changes in all abiotic factors caused small but significant changes in the relative ratios among the different compounds (quality) in the induced odor blends, except for air humidity. Hence, climatic conditions and nutrient availability can be important factors in determining the intensity and variability in the release of induced plant volatiles.

Recruitment of predators and parasitoids by herbivore-injured plants

Introduction In recent years, induced plant defenses have received widespread attention from biologists in a variety of disciplines. The mechanisms underlying these defenses and the interactions that mediate them appeal not only to plant physiologists, ecologists, and evolutionary biologists but also to those scientists that search for novel strategies in plant protection. Several recent books (Karban and Baldwin, 1997; Agrawal et al. , 1999) and reviews (Baldwin, 1994; Karban et al. , 1997; Agrawal and Rutter, 1998; Agrawal and Karban, 1999; Baldwin and Preston, 1999; Dicke et al. , 2003) have been devoted entirely to the subject of induced plant defenses. Various forces, ranging from abiotic stresses to biotic factors such as pathogens, arthropods, or higher organisms, may trigger different plant defense responses. Yet, the biochemical pathways that are involved appear to show considerable similarities. This is also true for the so-called indirect defenses. The term indirect defense refers to those adaptations that result in the recruitment and sustenance of organisms that protect the plants against herbivorous attackers. The early published examples of indirect defenses involved intimate plant–ant interactions, in which myrmecophilous plants were shown to have evolved a range of adaptations providing ants with shelter (domatia) and various food sources (Belt, 1874; Janzen, 1966). In return, these plants may obtain a range of benefits because ants can provide nutrition (Thomson, 1981) or more commonly, protection against herbivores, pathogens, and competing plants (e.g. Koptur, 1992; Oliveira, 1997).

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.

A maize (E)-beta-caryophyllene synthase implicated in indirect defense responses against herbivores is not expressed in most American maize varieties.

The sesquiterpene (E)-β-caryophyllene is emitted by maize (Zea mays) leaves in response to attack by lepidopteran larvae like Spodoptera littoralis and released from roots after damage by larvae of the coleopteran Diabrotica virgifera virgifera. We identified a maize terpene synthase, Terpene Synthase 23 (TPS23), that produces (E)-β-caryophyllene from farnesyl diphosphate. The expression of TPS23 is controlled at the transcript level and induced independently by D. v. virgifera damage in roots and S. littoralis damage in leaves. We demonstrate that (E)-β-caryophyllene can attract natural enemies of both herbivores: entomopathogenic nematodes below ground and parasitic wasps, after an initial learning experience, above ground. The biochemical properties of TPS23 are similar to those of (E)-β-caryophyllene synthases from dicotyledons but are the result of repeated evolution. The sequence of TPS23 is maintained by positive selection in maize and its closest wild relatives, teosinte (Zea sp) species. The gene encoding TPS23 is active in teosinte species and European maize lines, but decreased transcription in most North American lines resulted in the loss of (E)-β-caryophyllene production. We argue that the (E)-β-caryophyllene defense signal was lost during breeding of the North American lines and that its restoration might help to increase the resistance of these lines against agronomically important pests.

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.

Timing of induced volatile emissions in maize seedlings

Abstract. Maize (Zea mays L.) releases specific volatiles in response to herbivory by caterpillars. These volatiles are known to serve as cues for parasitic wasps to locate the herbivores. In the present study the exact time of volatile emission after simulated herbivory (mechanical damage and treatment with caterpillar regurgitant) was measured for seedlings of the cultivars “Ioana Sweet Corn” and “LG11”. Odours were collected every 0.5 h for a total of 12 h. Typical “green leaf odours”, (Z)-3-hexenal, (E )-2-hexenal, (Z)-hexen-1-o1, and (Z)-3-hexen-1-yl acetate, were emitted immediately upon damage and their amounts dropped rapidly after the first collections. Several of the induced compounds were released within 2 h after treatment, while others (mainly sesquiterpenoids) started to be released after 4 h. The LG11 seedlings emitted several compounds (e.g. β-myrcene, (Z)-β-ocimene, benzyl acetate, β-caryophyllene, (E,E )-α-farnesene) that were not detected for Ioana. (E,E )-α-farnesene was continuously emitted by LG11 seedlings, even by undamaged plants. Timing of the release of volatile compounds that the two varieties had in common did not differ significantly, with the exception of indole for which the peak production was considerably earlier for LG11. These findings are discussed in the context of biosynthetic pathways and mechanisms involved in induced emissions of plant volatiles and the exploitation of the resulting odour by parasitoids and predators of herbivores.

High Genetic Variability of Herbivore-Induced Volatile Emission within a Broad Range of Maize Inbred Lines

Maize plants (Zea mays) attacked by caterpillars release a mixture of odorous compounds that attract parasitic wasps, natural enemies of the herbivores. We assessed the genetic variability of these induced volatile emissions among 31 maize inbred lines representing a broad range of genetic diversity used by breeders in Europe and North America. Odors were collected from young plants that had been induced by injecting them with caterpillar regurgitant. Significant variation among lines was found for all 23 volatile compounds included in the analysis: the lines differed enormously in the total amount of volatiles emitted and showed highly variable odor profiles distinctive of each genotype. Principal component analysis performed on the relative quantities of particular compounds within the blend revealed clusters of highly correlated volatiles, which may share common metabolic pathways. European and American lines belonging to established heterotic groups were loosely separated from each other, with the most clear-cut difference in the typical release of (E)-β-caryophyllene by European lines. There was no correlation between the distances among the lines based on their odor profiles and their respective genetic distances previously assessed by neutral RFLP markers. This most comprehensive study to date on intraspecific variation in induced odor emission by maize plants provides a further example of the remarkably high genetic diversity conserved within this important crop plant. A better understanding of the genetic control of induced odor emissions may help in the development of maize varieties particularly attractive to parasitoids and other biological control agents and perhaps more repellent for herbivores.

Restoring a maize root signal that attracts insect-killing nematodes to control a major pest

When attacked by herbivorous insects, plants emit volatile compounds that attract natural enemies of the insects. It has been proposed that these volatile signals can be manipulated to improve crop protection. Here, we demonstrate the full potential of this strategy by restoring the emission of a specific belowground signal emitted by insect-damaged maize roots. The western corn rootworm induces the roots of many maize varieties to emit (E)-β-caryophyllene, which attracts entomopathogenic nematodes that infect and kill the voracious root pest. However, most North American maize varieties have lost the ability to emit (E)-β-caryophyllene and may therefore receive little protection from the nematodes. To restore the signal, a nonemitting maize line was transformed with a (E)-β-caryophyllene synthase gene from oregano, resulting in constitutive emissions of this sesquiterpene. In rootworm-infested field plots in which nematodes were released, the (E)-β-caryophyllene-emitting plants suffered significantly less root damage and had 60% fewer adult beetles emerge than untransformed, nonemitting lines. This demonstration that plant volatile emissions can be manipulated to enhance the effectiveness of biological control agents opens the way for novel and ecologically sound strategies to fight a variety of insect pests.