We found that adult beet armyworms (BAW) oviposited more and larvae fed significantly more and performed better on white mold infected plants than on healthy plants. The volatile profile released by white-mold infected peanuts was significantly different from those emitted by undamaged plants. Also, peanut plants infected with the white mold and then exposed to BAW damage released all the volatiles emitted by healthy plants fed on by BAW, and those emitted by plants in response to white mold infection alone. The BAW larval parasitoid Cotesia marginiventris, landed more frequently on infected than on healthy plants exposed to BAW damage.
Herbivorous insects and plant diseases present a continuos threat to the agricultural environment because they reduce yield and the quality of crops. In particular, plant diseases, caused by infectious viruses, bacteria, phytoplasmas, fungi and nematodes, present serious problems in agriculture. These problems include reduced yield, lower product shelf-life, decreased aesthetic and nutritional value. In addition to the direct damage to the plant, attack by some pathogen strains and insect species also results in the production and accumulation of secondary metabolites and toxins that can cause disease in humans and animals. Control of plant diseases and herbivorous insects is vital for providing an adequate supply of food, feed, and fiber to cope with the increasing human population and its demands. In Florida alone, more than a dozen plant disease epidemics occurred from 1970-1990 (Kucharek 1990). Growers currently spend large sums of money to control pathogens and insects that attack their crops. Nevertheless, crop and commodity losses due to disease and herbivore damage cost billions of dollars each year. In the United States, it is estimated that the yearly economic losses are approximately 9.1 billion dollars for plant diseases, and approximately 7.7 billion dollars for insect damage, all this after the application of control measures practiced under modern agriculture (Agrios, 1997). Thus, information about how insect and pathogen pests interact with crops, and how this interaction affects the economic value and quality of agricultural products is important for establishing the economic thresholds for managing pest populations, minimizing pest damage, developing new methods of insect and pathogen prevention and control, and improving host-plant resistance and other mechanisms for tolerance to insect and pathogen pests.
Plants play an active role in the interactions taking place in their ecosystem, they possess a number of chemical defense mechanisms that may be triggered by herbivore and/or pathogen attack. These chemical defenses can directly modify the development and survival of the attacking organism (e.g. phytoalexins, proteinase inhibitors) (Mur et al. 1997), or serve as attractants to natural enemies of the pest (i.e., release of herbivore-induced synomones) (Turlings and Tumlinson 1991, Turlings et al. 1991, 1993). Oxilipins are oxygenated fatty acids found in higher plants that activate transcriptional genes in response to herbivory or pathogen infection. Jasmonic acid is an oxylipin that is derived from oxidation of linolenic acid and is dramatically increased after insect damage, turning on genes necessary for the production of phytoalexins and proteinase inhibitors (reviewed in Choi et al. 1994). Arachidonic acid, a fatty acid found in the fungus Phytophtora infestans (potato late blight), has also been found to activate phytoalexin-encoding genes but these are different from those activated by jasmonic acid in potato discs (Choi et al. 1994). This finding may be an indication that the biochemical pathways involved in plant defense against herbivores is different from those involved in the defense against pathogens.
In addition to the production of internal defense compounds in response to insect and pathogen attack, plants may also produce volatile substances that are released externally. Plants release a mixture of such compounds in response to attack by herbivores (McCall et al. 1994, Loughrin et al. 1995, Röse et al. 1996, Pare and Tumlinson 1997). These chemical signals are attractive to parasitoids of the pests (Turlings et al. 1991, 1993, Röse et al. 1998) and, since both the emitter (plant) and the recipient (parasitoid) benefit from these infochemicals, they are categorized as synomones. In addition to direct herbivore feeding, some plants also release synomones in response to herbivore oral secretions or regurgitate when this is either applied topically to a mechanically-damaged leaf or fed through the stem of excised plants. The production of herbivore induced synomones by plants raises questions about the processes involved in the induction and production of such chemicals. Compounds responsible for eliciting the emission of plant volatiles have been isolated and identified in recent years. Examples of these are volicitin, a component found in Spodoptera exigua regurgitant (Alborn et al. 1997), and beta-glucosidase, found in oral secretions of Pieris bassicae (Mattiacci et al. 1995), which elicit volatile emissions in corn and cabbage, respectively.
Pathogens and pathogen derived substances can also elicit the production and release of volatiles from the affected plant hosts. Case in point, Brassica rapa seedlings were found to release volatile products of glucosinolate degradation when infected by the fungus Alternaria brassicae (Doughty et al. 1996). In another study with beans (Phaseoulus vulgaris L.) release of volatile linolenic acid derivatives ensued 15-24 h post-inoculation with Pseudomonas syringae pv. phaseolicola (Croft et al. 1993). It has also been suggested that ethylene, a volatile phytohormone, is involved in the induction of systemic acquired resistance (SAR), which confers protection against subsequent pathogen and herbivore attacks. The production of ethylene in plants is induced by various factors such as, mechanical wounding, exogenous auxin
applications, and herbivore and pathogen attack. Chaudhry et al. 1998 found that the production and release of ethylene increased in tobacco plants 48 h post-inoculation with cucumber mosaic virus (yellow strain). Additionally, it was observed that the increase in ethylene was positively correlated with an increase in the concentration of the enzymes required for its synthesis (ACC
synthase and ACC oxydase) (Chaudhry et al. 1998). Pathogen derived compounds such as cellulysin, which is a crude cellulose extract from the fungus Trichoderma viride, have been found to induce volatile production in tobacco, lima bean, and corn plants (Piël et al. 1997, Koch, et al. 1999). Piel et al. 1997 found that a 50 Fg/ml concentration of cellulysin elicited the emission of hexenyl acetate, ocimene, linalool, nonatriene, indole, bergamotene, beta-farnescene, nerolidol and tridecatetraene similar to those elicited by applications of jasmonic acid. These compounds have also been reported to be produced by plants in response to insect damage, however, caryophyllene which is a compound induced in corn by insect damage, was not present in the emissions of this plant in response to cellulysin, and bergamotene, another herbivore-induced compound, was only present in relatively small proportion. Similarly, coronatin, a phytotoxin isolated from Pseudomonas syringae bacteria, also elicits the release of volatiles in plants. Coronatin toxicity in plants exhibits symptom similar to those observed with high doses of jasmonic acid, such as chlorosis, accelerated senescence, and ethylene release (Weiler et al. 1994, Boland et al. 1995). However, treatment of plant cell cultures with coronatin did not induce an increase in endogenous jasmonates even though it was in some cases more active than jasmonates, and the coronatin structure strongly resembles that of 12-oxo-phytodienoic acid, which is a precursor of jasmonates (Weiler et al. 1994). Thus, it was concluded that coronatin is not an elicitor of plant responses but rather a close analogue of the octadecanoic precursor of jasmonates.
Many insect herbivores use volatile isothiocyanates to locate their brassicaceae hosts, and their parasitoids in turn use these chemicals to locate their herbivore hosts. So, the released volatiles emitted in response to attack by pathogens may play a role in oviposition site selection by herbivore females and in the host-searching process by natural enemies (Doughty et al. 1996). Plant volatiles also affect pathogens. The germination and growth of white mold (Sclerotium rolfsii) are stimulated by the release of methanol and other volatile compounds emanated from moist peanut hay. Volatiles from ground-up healthy corn kernels resistant to Aspergillus flavus, however, have been found to inhibit the growth and aflatoxin production in colonies of A. flavus (Zeringue et al. 1996). In Cotton the lipoxygenase-derived volatile trans-2-hexenal inhibited, while alpha and beta pinene stimulated the growth of A. flavus (Zeringue and McCormick 1989, 1990). Other compounds like 3-methyl-1-butanol and 3-methyl-2-butanol were found to decrease fungal growth but increase aflatoxin production (Zeringue et al. 1990). In the case of coronatin-induced volatiles in beans, the compounds emitted were trans-2-hexenal, which has high bactericidal activity and cis-3-hexenol, which is also bactericidal but only at much higher doses (Croft et al. 1993). Furthermore, these compounds were released in larger quantities from resistant varieties compared with susceptible ones (Croft et al. 1993).
Enhancing resistance to herbivores and disease in plants is an excellent management option and is often very cost-effective and environmentally safe. This approach, however, depends on our ability to identify and characterize the sources for resistance in crop species and in closely related plants. It is now known that plant volatiles play an important role in plant defense against both herbivorous and pathogenic organisms and thus may have a significant role in the regulation of
the behavior, development, and survival of these organisms. Therefore, the study of the
chemically-mediated interactions; the identification of the chemical compounds involved in the mediation between pathogen and pests with their host plant species; the effect of these interactions on agricultural ecosystems; and the environmental implications are critical for the
development of ecologically sound integrated management programs.
Detection of food or feed spoilage due to fungal growth was, for many years, conducted through the use of human noses, the recent development of electronic sensors has provided a far more sensitive alternative (Schnürer et al. 1999). This practice could potentially be used for the detection of pathogens and pathogen- or herbivore-damaged plants. Knowledge on the effect of plant emitted volatiles on insect behavior will enable the utilization of such chemicals for the detection of infected plants in the field, or even pathogen themselves by detection of pathogen-produced volatiles, by means of biological or mechanical devices. This will also enable the implementation of attractants for recruitment of parasitoids of herbivorous insects. Comparison of plant volatile profiles induced by pathogens and insects, alone and in combination, will give us a better idea, based on the nature of the compounds produced, of whether plant defense against these agents share the same biochemical pathways. Furthermore, the results obtained from such investigations will provide the basis for additional studies on the value of these plant-derived volatiles as possible antimicrobial agents, and for the identification of plant varieties with an enhanced chemical arsenal against pests.
The overall objective of this project is to investigate the production of volatile compounds by plants under pathogen attack, and to evaluate the effect of simultaneous pathogen/herbivore challenge on the volatile emission by host plant. The specific research objectives are to:
1) Analyze, identify, and compare compounds from head space collections from diseased and healthy plants.
2) Determine the effect of pathogen defense induction on the production and release of herbivore-induced volatiles by the host plant
3) Evaluate the effect of volatiles emitted from healthy, diseased, herbivore-damaged, and the combination of disease and herbivore damage on insect herbivore performance and on the oviposition site selection by adult herbivores and on the host searching behavior of parasitoids .
S. rolfsii infection on peanuts. Peanut plants were infected with the fungus by distributing four culture plugs along the main stem. The plugs were positioned so the fungus was in direct contact with the stem. Fungal plugs were pressed against the stem so they remained in place. Each plant was then individually covered with a 3.78-L plastic storage bag (Ziploc DowBrands L. P., Indianapolis, IN) to provide adequate humidity and temperature conditions for fungal growth and colonization of the plant’s stem. The plants were incubated for 3 d, after which time lesions of approximately 1-cm long could be observed at the point of fungal contact with the stem. After this incubation period, bags were removed from the plants 24 h before being used for the experiment. S. rolfsii is a non-systemic pathogen and only the stems of the plants were in contact with the fungus and therefore, the leaves that were consumed by the caterpillars were not infected by the fungus.
Volatile collections from fungus and BAW-damaged peanuts. In this experiment plant treatments consisted of a) control (uninfected/undamaged), b) BAW-damaged, c) fungus-infected and, d) fungus-infected plus BAW damage. Plants were inoculated with the fungus as described in the first experiment. BAW-damaged plants were exposed to feeding by six third instar larvae within the volatile collection chambers 3 d after pathogen inoculation and 12 h before the start of the first sampling period.
The aerial portion of the plant was contained within glass sleeves with teflon base, whith an opening that closed around the plant stem (Röse et al., 1996). Purified air was pumped in at the top of the chamber at a rate of 5 L min-1. Air within each of the chambers was sampled daily, at a rate of 1 L min-1, for 4 d in three consecutive periods: 1) 6:00 am-12:00 pm, 2) 12:00 pm-6:00 pm, and 3) 6:00 pm-6:00 am. Compounds emitted were collected at the downwind end of the chambers in adsorbent traps containing 25 mg Super Q (800-100 mesh) (Alltech, Deerfield, IL). All volatiles collections were conducted in the greenhouse were plants were grown. The experiment was set up in duplicate and repeated three times on different days for a total of six replicates.
BAW feeding on S. rolfsii-infected peanuts. The feeding preference of BAW larvae on old and young leaves of healthy and fungus-damaged plants was evaluated to determine whether insects were deterred from feeding on tissues of S. rolfsii-infected plants. Peanut plants were infected with the fungus by distributing four culture plugs along the main stem. The plugs were positioned so the fungus was in direct contact with the stem. Fungal plugs were pressed against the stem so they remained in place. Each plant was then individually covered with a 3.78-L plastic storage bag (Ziploc DowBrands L. P., Indianapolis, IN) to provide adequate humidity and temperature conditions for fungal growth and colonization of the plant’s stem. The plants were incubated for 3 d, after which time lesions of approximately 1-cm long could be observed at the point of fungal contact with the stem. After this incubation period, bags were removed from the plants 24 h before being used for the experiment. S. rolfsii is a non-systemic pathogen and only the stems of the plants were in contact with the fungus and therefore, the leaves used for the experiments were not infected.
The second-oldest (old) or newest (young) fully developed tetrafoliate leaves on the mainstem of an infected plant were paired with their counterparts from a healthy plant by confining them within petri dish clip-cages. The leaves (four leaflets each), still attached to each of the plants, were placed side by side within a clip-cage (Alborn et al., 1996) so the caterpillars had equal access to them. Insects were deprived of food for 6 h before the start of the experiment to ensure immediate feeding on the plant tissues. Young peanut leaves are larger than old leaves, so six third instar larvae were confined with the young leaves and three larvae of the same stage were confined to the old leaves. Caterpillars were removed from the leaves after 24 h. Leaves exposed to the feeding were carefully labelled, removed from the plants, brought into the laboratory, and photocopied to estimate feeding damage. Leaf images were scanned and imported into an imaging software program (ImagePC beta version 1, Scion Corporation, Frederick, MD) to estimate leaf area eaten and leaf area remaining, these measurements were used to calculate the leaf area consumed by the insects on each of the treatments. Six replicates of this experiment were set up at one time in the greenhouse, under conditions described above. This experiment was repeated twice more for a total of 18 replicates.
BAW Performance on Healthy and Fungus-Infected Peanut. Healthy and fungus-infected plants were individually placed within 18″×18″×18″ plexiglass cages into which 6 BAW were introduced. Cages were kept in the greenhouse throughout the duration of the experiment. Insects were observed daily and they were removed from the plants when no more feeding activity was observed. At this time, the number of total surviving insects and their number in the wandering, pre-pupal, and pupal stages was recorded. All insects were placed into petri plates and kept in an incubator under the conditions described above, finally, their weight was determined and recorded . Four replicates of this experiment were set up at one time in the greenhouse. The experiment was repeated twice more for a total of 12 replicates.
Effect of white mold infection on peanut on host searching behavior by adult BAW and their parasitoids. To test the oviposition preference of BAW moths, three healthy and three fungus-infected plants were placed within a 4×2 ×3 m screen cage. Plants were distributed in two rows along the length of the cage so there was approximately a 3 ft distance in between plants. Healthy and infected plants were placed in an alternate fashion. Additionally, to account for any environmental differences within the cage, plants were moved one space to the north and one to the east daily for the duration of the experiment. Sixteen 5-7 d-old adult BAW, 8 females and 8 males, were released in the center of the cage and were allowed access to the plants for 3 days. At the end of the experiments, leaves from each treatment containing egg masses were removed from the plants and brought into the laboratory to determine the number of masses and individual eggs, with the aid of a stereo microscope, laid on plants with the two treatments. This experiment was repeated over time for a total of 6 replicates.
To test the effect of white mold infection on peanut attraction to BAW larval parasitoids, infected and healthy peanut plants were exposed to feeding by 10 3rd instar BAW larvae for 24 h. After this feeding exposure, insects were removed from the plants, and plants were placed 4 ft apart in a 6′ × 2′ × 2′ plexiglass cage. Ten 4-7 day-old mated female parasitoids were released in the middle of the cage. Parasitoids were allowed to settle down for approximately 30 min and then they were observed for 15 minutes during which, the number of insects landing on each of the plants was recorded. At the end of this period, insects were removed from the cage, the position of the plants was switched and new set of insects was introduced and watched for another 15 minutes. This procedure was repeated twice more during the same day to obtain a total of 4
repetitions. This experiment was repeated once more at a different time with a different set of plants, this yielded a total of 8 replicates.
Statistical Analyses. Data for performance of BAW on healthy and feeding preference were analyzed by t-test (Proc MEANS, SAS Institute, 1996). Data for BAW feeding and oviposition preference were analyzed by paired t-test (Proc MEANS, SAS Institute, 1996). Paired t-test (Proc MEANS, SAS Institute, 1996) were also used to analyze data on parasitoid landing preference.
Uninfected, undamaged control plants (Fig 1A) released small amounts of volatiles, compared to those released by white mold-infected or BAW-damaged peanut plants, throughout the duration of the experiment. Plants infected with the white mold fungus alone released (Z)-3-hexenyl acetate, linalool and large amounts of (E)-4, 8-dimethyl-1,3,7-nonatriene (Fig 1B), which were also present in emissions from BAW-damaged plants. The compounds methyl salicylate and 3-octanone were only present in volatile emissions from white mold-infected plants (Fig 1C) and not in those of plants damaged by BAW alone (Figs 1 B, D). Non infected peanut plants exposed to feeding by BAW released large amounts of lipoxygenase products, monoterpenes, indole, and sesquiterpenes (Fig 1C). The emission of volatiles from peanuts in response to BAW-damage was not negatively affected by infection of white-mold fungus on the plant. White mold infected peanuts damaged by BAW (Fig 1D ) released all the volatiles typical of a healthy plant damaged by BAW. Furthermore, the amounts of some released compounds, such as (E)-2-hexenal, (Z)-3-hexen-1-ol, myrcene, and (E)-4, 8-dimethyl-1,3,7-nonatriene, were higher than those emitted from non infected plants in response to BAW feeding (Figs 1 C, D). Also, headspace collections from white mold/BAW-damaged plants also contained volatiles produced by plants in response to white mold infection alone (Fig 1 B).
BAW feeding was not negatively affected by infection of S. rolfsii on peanut plants. To the contrary, larvae showed a significant preference for young and old leaves from fungi-infected plants over those from healthy, non-infected plants (Fig 2). This preference was more prevalent when insects were confined to young versus old leaves (Fig 2). In a no-choice situation, BAW larvae reared on fungus-infected peanut plants showed significantly higher survival rate (t=3.37; df=53; P=0.0014) (Fig 1A), developed significantly faster (t=29.9; df=51; P=0.0001) (Fig 1B), and produced significantly heavier pupae (t=2.96; df=53; P=0.0045) (Fig 1C) than on their healthy counterparts (Fig 3 a, b and c). Thus it seems that peanut plants infected with white mold are better suited as hosts for BAW. This is confirmed by the fact that, in choice tests, adult BAW oviposited significantly more on peanut plants that were infected by the white mold compared to healthy ones (Fig 4a). The BAW larval parasitoid Cotesia marginiventris landed comparatively more (on plants that were damaged by BAW larvae when plants were also white mold infected than when they were healthy (Fig 4b).
Data obtained from this study confirm that peanut plants release chemical signals in response to infection by the white mold fungus. We present conclusive evidence that the volatile profile emitted by these plants differs qualitatively and quantitatively from those from healthy plants and from those emitted in response to BAW damage. Additionally, previous infection of the plant by the white mold fungus does not interfere with the emission of volatiles by the diseased plant in response to BAW attack; rather it seems to induce release of some compounds in relatively higher quantities.
The significant quantitative difference in the volatile profile of peanut plants attacked by either the fungus or the insect, added to the fact that emission of compounds was not suppressed when both organisms were simultaneously attacking the plant, provides a clear indication that the activation and regulation of plant biosynthetic pathways is dependant upon the type of threat perceived by the plant. Four major biosynthetic pathways are believed to be involved in the production of plant volatiles in response to damage by lepidopterous larvae (Paré et al., 1997). The lipoxygenase (LOX) pathway, via jasmonate production, has been directly linked to direct plant defense responses to wounding and damage by herbivorous insects (Ryan, 1990; Farmer and Ryan, 1990; Farmer et al., 1992). Jasmonic acid and methyl jasmonate have also been found to induce volatile emissions similar to those resulting from herbivory in many plants (Boland et al., 1995; Dicke et al, 1999). Thus, it has been suggested that they mediate volatile production. In our study, the emission of volatiles from peanut plants previously infected with S. rolfsii indicates that, unlike direct defenses, volatile production is not compromised by pathogen infection. The latter may be an indication that peanut production of volatile signals in response to insect and pathogen attack is not jasmonate-dependant. However, the precise combination and regulation of pathways activated in response to an individual or combination of stressors may vary widely between plant species. Therefore, the role of jasmonic acid and salicylic acid on the production and release of induced volatiles by plants in response to insect and pathogen attack needs to be investigated further. The dynamics of volatile emission in response to insect herbivores, phytopathogens, and their combined effect upon other plant species also merit additional attention.
The fact that 3-octanone was recovered from fungal culture and diseased plant samples is encouraging because this compound could potentially be used to determine the presence of the pathogen with biological or mechanical devices, such as trained animals or electronic noses. Additionally, the presence of 3-octanone and methyl salicylate could be used in the future as an indicator of infection in plants, and this could facilitate treatment or elimination of affected plants before the disease has a chance to spread and cause significant damage in the field.
Host plant selection and feeding preference of insect herbivores is influenced by a number of factors, including feeding stimulants and deterrents within plants. The level of these compounds may vary from plant to plant and can be influenced by stress factors such as pathogen infection (Hatcher, 1995). Major nutrients like carbohydrates and proteins are listed as one of the most important phagostimulants for phytophagous insects; whereas, secondary plant defensive compounds such as tannins and protease inhibitors can have the opposite effect (Slanky and Scriber, 1985). In the dual-choice feeding experiments, BAW preferentially fed on leaves from white mold-infected peanut plants. BAW and corn earworm, Helicoverpa zea, have been reported to feed more upon tomato plants that had been previously treated with benzothiadiazole-carbothionic acid S-methyl ester (BTH), an elicitor of systemic acquired resistance via the salicylic acid pathway (Stout at al., 1999). Tomato plants treated with BTH were found to have compromised direct defenses, based on expression of genes encoding for proteinase inhibitors (Fidantsef et al., 1999). In our study, the presence of methyl salicylate in headspace samples of white mold infected plants indicates activity of the salicylic acid pathway in peanuts in response to S. rolfsii infection. Thus, the feeding preference observed in BAW towards leaves from white mold infected peanut plants may be caused by a reduction in direct defenses of the plant due to fungal infection. Alternatively, the feeding preference of BAW for leaves from fungus infected plants could be due to changes in the nutritional quality of the tissues caused by the pathogen infection. Different pathogen species have been found to either up-or down-regulate accumulation of photo-assimilates, namely, nitrogen, non-structural carbohydrates, starch, protein and free amino acid, as well as their composition, on their respective host (reviewed in Hatcher, 1995). Although most of the studies included in the latter review evaluated the effect of leaf-infecting pathogens on the suitability of plant tissue to insect herbivores, it is not inconceivable that a stem-infecting pathogen such as S. rolfsii can have similar consequences upon its hosts. However, this is an aspect that remains to be elucidated. The enhanced performance of BAW larvae reared on and oviposition preference of BAW adults for white mold infected plants be due to an interference of the fungal infection with the plant’s defenses against the herbivores or an increase in the nutritional quality of the plant tissue following infection.
Studies have shown that foraging behavior of insect parasitoids rely on a variety of cues to locate hosts (Geervliet 1994; Turlings et al. 1991, 1993, Röse et al. 1998). Chemical compounds play a major role in guiding the host searching behavior by natural enemies. These chemical signals may derive from the herbivore host, from the intact plants, or from the herbivore-damaged plant. Herbivores have evolved ways to evade their natural enemies, thus it can be expected that they would emit very little in terms of odors that parasitoids could use to locate them. Thus, long distance location by parasitoids seems to be dependent upon information provided by other sources such as the host plants the herbivore is feeding upon. Plants seem to provide a more reliable source of long-range, easily tractable volatile cues for foraging parasitoids. Recently it has been found that plants under attack by herbivores release a chemical profile that is specific for the plant part being attacked and for the herbivore species causing the damage (McCall et al. 1994, Loughrin et al. 1995, Röse et al. 1996, Pare and Tumlinson 1997). For example, cotton leaves damaged by two different herbivore species emit a unique volatile profile in response to each of the herbivores. However variable plant-derived signals may be, parasitoids are capable of detecting and exploiting these plant-derived volatiles to locate their prey, which makes them more successful in their foraging. We believe that the higher attraction of Cotesia wasps to white mold infected peanut plants may be due to the fact that caterpillars feed more on infected plants, which may in turn be responsible for an increase in volatile emission causing the plant to be more attractive to the parasitoids.
Educational & Outreach Activities
Cardoza, Y. J., H. T. Alborn and J. H. Tumlinson. 2002. In vivo volatile emissions of peanut plants induced by fungal infection and insect damage. J. Chem. Ecol. 28: 161-174
Cardoza, Y. J., C. G. Lait, E. A. Schmelz, J. Huang and J. H. Tumlinson. Enhanced Larval Performance of Spodoptera exigua on Peanut Plants Infected with Sclerotium rolfsii. (In preparation)
Plant release of volatile compounds in response to attack by herbivores, and the role of such compounds in attracting parasitoids of the herbivores, have been studied extensively in recent years (Turlings et al., 1991, 1993; McCall et al., 1994; Loughrin et al., 1995; Röse et al., 1996, 1998; Pare and Tumlinson, 1997). In contrast, the emission of volatiles in response to pathogen invasion has been the subject of a limited number of studies (Croft et al., 1993; Doughty et al., 1996; Shualev et al., 1997; Chaudry et al., 1998; Buonaurio and Servili, 1999). All of these studies have examined the induction of plant volatile emission by either herbivore damage or pathogen infection on different plant systems or using excised plant parts. Thus, we have no clear knowledge of whether the regulation of volatile production in response to these organisms is affected in any way by the simultaneous attack of herbivores and phytopathogens on the same
plant. Additionally, the effect that pathogen infection might have on its ability to attract natural enemies of herbivorous insects has not been studied previously. To our knowledge, this is the first in planta study in which the production of volatiles by a single host system in response to both insect herbivores and pathogens has been evaluated simultaneously. Additionally, it is the first time in which the effect of previous pathogen infection on the production of plant volatiles in response to insect damage has been studied. The study of plant volatile defenses may improve our understanding of plant resistance mechanisms to disease and insect herbivores. The identification of specific pathogen- and herbivore-induced plant volatiles will greatly contribute to the development, improvement and implementation of host-plant resistance and other control methods for insect and pathogen pests.
BAW feeding and oviposition preference and larval performance were all enhanced by white mold infection on peanut plants. This may indicate an interference of the fungal infection with the plant’s direct chemical defenses against the herbivores.
Peanut plants release a specific set of chemical signals in response to S. rolfsii infection. This volatile profile differs not only qualitatively, but also quantitatively from signals emitted in response to BAW damage.
Previous infection on the plant by the white mold fungus does not interfere with the emission of volatiles by the diseased plant in response to BAW attack.
The white mold-derived compound 3-octanone and the plant-produced methyl salicylate were only recovered from plants infected with S. rolfsii, thus the presence of these compounds could potentially be used in the future for the detection of infected plants in the field.
Although herbivore preference and performance was enhanced by white mold infection on peanut plants, it does not appear that previous infection of the plant has any negative effect on the parasitoid’s host searching ability. Parasitoids were actually observed landing more frequently on infected than on healthy peanut plants. This may be a response to higher amounts of volatiles being released by infected plants either caused by increased sensitivity due to the fungal infection or to increased feeding by the BAW.