The timing of induced resistance and induced susceptibility in the soybean-Mexican bean beetle system

Induced plant responses to herbivory have been demonstrated in many systems. It has been suggested that the timing of these responses may influence the impact of induced resistance on herbivore populations, and may affect the evolution of induced defenses. This study used a bioassay to characterize the time course of systemic induced responses to Mexican bean beetle herbivory in four genotypes of soybeans. The results suggest that the time course of induced responses in this system is more complex than most previous studies have indicated. Herbivory provoked both rapid induced resistance and subsequent induced susceptibility to beetle feeding. All four genotypes of soybean induced significant resistance to beetle damage (beetles preferred undamaged to damaged plants) by 3 days after damage. By 15 days after damage, this resistance had decayed (beetles showed no preference for undamaged over damaged plants), and by 20 days after damage, all four genotypes exhibited significant induced susceptibility (beetles preferred previously damaged plants over undamaged plants). The magnitude of induced resistance in each genotype correlated strongly with the magnitude of induced susceptibility in that genotype. Received: 28 September 1997 / Accepted: 1 December 1997     

Induced resistance to Mexican bean beetles in soybean: variation among genotypes and lack of correlation with constitutive resistance

Fourteen genotypes (varieties) of soybean (Glycine max) were screened for levels of induced resistance to Mexican bean beetle (Epilachna varivestis) damage, and a subset of 6 of those varieties was screened for levels of constitutive resistance to Mexican bean beetles. Experiments were carried out in the greenhouse, with damage imposed by Mexican bean beetle larvae, and levels of resistance measured by a choice test bioassay with adult beetles. We found significant variation among soybean genotypes in levels of both induced and constitutive resistance. We found no significant correlation between levels of induced and constitutive resistance measured in the same genotypes. We compare these results to past work on resistance in the soybean-Mexican bean beetle system, consider the implications of variation in both types of resistance for plant-herbivore interactions in agricultural and natural systems, and discuss the relationship between induced and constitutive resistance. Received: 30 November 1998 / Accepted: 25 June 1999     

Comparing the consequences of induced and constitutive plant resistance for herbivore population dynamics

Although it has been suggested that induced and constitutive plant resistance should have different effects on insect herbivore population dynamics, there is little experimental evidence that plant resistance can influence herbivore populations longer than one season. We used a density-manipulation experiment and model fitting to examine the effects of constitutive and induced resistance on herbivore dynamics over both the short and long term. We used likelihood methods to fit population dynamic models to recruitment data for populations of Mexican bean beetles on soybean varieties with no resistance, constitutive resistance, or induced resistance. We compared model configurations that fit parameters for resistance types separately to models that did not account for resistance type. Models representing the hypothesis that the three resistance types differed in their effects on beetle dynamics received the most support. Induced resistance resulted in lower population growth rates and stronger density dependence than no resistance. Constitutive resistance resulted in lower population growth rates and stronger density dependence than induced resistance. Constitutive resistance had a stronger effect on both short-term beetle recruitment and predicted beetle population dynamics than induced resistance. The results of this study suggest that induced and constitutive resistance can differ in their effects on herbivore populations even in a relatively complex system.     

Bioassay versus chemical assay: measuring the impact of induced and constitutive resistance on herbivores in the field

Studies of induced plant resistance usually either examine physiological/chemical mechanisms or explore the ecological and evolutionary role of induced resistance. To connect these two methods of study, data are needed that address the relationships between plant chemistry and effects of induced resistance on herbivores under field conditions. In this paper we combine the results of a greenhouse experiment and a field experiment to try to make such a connection. Levels of induced and constitutive resistance to Mexican bean beetles in several soybean genotypes were measured in a greenhouse experiment using a behavioral bioassay. In a field experiment, beetle performance and induced and constitutive levels of cystein proteinase inhibitor activity were measured for these same genotypes. Greenhouse bioassay ratings of induced and constitutive resistance and induced and constitutive levels of proteinase inhibitor activity agree closely for individual genotypes, suggesting that proteinase inhibitors (PIs) are involved in induced resistance. However, while greenhouse bioassay measures of constitutive and induced resistance were good predictors of beetle performance in the field, proteinase inhibitor activity did not predict beetle performance well. While PIs appear to be involved in induced resistance, they were not strongly correlated to Mexican bean beetle performance in the field in this study. These results suggest that measurement of single aspects of plant chemistry may not encompass all ecologically important aspects of plant resistance.     

Between-Population Outbreeding Affects Plant Defence

Between-population crosses may replenish genetic variation of populations, but may also result in outbreeding depression. Apart from direct effects on plant fitness, these outbreeding effects can also alter plant-herbivore interactions by influencing plant tolerance and resistance to herbivory. We investigated effects of experimental within- and between-population outbreeding on herbivore resistance, tolerance and plant fitness using plants from 13 to 19 Lychnis flos-cuculi populations. We found no evidence for outbreeding depression in resistance reflected by the amount of leaf area consumed. However, herbivore performance was greater when fed on plants from between-population compared to within-population crosses. This can reflect outbreeding depression in resistance and/or outbreeding effects on plant quality for the herbivores. The effects of type of cross on the relationship between herbivore damage and plant fitness varied among populations. This demonstrates how between-population outbreeding effects on tolerance range from outbreeding depression to outbreeding benefits among plant populations. Finally, herbivore damage strengthened the observed outbreeding effects on plant fitness in several populations. These results raise novel considerations on the impact of outbreeding on the joint evolution of resistance and tolerance, and on the evolution of multiple defence strategies.     

GENOTYPIC VARIATION IN LEAF DAMAGE IN PIPER ARIEIANUM (PIPERACEAE) BY A MULTISPECIES ASSEMBLAGE OF HERBIVORES

The shrub Piper arieianum (Piperaceae) has a diverse herbivore fauna (95 species total) in Costa Rican rain forest. The effect of plant genotype on leaf damage by individual herbivore species and total leaf area removed was studied in P. arieianum through a cloning experiment. Damage patterns were measured over 3.5 years for two plots, four genotypes per plot, in the understory of lowland rain forest. In both plots, there were significant differences among genotypes in total leaf area missing throughout the study period. Rankings of genotypes based on overall damage remained constant over time in plot 1 but changed in plot 2. Certain individual herbivore species caused significantly higher damage in some genotypes than in others; the change in genotype rankings in plot 2 was associated with increased damage to particular genotypes by specific herbivore groups. The genotype most heavily damaged by a given insect species varied depending on the herbivore species; thus, resistance to one herbivore species did not necessarily confer resistance against all species. Those herbivore species causing the greatest proportion of damage for a given plant changed over time. Because total damage resulted from the summation of losses to individual herbivore species, whether an individual plant lost more leaf area than its neighbors depended on the relative abundance of the herbivore species at any one time. Finally, for a portion of the study period in each plot, more heavily damaged clones grew less than lesser damaged clones. Together with previous reports that naturally growing plants differ significantly in damage and that these differences are sufficient to cause fitness differences, the results presented here suggest that the herbivores of P. arieianum represent a selective force for changes in resistance but that this selective force changes both in intensity and quality over time.     

Herbivore community promotes trait evolution in a leaf beetle via induced plant response

Several recent studies have emphasised that community composition alters species trait evolution. Here, we demonstrate that differences in composition of local herbivore communities lead to divergent trait evolution of the leaf beetle Plagiodera versicolora through plant‐mediated indirect interactions. Our field surveys, genetic analyses and community‐manipulation experiments show that herbivore community composition determines the degree of herbivore‐induced regrowth of willows (Salicaceae), which in turn, promotes the divergent evolution of feeding preference in the leaf beetle from exclusive preference for new leaves to a lack of preference among leaf‐age types. Regrowth intensity depends both on the differential response of willows to different herbivore species and the integration of those herbivore species in the community. Because herbivore‐induced regrowth involves phenological changes in new leaf production, leaf beetle populations develop divergent feeding preferences according to local regrowth intensity. Therefore, herbivore community composition shapes the selection regime for leaf beetle evolution through trait‐mediated indirect interactions.     

Leaf phenology mediates provenance differences in herbivore populations on valley oaks in a common garden

1. Plants from different populations often display a variation in herbivore resistance. However, it is rarely understood what plant traits mediate such differences. 2. It was tested how leaf phenology affects herbivore populations in a 15‐year‐old common garden of valley oaks (Quercus lobata Née) with different populations and maternal parents from throughout the Q. lobata range. 3. The abundance of leaf miners (Stigmella sp. Shrank) and leaf phenology of oaks in the common garden was measured. 4. Leaf miner abundance varied among provenance locations (population), but not among maternal parents within populations. Leaf phenology varied by provenance location and maternal parent, and trees that leafed out earlier accrued higher leaf‐miner abundance. Path analysis indicated that leaf phenology was the likely driver of provenance and parental differences in resistance to leaf miners. 5. Understanding population differences is particularly important when considering transport of genotypes for ornamental or restoration purposes. The present study suggests that similarity in leaf phenology may be one factor that could be used to find genotypes with a similar herbivore resistance to local genotypes.     

Plant genotype and induced responses affect resistance to herbivores on evening primrose (Oenothera biennis)

Abstract. 1. Although both genotype and induced responses affect a plant's resistance to herbivores, little is known about their relative and interactive effects. This study examined how plant genotype of a native plant (Oenothera biennis) and induced plant responses to herbivory affect resistance to, and interactions among, several herbivores. 2. In a field experiment, genetic and environmental variation among habitats led to variation in the amount of early season damage and plant quality. The pattern of variation in early season infestation by spittlebugs (Philaenus spumarius, a piercing–sucking herbivore) negatively correlated with oviposition preference by a later feeding specialist weevil (Tyloderma foveolatum, a leaf‐chewer). 3. To determine if plant genotype and induced responses to herbivory might be responsible for these field patterns, we performed no‐choice and choice bioassays using four genotypes of O. biennis that varied in resistance. Plants were induced by either spittlebugs or weevils and assays measured the responses of the same specialist weevil as well as a generalist caterpillar (Spodoptera exigua). 4. Resistance to adult weevils was largely unaffected by plant genotype, while they experienced induced resistance following damage by conspecific weevils in no‐choice assays. Caterpillars were more strongly affected by plant genotype than induced responses in both no‐choice and choice assays, but they also fed less and experienced higher mortality on plants previously damaged by weevils. In contrast to the pattern suggested by the field experiment, spittlebugs did consistently induce resistance against either weevils or caterpillars in the bioassay experiment. 5. These results support recent findings that show herbivore species can compete via induced plant responses. Additionally, a quantitative review of the literature demonstrates that plant genotype tends to be more important than interspecific competition among herbivores (plant‐mediated or otherwise) in affecting herbivore preference and performance.     

Small-scale distributional dynamics of the yellow water-lily and its herbivore Galerucella nymphaeae (Coleoptera: Chrysomelidae)

Summary I followed the within-generation distribution of a chewing specialist herbivore, the water-lily beetle, on individually marked leaves of the yellow water-lily. Yellow water-lilies produced new leaves steadily throughout the growth season. Average leaf longevity was 3–4 weeks, much shorter than the developmental time of the beetle. The average egg-load of leaves was about 120. Leaf longevity was independent of egg density, but migration rate of the first instar larvae was density-dependent. Beetles occurred in every leaf, but consumed only a fraction (17%) of the available leaf area. However, this caused the leaf to lose its floating ability, so even this low rate of consumption made the leaf unavailable to herbivores. The herbivore population had to redistribute itself throughout the summer, escaping from the drowning leaves to fresh ones. No beetle could survive from egg to adult on a single leaf. The small-scale redistribution of the herbivore strongly affected the damage experienced by the host plant. In general, ability to redistribute depends on the dispersal ability of the herbivore, and thus migration ability may strongly affect the plant-herbivore interaction.     

CO2‐mediated changes of plant traits and their effects on herbivores are determined by leaf age

1. Concentration of atmospheric CO₂ is predicted to double during the 21st century. However, quantitative effects of increased CO₂ levels on natural herbivore–plant interactions are still little understood. 2. In this study, we assess whether increased CO₂ quantitatively affects multiple defensive and nutritive traits in different leaf stages of cyanogenic wildtype lima bean plants (Phaseolus lunatus), and whether plant responses influence performance and choice behaviour of a natural insect herbivore, the Mexican bean beetle (Epilachna varivestis). 3. We cultivated lima bean plants in climate chambers at ambient, 500, 700, and 1000 ppm CO₂ and analysed cyanogenic precursor concentration (nitrogen‐based defence), total phenolics (carbon‐based defence), leaf mass per area (LMA; physical defence), and soluble proteins (nutritive parameter) of three defined leaf age groups. 4. In young leaves, cyanide concentration was the only parameter that quantitatively decreased in response to CO₂ treatments. In intermediate and mature leaves, cyanide and protein concentrations decreased while total phenolics and LMA increased. 5. Depending on leaf stage, CO₂‐mediated changes in leaf traits significantly affected larval performance and choice behaviour of adult beetles. We observed a complete shift from highest herbivore damage in mature leaves under natural CO₂ to highest damage of young leaves under elevated CO_2. Our study shows that leaf stage is an essential factor when considering CO₂‐mediated changes of plant defences against herbivores. Since in the long run preferred consumption of young leaves can strongly affect plant fitness, variable effects of elevated CO₂ on different leaf stages should receive highlighted attention in future research.     

Negative interactions between chemical resistance and predators affect fitness in soybeans

Abstract 1. Plants may benefit from both chemical resistance traits and the presence of predators of herbivores. In past studies, the interaction between resistance and predators varies from complementary to antagonistic among different systems. However, this interaction has primarily been quantified by effects on predator abundance or vigor, not effects on plant fitness. 2. In this study, the combined effects of chemical resistance and predators on plant fitness were examined using soybeans (Glycine max), herbivorous Mexican bean beetles (Epilachna varivestis), and predaceous spined soldier bugs (Podisus maculiventris). Mexican bean beetles were reared in field cages in the presence or absence of spined soldier bugs on soybeans with or without strong constitutive chemical resistance. 3. Spined soldier bugs were more likely to feed on Mexican bean beetles that fed on susceptible than on resistant plants. 4. Susceptible plants with predators produced significantly more seeds than those without predators, while resistant plants did not produce significantly different numbers of seeds based on the presence or absence of predators. 5. Selection for the production of some types of chemical resistance in plants would thus be expected to be stronger with lower predation rates. 6. These results also suggest predator introductions would be more effective on plants without a strong constitutive chemical resistance to herbivores.     

Herbivore-induced resistance in Betula pendula: the role of plant vascular architecture

We studied the role of plant vascular architecture in the determination of the spatial extent of herbivore induced responses within Betula pendula Roth saplings. The induced responses were measured in bioassays in terms of the relative growth rate of larvae of a geometrid moth, Epirrita autumnata. We hypothesised that the level of induced resistance of a certain leaf would be determined by the degree of vascular connectivity between the leaf in question and a damaged leaf, as suggested by recent theoretical and empirical studies. A comparison of the control plants with the damaged plants indicated that damaging one leaf of a sapling was sufficient to induce an increase in the resistance level. There were also differences among the leaves within a plant in the resistance level, but these differences could not be explained by the degree of vascular connectivity with the damaged leaf. These results suggest that the vascular connections have low power as explanations of the spread and spatial extent of the induced resistance in Betula pendula saplings Instead, the resistance level of all leaves within a sapling increased following the damage. We suggest that the pattern of increased resistance observed in this experiment may be beneficial for the young saplings studied. For young saplings at their early stages of development, it may be beneficial to be able to distribute the induction signal to all leaves as fast as possible and thus repel the herbivore totally. For a young sapling, the capability of repelling the herbivore totally might thus be a feasible strategy whereas an older sapling may tolerate localised damage better and compensate for the damage within the undamaged plant parts.     

Plant genotypic diversity reduces the rate of consumer resource utilization

While plant species diversity can reduce herbivore densities and herbivory, little is known regarding how plant genotypic diversity alters resource utilization by herbivores. Here, we show that an invasive folivore—the Japanese beetle (Popillia japonica)—increases 28 per cent in abundance, but consumes 24 per cent less foliage in genotypic polycultures compared with monocultures of the common evening primrose (Oenothera biennis). We found strong complementarity for reduced herbivore damage among plant genotypes growing in polycultures and a weak dominance effect of particularly resistant genotypes. Sequential feeding by P. japonica on different genotypes from polycultures resulted in reduced consumption compared with feeding on different plants of the same genotype from monocultures. Thus, diet mixing among plant genotypes reduced herbivore consumption efficiency. Despite positive complementarity driving an increase in fruit production in polycultures, we observed a trade-off between complementarity for increased plant productivity and resistance to herbivory, suggesting costs in the complementary use of resources by plant genotypes may manifest across trophic levels. These results elucidate mechanisms for how plant genotypic diversity simultaneously alters resource utilization by both producers and consumers, and show that population genotypic diversity can increase the resistance of a native plant to an invasive herbivore.     

Friend or foe?: a plant's induced response to an omnivore

Omnivorous natural enemies of herbivores consume plant-based resources and may elicit induced resistance in their host plant. A greater induction threshold for damage produced by omnivorous predators than for strict herbivores might be expected if omnivore performance is enhanced on noninduced plants, allowing them to reduce future levels of herbivory. Currently, it is not known if a plant responds to feeding by omnivorous predators and by herbivores similarly. To examine this question, we chose herbivore and omnivore species that produce the same kind of quantifiable damage to cotton leaves, enabling us to control statistically for the intensity of plant damage, and ask whether plant responses differed depending on the identity of the damaging species. We first compared changes in plant peroxidase activity, gossypol gland number and density, and leaf area in response to feeding by the spider mite Tetranychus turkestani (Ugarov and Nikolski) (an herbivore) and by one of the mite's principal natural enemies, the western flower thrips Frankliniella occidentalis (Pergande) (an omnivore). Both species increased the activity of peroxidase, but when we controlled for the amount of damage, the peroxidase activity of mite-damaged plants was higher than that of thrips-damaged plants. We also found that thrips, but not spider mites, increased the density of gossypol glands in the second true leaf. In a second experiment we included an additional herbivore, the bean thrips Caliothrips fasciatus (Pergande), to see if the different responses of cotton to thrips and mite herbivory we first observed were attributable to differences in trophic function (herbivore versus omnivore) or to other differences in feeding generated by thrips versus mites. Cotton plants exhibited the same pattern of induced responses (elevated peroxidase, increased number of glands, reduced leaf area) to herbivory generated by the bean thrips (an herbivore) and western flower thrips (an omnivore), suggesting that trophic function was not a key determinant of plant response. Thrips-damaged plants again showed a significantly higher density of gossypol glands than did mite-damaged plants. Overall, our results suggest that (1) an omnivorous predator systemically induces resistance traits in cotton and (2) whereas there is evidence of taxonomic specificity (thrips versus mites), there is little support for trophic specificity (herbivorous thrips versus omnivorous thrips) in the elicitation of induced responses.     

Within-plant distribution of induced resistance in apple seedlings: rapid acropetal and delayed basipetal responses

Induction of plant resistance by herbivory is a complex process, which follows a temporal dynamic and varies spatially at the within-plant scale. This study aimed at improving the understanding of the induction process in terms of time scale and within-plant allocation, using apple tree seedlings (Malus × domestica) as plant model. Feeding preferences of a leaf-chewing insect (Spodoptera littoralis) for previously damaged and undamaged plants were assessed for six different time intervals with respect to the herbivore damage treatment and for three leaf positions. In addition, main secondary defense compounds were quantified and linked to herbivore feeding preferences. Significant herbivore preference for undamaged plants (induced resistance) was first observed 3 days after herbivore damage in the most apical leaf. Responses were delayed in the other leaf positions, and induced resistance decreased within 10 days after herbivore damage simultaneously in all tested leaf positions. Chemical analysis revealed higher concentrations of the flavonoid phloridzin in damaged plants as compared to undamaged plants. This indicates that herbivore preference for undamaged apple plants may be linked to phloridzin, which is the main secondary metabolite of apple leaves. The observed time course and distribution of resistance responses within plants contribute to the understanding of induction processes and patterns, and support the optimal defense theory stating young tissue to be prioritized. Moreover, induced resistance responses occurred also basipetally in leaves below the damage site, which suggests that signaling pathways involved in resistance responses are not unidirectional.     

Effects of soil nitrogen levels on growth and defense of the native and introduced genotypes of alligator weed

土壤氮水平对喜旱莲子草原产地和引入地基因型生长和防御的影响 同种植物生长在资源丰富生境中的个体,其防御水平被认为低于生长在资源匮乏生境中的个体。然而,生境的养分水平如何影响植物的诱导抗性和耐受性,以及这种影响在入侵植物的原产地和引入 地种群间是否存在差异,目前均知之甚少。本研究以入侵植物喜旱莲子草(Alternanthera philoxeroides)的原产地阿根廷和引入地美国的基因型为研究对象设计同质园实验,以探究土壤氮水平对植物的生长、组成和诱导性[莲草直胸跳甲(Agasicles hygrophila)取食诱导]化学防御以及耐受性的影响。实验中,我们测定了植物总生物量、伸长速率(生长速率的表征)以及叶片和根系中总碳、总氮和三萜皂苷(化学防御物质)的含量。研究结果显示,植物在低土壤氮水平下表现出较高的组成抗性(植物在低土壤氮水平下的叶片三萜皂苷含量高于其在高土壤氮水平的33%)和耐受性[植物被取食后总生物量下降的程度更低(植物在高土壤氮水平和低土壤氮水平下被取食后总生物量分别下降了24%和15%)],而在高土壤氮水平下表现出较高的诱导抗性(在高土壤氮水平下的植物被取食后叶片三萜皂苷含量与空白对照的植物相比升高了24%)。植物的组成抗性和耐受性与生长速率存在权衡,但诱导抗性与生长速率存在显著的正相关性。此外,引入地基因型在低土壤氮水平下叶片碳含量显著低于原产地基因型(-6%),但这种差异在高土壤氮水平下消失。这些结果表明,土壤氮水平 影响植物对不同防御策略的选择偏好,并且在决定引入地基因型的表现时与植食作用存在交互作用。     

The effect of frequency‐dependent selection on resistance and tolerance to herbivory

Negative frequency‐dependent selection (FDS), where rare genotypes are favoured by selection, is commonly invoked as a mechanism explaining the maintenance of genetic variation in plant defences. However, empirical tests of FDS in plant–herbivore interactions are lacking. We evaluated whether the oviposition preference of the specialist herbivore Lema daturaphila is a mechanism through which this herbivore can exert FDS on its host plant Datura stramonium. The frequency of contrasting resistance–tolerance strategies was manipulated within experimental plots, and the plants were exposed to a similar initial density of their natural herbivore. Herbivore oviposition preference and final density, as well as plant damage and seed production, were estimated. Overall, we found that the high‐resistant–low‐tolerant genotypes produced four times more seeds when common than when rare, whereas the high‐tolerant–low‐resistant genotypes achieved twice its fitness when rare than when common. This pattern was the result of differential oviposition preferences. In addition, when the high‐resistant–low‐tolerant genotypes were common, there was a three‐fold decreased in herbivore final density which led to a decrease in damage level by 10%. Thus, in our experiment positive FDS seems to favour resistance over tolerance. We discuss how this result would change if the extent of herbivore local adaptation and damage modify the pattern of positive FDS acting on resistance and the optimal allocation to tolerance.     

The jasmonate pathway alters herbivore feeding behaviour: consequences for plant defences

The jasmonate pathway is a highly conserved defensive cascade in plants that regulates the induction of resistance against herbivores; however, its role in herbivore feeding behaviour remains unknown. We used a mutant tomato plant (def‐1) deficient in the production of jasmonate‐related defensive proteins to test the hypothesis that genotypes with a reduced ability to induce resistance have a higher and more concentrated pattern of herbivore damage. Wild‐type and def‐1 plants received either damage by Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae) caterpillars or no damage. After treatment, we tested for systemic responses by allowing a free roaming S. exigua caterpillar to feed on the undamaged portions of plants. Weight‐gain and leaf consumption of S. exigua were highest on def‐1 plants, regardless of prior herbivore damage. Def‐1 plants also had fewer numbers of leaves and leaflets eaten, and fewer feeding holes, which was indicative of a more concentrated distribution of damage on mutant compared to wild‐type plants. Following these results, we mimicked the amount and distribution of feeding damage that wild‐type or jasmonate‐deficient plants would receive on wild‐type plants to test whether changes in feeding behaviour may feedback to influence the expression of induced resistance. We mimicked the distribution of damage in wild‐type and jasmonate‐deficient plants by allowing caterpillars to feed on either one (leaf 1 or 2) or two leaves (leaf 1 and 2). Increased herbivore damage resulted in higher proteinase inhibitor (PI) activity, a jasmonate‐regulated defensive protein, and lower S. exigua performance on wild‐type but not jasmonate‐deficient plants. Compared to undamaged plants, a concentrated pattern of herbivore damage increased systemic resistance; these induced responses were greater on leaflets with stronger vascular connections to the damaged leaf. A more dispersed pattern of caterpillar damage altered the expression of induced responses, but the outcome depended on the specific pattern of damage. When leaf 1 was damaged and then leaf 2, the undamaged (third) leaf (which is more strongly connected to leaf 1 than 2) expressed reduced the PI activity compared to plants receiving concentrated damage to leaf 1; whereas in plants where leaf 2 was first damaged and then leaf 1, there were no differences in PI activity in leaf 3 compared to plants receiving concentrated damage to leaf 2. Thus, induction of the jasmonate pathway may not only determine the amount and distribution of feeding damage by herbivores, but this may feedback to affect the subsequent expression of plant defence.     

A fungal root symbiont modifies plant resistance to an insect herbivore

Vesicular-arbuscular mycorrhizal (VAM) fungi are common root-colonizing symbionts that affect nutrient uptake by plants and can alter plant susceptibility to herbivores. I conducted a factorial experiment to test the hypotheses that colonization by VAM fungi (1) improves soybean (Glycine max) tolerance to grazing by folivorous Mexican bean beetle (Epilachna varivestis), and (2) indirectly affects herbivores by increasing host resistance. Soybean seedlings were inoculated with the VAM fungus Glomus etunicatum or VAM-free filtrate and fertilized with high-[P] or low-[P] fertilizer. After plants had grown for 7 weeks first-instar beetle larvae were placed on bagged leaves. Growth of soybean was little affected by grazing larvae, and no effects of treatments on tolerance of soybeans to herbivores were evident. Colonization by VAM fungus doubled the size of phosphorus-stressed plants but these plants were still half the size of plants given adequate phosphorus. High-[P] fertilizer increased levels of phosphorus and soluble carbohydrates, and decreased levels of soluble proteins in leaves of grazed plants. Colonization of grazed plants by VAM fungus had no significant effect on plant soluble carbohydrates, but increased concentration of phosphorus and decreased levels of proteins in phosphorus-stressed plants to concentrations similar to those of plants given adequate phosphorus. Mexican bean beetle mass at pupation, pupation rate, and survival to eclosion were greatest for beetles reared on phosphorus-stressed, VAM-colonized plants, refuting the hypothesis that VAM colonization improves host plant resistance. VAM colonization indirectly affected performance of Mexician bean beetle larvae by improving growth and nutrition of the host plant. Received: 28 February 1997 / Accepted: 23 June 1997