Plant Stress Hypothesis Research Articles

Radon as a possible link between peak spring tides and lemming cycles

There is still no consensus regarding the ultimate cause of the famous 3–4-year population cycles of lemmings. According to the plant stress hypothesis, herbivore population peaks are caused by stress factors that force plants to reallocate stored defensive proteins to transportable and easily digestible N-compounds. One possible plant stress factor is ionization caused by exhalation of the radioactive noble gas radon, which is enhanced after spring tides, i.e., tides at new or full Moon. I hypothesized that increased ionization caused by radon accumulation in the subnivean space after peak spring tides, which occur close to perihelion (31 December–05 January, when the Earth–Sun distance is shortest), and at 3.8-year intervals, results in increased protein digestibility of mosses, which are important food for lemmings. Population outbreaks of Norway lemming Lemmus lemmus occurred two years after peak spring tides during 1871–1910, when also perigee (the time when the Earth–Moon distance is shortest) coincided with spring tides and perihelion. Thereafter this relationship weakened, and lemming peaks became less pronounced and more related to plant seed indices. As we have now entered a new 30-year period with coincidence of spring tide, perihelion and perigee, I predict more regular and pronounced lemming outbreaks in the next decades, unless radon accumulation is prevented by frequent lack of a stable snow cover due to global warming.

The lunar nodal phase cycle and winter atmospheric pressure as possible determinants of moth abundance: Analyses of a30-year time series from South Norway.

According to the plant stress hypothesis, population peaks of herbivores such as moths are caused by plant stress factors that force plants to reallocate stored defensive proteins to transportable and easily digestive N‐compounds. A suggested plant stress factor is ionization caused by cosmic ray muons, which are modulated by the 9.3‐year lunar nodal phase cycle, solar activity, and atmospheric pressure. Vascular plants are more sensitive to ionization than are bryophytes, and woody plants are more sensitive than are herbaceous plants, but the difference may be less during dormancy in winter. We selected the 14 most common moth species from a 30‐year light‐trapping study in southern Norway to test whether the fluctuation patterns of species from three different feeding guilds were correlated with lunar/solar cycles, or with atmospheric pressure in winter, when muon fluxes are higher than in other seasons. The population indices of three species feeding on deciduous woody plants were positively correlated with the lunar nodal phase index, and there was a similar tendency for the remaining three species. No positive correlations with the lunar index were found for species feeding on herbs or mosses. For nine species, that is, from all three guilds, there was a significant negative correlation between the population index and winter atmospheric pressure in the previous year. The results are in accordance with predictions deduced from the cosmic ray hypothesis, but thorough investigations of the proposed physiological mechanisms are needed for the hypothesis to be widely accepted.

Environmental Pressures on Top-Down and Bottom-Up Forces in Coastal Ecosystems

Global change is manifesting new and potent pressures that may determine the relative influence of top-down and bottom-up forces on the productivity of plants that undergird coastal ecosystems. Here, I present a meta-analysis conducted to assess how herbivory, nitrogen enrichment, and elevated salinity influence plant productivity according to the salinity regimes of coastal ecosystems. An examination of 99 studies representing 288 effect sizes across 76 different plant species revealed that elevated salinity negatively affected productivity across all environments, but particularly in freshwater ecosystems. Nitrogen enrichment, on the other hand, positively affected productivity. In agreement with the plant stress hypothesis, herbivory had the greatest negative impact in saline habitats. This trend, however, appears to reverse with nitrogen enrichment, with maximum losses to herbivory occurring in brackish habitats. These findings demonstrate that multiple stressors can yield complex, and sometimes opposite outcomes to those arising from individual stressors. This study also suggests that trophic interactions will likely shift as coastal ecosystems continue to experience nutrient enrichment and sea level rise.

Effect of light stress on Crotalaria spectabilis (Fabaceae) and on its herbivore insect, the moth Utetheisa ornatrix (Erebidae: Arctiinae)

ABSTRACT The Plant Stress Hypothesis predicts that stressed plants are more attacked by herbivorous insects. In this work, we investigated the influence of light stress on Crotalaria spectabilis Roth (Fabaceae) and on its main herbivore, the moth Utetheisa ornatrix (L., 1758) (Erebidae: Arctiinae). Specifically, we verified whether plants stressed by shading differ from non-stressed plants in terms of productivity, morphological characteristics and water percentage. We also evaluated the performance of moths in stressed and non-stressed plants. Seeds were sown in pots. When the plants reached 50 cm in height, they were randomly divided into two groups: stressed plants (treatment group) and non-stressed plants (control group). The stressed plants were covered by a black mesh, providing 50% of shading. Eight characteristics of stressed and non-stressed C. spectabilis plants were evaluated: height, fresh and dry aerial biomass, number of pods and seeds, leaf hardness, number of trichomes, leaf area, specific leaf mass and percentage of leaf water. Moths were raised individually on leaves of stressed and non-stressed plants and we obtained the larval survival, larval development time, pupal weight and female fecundity. The non-stressed plants had significantly higher percentage of water in the leaves, greater fresh aerial biomass and a higher number of trichomes than the stressed plants. The survival rate was 98% for larvae raised on leaves from stressed plants and 92% on leaves from non-stressed plants. The larval developmental time was significantly shorter and the weight of female pupae significantly higher in non-stressed plants than in stressed plants. Thus, the Plant Stress Hypothesis was only corroborated by two tested variables: number of trichomes (lower in stressed plants) and larval survival (higher in stressed plants). Trichomes are among the main types of plant defenses against herbivory and reducing their number on leaves would make stressed plants more susceptible to attack by moth larvae, a fact corroborated by a greater larval survival. One of the possible explanations for the lack of corroboration of the Plant Stress Hypothesis for most of the variables tested is that other characteristics can be changed under stress conditions, such as the concentration of secondary compounds.

Annual change in forest grouse in southern Norway: variation explained by temperatures, bilberry seed production and the lunar nodal phase cycle

Forage quality is rarely the focus of studies on grouse performance. The plant stress hypothesis predicts that the protein digestibility of a focal food plant for grouse in Norway, the bilberry Vaccinium myrtillus should be highest after a peak in seed production. Protein digestibility may also be affected positively by cosmic ray (muon) fluxes, which fluctuate with the 9.3-year lunar nodal phase cycle, and negatively by high temperatures in the preceding summers. The annual change (first difference) in ln-transformed black grouse Tetrao tetrix and capercaillie T. urogallus hunting statistic indices from southern Norway 1999–2017 was positively related to the mean June temperature, which is assumed to have a direct positive impact on chick survival. In multiple regression models, there was also a positive relationship with the seed production of bilberry in the preceding year, the lunar nodal phase index with a two-year time lag, and a negative relationship with the combined mean June–September temperature of the two preceding years. Small rodent numbers did not contribute significantly in these models. In further monitoring of grouse populations, the seed production of bilberry or other important food plants should also be recorded.

The responses of cucumber plants subjected to different salinity or fertilizer concentrations and reproductive success of Tetranychus urticae mites on these plants.

The plant stress hypothesis posits that a herbivore's reproductive success increases when it feeds on stressed plants, while the plant vigor hypothesis predicts that a herbivore preferentially feeds on more vigorous plants. We examined these opposing hypotheses by growing spider mites (Tetranychus urticae) on the leaves of stressed and healthy (vigorous) cucumber plants. Host plants were grown under controlled conditions at low, moderate, and high concentrations of NaCl (to induce salinity stress), at low, moderate, and high fertilizer concentrations (to support growth), and without these additions (control). The effects of these treatments were evaluated by measuring fresh and dry plant biomass, carotenoid and chlorophyll content, antioxidant enzyme activity, and concentrations of PO43-, K+, and Na+ in plant tissues. The addition of low concentrations of fertilizer increased dry mass, protein, and carotenoid content relative to controls, suggesting a beneficial effect on plants. The highest NaCl treatment (2560mgL-1) resulted in increased Na+ and protein content relative to control plants, as well as reduced PO43-, K+, and chlorophyll levels and reduced catalase and ascorbate peroxidase enzyme activity levels. Analysis of life table data of T. urticae mites raised on leaves from the aforementioned plant groups showed the intrinsic rate of increase (r) for mites was 0.167day-1 in control specimens, 0.125day-1 for mites reared on plants treated with a moderate concentration of fertilizer (10mLL-1), and was highest (0.241day-1) on plants grown under moderate salinity conditions (1920mgL-1 NaCl). Reproductive success of T. urticae did not differ on plants watered with a moderate concentration of NaCl or a high concentration of fertilizer. The moderately-stressed plants formed a favorable environment for the development and reproduction of spider mites, supporting the plant stress hypothesis.

The insects as an assessment tool of ecotoxicology associated with metal toxic plants

In this article, the assessment of lethal effects of Copper (Cu) on Luffa acutangula and Spinacia oleracea plants investigated in relation to the presence of insect species Oxycarenus hyalinipennis. The analysis of Cu-treated plants displays the information of rapid growth of Oxycarenus hyalinipennis species in triplicate. However, results showed that the impact of metal toxicity appeared as the reduced growth rate of plants, and dense growth of the insect species Oxycarenus halinipennis followed by the chewing/degradation of the toxic plant. The insect's inductees into polluted plants were justified by morphological and primary molecular level using plant stress hypothesis through analysis of the primary chemistry of leaves and roots. That includes various sugar contents which substantiated that these compounds act as the best feeding stimulant from oviposition to adult stage of the insects and accountable for the enactment of insects in the toxic plants. The relationship of these insects to the toxic plants linked with the higher contents of glucose, carbohydrates, and cellulose. The higher carbohydrate and cellulose content in both plants species under Cu accumulation exhibited more signs of insect mutilation over control plants and the lack of chemical resistances allowed the adult insects to spread, survive, reproduce and live long. The presence of insects developed relationships that assimilate all developmental, biological, and the interactive toxicity of Cu in both plant species which indicate the risk associated with these plants.

Higher Fertilizer Inputs Increase Fitness Traits of Brown Planthopper in Rice

Rice (Oryza sativa L.) is the primary staple food source for more than half of the world’s population. In many developing countries, increased use of fertilizers is a response to increase demand for rice. In this study, we investigated the effects of three principal fertilizer components (nitrogen, phosphorus and potassium) on the development of potted rice plants and their effects on fitness traits of the brown planthopper (BPH) [Nilaparvata lugens (Stål) (Homoptera: Delphacidae)], which is a major pest of rice in Bangladesh and elsewhere. Compared to low fertilizer inputs, high fertilizer treatments induced plant growth but also favored BPH development. The BPH had higher survival, developed faster, and the intrinsic rate of natural increase (rm) was higher on well-fertilized than under-fertilized plants. Among the fertilizer inputs, nitrogen had the strongest effect on the fitness traits of BPH. Furthermore, both the “Plant vigor hypothesis” and the “Plant stress hypothesis” were supported by the results, the former hypothesis more so than the latter. These hypotheses suggest that the most suitable/attractive hosts for insect herbivores are the most vigorous plants. Our findings emphasized that an exclusive focus on yield increases through only enhanced crop fertilization may have unforeseen, indirect, effects on crop susceptibility to pests, such as BPH.

Upscaling hypotheses on herbivore damage in plants facing environmental stress: Variation among scales and plant enemies in a relict tree

The few existing analyses of broad-scale variation in plant–herbivore interactions are largely exploratory and not based on hypotheses on the effect of varying environmental conditions on the patterns of herbivore damage. However, there are two established hypotheses that relate herbivore damage and environmental stress mostly tested at the within-population level: the Plant Stress Hypothesis (PSH) and the Plant Vigor Hypothesis (PVH). To examine if support for the PSH holds true at broader scales, we tested it at the micro- (among individuals), meso- (between habitats) and macroscale (between geographical regions, Iberia and Macaronesia). We measured plant stress and leaf damage by different natural enemies (chewers, miners and pathogens) in 12 populations across the range of the relict tree Prunus lusitanica. Results regarding the proportion of leaves attacked by any enemy showed no support for the PSH at either micro or mesoscale, observing instead higher damage in less stressed individuals within populations and in high-quality habitats. However, the latter was true exclusively within the Macaronesian region, highlighting the important effect of the type of abiotic stress on plant susceptibility to herbivory. At the macroscale, alternatively, results showed support for the PSH: we observed a higher proportion of leaves attacked in Macaronesia than in Iberia, where plants unexpectedly exhibited greater stress symptoms. Among-scale inconsistencies may respond to the effect of distinct climatic factors governing both plant and herbivore dynamics at large scales, particularly in species with distributional ranges across contrasting climatic regions such as P. lusitanica. Finally, different plant enemies showed different patterns of damage, corroborating the need to study them separately to achieve consistent generalizations of these hypotheses.

Drought Impacts on Competition in Phoetaliotes nebrascensis (Orthoptera Acrididae) in a Northern Mixed Grassland.

Global climate change is predicted to significantly modify patterns of precipitation, making it critical to develop a better understanding of how this will modify biotic interactions. Short-term to decadal-scale weather patterns can impact grasshopper population dynamics, but drought impacts on grasshoppers have rarely been studied in manipulative experiments. A cage experiment was conducted in eastern Montana to examine the impact of intra- and interspecific competition and precipitation manipulation treatments on performance of a common melanopline grasshopper Phoetaliotes nebrascensis (Thomas). High-density and drought treatments had similarly strong negative impacts on food availability. Proportional grasshopper survival did not differ significantly by treatment, but density dependence was evident in both body size and reproductive traits. The impact of precipitation and density treatments on grasshopper body size and reproduction were typically similar in magnitude and much larger than interspecific competition, with the exception of male femur length. Even with high late summer precipitation, drought had strong effects on individual body size and future reproduction. This study provides valuable information on population dynamics of an abundant grasshopper, with moderate precipitation reductions negatively affecting reproduction and body size. No positive impacts of drought as predicted by the plant stress hypothesis were observed. The study reinforces the need to examine drought manipulations to better predict grasshopper population changes due to changing climate conditions.

Herbivory-induced stress: Leaf developmental instability is caused by herbivore damage in early stages of leaf development

Herbivory is a major source of plant stress and its effects can be severe, decreasing plant fitness, or subtle, affecting the development of leaves by influencing the normal pattern of growth and expansion of leaf blades. Fluctuating asymmetry (FA) analysis is recognized as a measure of plant stress, and can be used to evaluate subtle effects of herbivory on the imperfect growth of bilaterally symmetrical traits, such as leaves. One general issue is that authors usually consider FA as an indicator of stress, which can attract herbivores (plant stress hypothesis), and studies showing that herbivores themselves affect leaf symmetry (herbivory-induced stress hypothesis) are scarce, with mixed results. Here, we investigated the relationship between herbivory by thrips and leaf FA in Banisteriopsis malifolia and Heteropterys escallonifolia (Malpighiaceae). Pseudophilothrips obscuricornis is a free-living, non-pest, sucking species that feeds mainly on leaf buds. We hypothesized that herbivory by thrips in the early stages of leaf development would provoke increased FA levels in mature leaves. The results showed that thrips herbivory rate was low, affecting barely more than 1% of the leaf blade. Nonetheless, thrips-attacked leaves of B. malifolia and H. escallonifolia presented increases of 15 and 27% in leaf asymmetry, respectively, compared to uninjured leaves, corroborating the herbivory-induced stress hypothesis. Since herbivory by thrips in leaf buds was related to significant increases in the stress of mature leaves, we assume that under these circumstances, FA can be used as a biomarker for plant stress following herbivory damage. To be useful as a biomarker of stress, FA in plants must be investigated with caution, taking into account the natural history of the herbivore species and timing of leaf damage.

Tree genotype modulates the effects of water deficit on a plant-manipulating aphid

Successful plant manipulation by herbivores requires a reconfiguration of the primary and secondary metabolisms of the host-plant. Water deficit is generally predicted to negatively affect the development of gall-inducing insects, by impairing their ability to remodel the primary metabolism of their host. We assessed whether host genotype could modulate water deficit effects on a gall-inducing aphid, Phloeomyzus passerinii, developing on poplars, which are known to be among the most susceptible trees to water deficit. Stem-cuttings of two poplar genotypes, with a different resistance level to P. passerinii, were grown under three irrigation treatments, and subsequently infested with the same aphid clone. Plant growth parameters and aphid colonies development were recorded, and the development and organization of the pseudogall induced by the aphid was observed using histological investigations. The three irrigation treatments resulted in three levels of water deficit. Mean predawn leaf water potential reached −0.10, −0.49 and −0.75MPa in the optimal, intermediate and low irrigation treatments respectively. In both genotypes, growth, architecture and photosynthesis were similarly affected. The severe water deficit reduced the development of aphids on both genotypes, which is in agreement with the general prediction for gall-inducing insects. Nonetheless, while the intermediate water deficit did not affect the development of aphids on the susceptible genotype, it enhanced their development on the resistant one. This latter observation contradicts the prediction for gall-inducing insects, but is in agreement with the plant stress hypothesis for sucking insects. The size of the pseudogall induced by the aphids in the bark of their host progressively diminished when water deficit increased. Lignification, believed to be an important component of poplar resistance to P. passerinii, also progressively decreased with water deficit. The involvement of physiological and architecture modifications due to water deficit on the development of aphids is discussed. The results demonstrate that host genotype can modulate water deficit effects on the development of a galling aphid, thereby compromising predictions on the outcome of droughts on stand resistance to plant-manipulating herbivores.

Bark beetles attacking conifer seedlings: picking on the weakest or feasting upon the fittest?

Summary Plant stress from causes such as drought is thought to increase the susceptibility of plants to herbivore attack and damage. Although this forms the basis of the widely cited plant stress hypothesis, there are few cases where this has been tested thoroughly. We compared the frequency of attack and damage of an invasive pine bark beetle Hylates ater, on seedlings of Pinus radiata, using a 2 × 2 factorial design with artificially induced stress and insecticide application to distinguish effects of beetle attack and direct effects of stress. Contrary to expectations based on the plant stress hypothesis, twice as many unstressed seedlings (22%) were attacked than stressed seedlings (11%). However, stressed seedlings were twice as likely to experience sustained bark beetle feeding resulting in girdling, than unstressed seedlings (31% and 15%, respectively). In response to beetle attack, unstressed seedlings showed much stronger resin production than stressed seedlings, which is likely to explain the greater survival of unstressed seedlings after attack. Experimental drought stress caused a 50% reduction in shoot growth. Synthesis and applications. Maximizing seedling vigour and minimizing (pre‐)planting stress are key for the resilience of planted forests. Managers should emphasize a stress‐free transition of seedlings from nursery to field as well as correct planting techniques, in order to safeguard this vulnerable stage. This is particularly crucial since drought stress and insect invasions are likely to increase as a result of global change.

Can we use plant traits and soil characteristics to predict leaf damage in savanna woody species?

Although several hypotheses aim to explain insect herbivory on plants, the relative importance of plant traits, environment, and organizational scale (i.e., individual or community) to herbivory damage level is not well understood. We used an approach based on a local scale, divided into individual and community levels, to test if plant traits, soil characteristics, and plant density explain leaf damage. We sampled 983 individuals in 49 plots distributed over dense and open savanna formations in Emas National Park. In order to explain plant damage, we performed a multi-model inference analysis of four plant traits associated with plant damage, five soil characteristics, and plant density. We did not find any support to plant vigor or plant stress hypotheses at individual plant level. However, the resource concentration hypothesis and plant stress hypothesis explained leaf damage at the community level. We found that increased availability of calcium (Ca) in soils reduced plant damage at the community level. Because soil Ca concentration is a major constraint to plant development in the Brazilian savanna, we postulated that its increasing availability permits plants to invest more in defense strategies. We demonstrate that plant density, Ca soil concentration, and leaf size can be used to predict the plant damage suffered by woody species in savannas at community level.

Population fluctuations of moths and small rodents in relation to plant reproduction indices in southern Norway

Trophic interaction hypotheses for multiannual population fluctuations of herbivores include the predation hypothesis, the induction hypothesis and the plant stress hypothesis. Spatial synchrony is best explained by plant stress, such as high reproduction, because weather may synchronize resource allocation in plants over large areas. We used a 29‐yr time series on moth light trapping (6 selected species) and small rodent snap trapping (2 selected species) from southern Norway to test whether herbivore population fluctuations can be related to sexual reproduction of their host plants. The fluctuation pattern of the moths Parashwammerdamia lutarea and Yponomeuta padella, feeding on leaves of rowan (Sorbus aucuparia), could be explained by fluctuations in an annual index of rowanberry production, whereas the fluctuation pattern of Yponomeuta evonymella, feeding on leaves of bird cherry (Prunus padus), could not. The moth Eulithis populata, feeding on bilberry (Vaccinium myrtillus) leaves, fluctuated in synchrony with the bank vole (Myodes clethrionomys), which feeds on bilberry twigs in winter. E. populata and the bank vole also fluctuated in synchrony with a shorter index on bilberry seed crops. Finally, two moths feeding on mosses, Eudonia truncicolella and Catoptria falsella, fluctuated in synchrony with the moss‐feeding wood lemming (Myopus schisticolor). The observed patterns could be caused by a positive effect of the supply of highly nutritious reproductive plant tissue, but the one‐year delayed response by the rodents, which to a large extent feed on the plant species in question during winter, suggests that feeding deterrents are involved.

Testing the plant stress hypothesis: stressed plants offer better food to an insect herbivore

AbstractTemperature and nutrition are among the most important environmental factors affecting ectotherm growth. As temperature and host‐plant quality often co‐vary in nature, the interaction between the two is of potentially high ecological importance for herbivorous insects. We here use the temperate‐zone butterfly Pieris napi L. (Lepidoptera: Pieridae) to investigate interactive effects of larval rearing temperature and host‐plant quality (by manipulating water availability) on larval growth. As growth rates have been hypothesized to govern stress tolerance, we additionally assessed adult starvation resistance. Butterflies followed the ‘temperature‐size rule’, which states that body size increases at lower developmental temperatures, proximately caused by differences in growth increment, which resulted from increased consumption at the lower temperature. Larvae benefitted from feeding on stressed plants from the low‐water regime by having higher body mass, growth rate, and food conversion efficiency, thus supporting the plant stress hypothesis, which predicts that plant quality for herbivores should increase if stress is imposed on plants. Some effects of host‐plant quality on larval growth parameters were as strong as or even stronger than effects of temperature, whereas interactive effects between temperature and food quality were scarce. At the low temperature, adult starvation resistance was higher than at the higher temperature and females were more resistant than males, whereas plant water regime had no clear impact. No evidence was found for a trade‐off between growth rate and starvation resistance. This study illustrates the importance of considering effects of host‐plant quality along with variation in other environmental factors for estimating the impact of environmental changes on herbivorous species.

The effects of drought and herbivory on plant–herbivore interactions across 16 soybean genotypes in a field experiment

Abstract As the Earth's climate continues to change, drought and insect population outbreaks are predicted to increase in many parts of the world. It is therefore important to understand how changes in such abiotic and biotic stressors might impact agroecosystems. The plant stress hypothesis predicts that, owing to physiological and biochemical changes, plants experiencing drought will be more susceptible to insect herbivory, which could have synergistic negative effects on plant performance. By contrast, the plant vigor hypothesis predicts that insects will preferentially feed on fast‐growing vigorous plants. These hypotheses were tested in a field experiment using 16 soybean (Glycine max (L.) Merr.) genotypes to determine: (i) the combined effects of drought and herbivory on plant performance; (ii) the impact of drought on soybean resistance to herbivores; and (iii) how genetically variable phenotypic traits in soybean correlate with these responses. It was found that drought had a greater effect on soybean performance than herbivory, and drought and herbivory did not interact to impact on any measure of plant performance. Drought caused decreased insect herbivory on average, suggesting that the plant vigor hypothesis is consistent with the effects of drought stress on soybean resistance to leaf‐chewing insect herbivores. This conclusion is further supported by genotypic correlations which show that plant growth rate is positively correlated with the amount of herbivory plants received. These results suggest that, although the effects of climate‐associated changes in drought and herbivory will have negative effects on soybean, these potential effects are quantifiable with simple experiments and can be mitigated through continued breeding of varieties that are tolerant and resistant to these abiotic and biotic stressors.

Fluctuating asymmetry of and herbivory on Poincianella pyramidalis (Tul.) L.P. Queiroz (Fabaceae) in pasture and secondary tropical dry forest

Plants subjected to stressful environments tend to be more asymmetric with reduced defenses and are therefore more vulnerable to herbivory. This study investigates the relationship between herbivory and fluctuating asymmetry (FA) in Poincianella pyramidalis in two contrasting habitat types in the Brazilian caatinga (shrublands). We tested the following hypotheses: that FA occurs in P. pyramidalis; that FA of P. pyramidalis leaves is greater in individuals located in pasture than in those located in secondary tropical dry forest; that herbivory by insects (leaf chewers and leaf miners) increases in parallel with increases in the level of FA; and that herbivory is more common in pasture than in secondary tropical dry forest. In each of the two environments, we sampled 20 plants and evaluated 400 leaflets. We submitted FA data to the Shapiro-Wilk test of normality, and we investigated the other variables using generalized linear models. We found that FA was present in all P. pyramidalis individuals evaluated but was greater in those located in the more degraded habitat (pasture). In addition, although herbivory was similar between the two habitats, there was positive relationship between FA and herbivory. This indicates that herbivores select plants that are more asymmetric, regardless of the type of habitat involved, which might be attributable to the mechanisms posited in the plant stress hypothesis.

Aphids in a changing world: testing the plant stress, plant vigour and pulsed stress hypotheses

The plant stress, plant vigour and pulsed stress hypotheses describe the relationships between drought stress, plant quality and herbivore performance. We used an aphid‐Brassica system to test these hypotheses under different drought treatments. The quantity of water added per plant/week was 75%, 50% and 25% of the control (unstressed) water regime for low, medium and high drought stress, respectively, and 50% applied fortnightly for pulsed drought stress. The performance of a ‘senescence’ (generalist) and a ‘flush’ feeder (specialist) aphid species and host plant quality were assessed. Drought treatments had a similar effect on the fecundity and intrinsic rate of increase of both aphid species. Aphid performance on unstressed and highly drought‐stressed plants was significantly lower compared with medium drought stress. On average, 20% greater fecundity and 40% greater intrinsic rates of increase were recorded for both aphid species at medium drought stress compared with unstressed plants. Plant biomass and relative water contents were significantly greater for unstressed plants compared with high and pulsed drought treatments. Foliar nitrogen concentration was significantly greater in the high drought stress and pulsed treatments, and the dominant glucosinolate (glucobrassicin) concentration was significantly greater in drought stress treatments. The present study supports the plant stress hypothesis, although the plant vigour and pulsed stress hypotheses are not supported by our data. The implications of these findings for plant–herbivore interactions under changing environmental conditions are discussed.

Moose recruitment in relation to bilberry production and bank vole numbers along a summer temperature gradient in Norway

The plant stress hypothesis states that plant stress factors other than herbivory improve herbivore performance due to changes in the content of nutritive or defensive compounds in the plants. In Norway, the bilberry (Vaccinium myrtillus) is important forage for the bank vole (Myodes glareolus) in winter and for the moose (Alces alces) in summer and autumn. The observed peaks in bank vole numbers after years with high production of bilberries are suggested to be caused by increased winter survival of bank voles due to improved forage quality. High production of bilberries should also lead to higher recruitment rates in moose in the following year. We predict, however, that there is an increasing tendency for a 1-year delay of moose indices relative to vole indices with decreasing summer temperatures, because low temperatures prolong the period needed by plants to recover in the vole peak year, and thus positively affect moose reproduction also in the succeeding year. In eight out of nine counties in south-eastern Norway, there was a positive relationship between the number of calves observed per female moose during hunting and a bilberry seed production index or an autumn bank vole population index. When dividing the study area into regions, there was a negative relationship between a moose-vole time-lag index and the mean summer temperature of the region. These patterns suggest that annual fluctuations in the production of bilberries affect forage quality, but that the effect on moose reproduction also depends on summer temperatures.