Oviposition site selection on oats: the effect of plant architecture,plant and leaf age,tissue toughness,and hardness on cereal leaf beetle,Oulema melanopus

In sequentially planted oat stands, the cereal leaf beetle (CLB), Oulema melanopus (L.) (Coleoptera: Chrysomelidae: Lemini), is found in greater numbers, and lays more eggs, on later planted (younger) oats (Avena sativa L.) (Poaceae). Plant characteristics that could explain this ovipositional preference were examined in a series of experiments. Cage and open field whole plant preference tests confirmed the attraction of ovipositing females to younger oats. A cage effect illustrated the role of plant architecture (plant height) in CLB host selection. Two multiple‐choice and one no‐choice excised leaf experiments determined that characteristics of individual leaves associated with leaf insertion level (leaf number from base to apex) and age influence ovipositional site selection. Leaves of higher insertion level have higher nitrogen content, but fewer eggs are laid on those leaves. Two experiments examining the interaction between total leaf nitrogen and leaf insertion level showed that only leaf insertion level affected oviposition choice. Published literature suggests variation in secondary plant compounds cannot explain O. melanopus ovipositional preference among leaves. Grass leaves of higher insertion level have more extensively developed cells associated with tissue toughness and hardness. The data and supporting literature suggest tissue toughness and hardness are deterring oviposition on oat leaves of higher insertion level. However, newly eclosed larvae are able to feed on leaves usually avoided as oviposition sites. The explanation for this result may be a lack of correlation between host suitability and ovipositional preference.     

Palatability, decomposition and insect herbivory: patterns in a successional old-field plant community

We tested the hypothesis that selective feeding by insect herbivores in an old‐field plant community induces a shift of community structure towards less palatable plant species with lower leaf and litter tissue quality and may therefore affect nutrient cycling. Leaf palatability of 20 herbaceous plant species which are common during the early successional stages of an old‐field plant community was assayed using the generalist herbivores Deroceras reticulatum (Mollusca: Agriolomacidae) and Acheta domesticus (Ensifera: Gryllidae). Palatability was positively correlated with nitrogen content, specific leaf area and water content of leaves and negatively correlated with leaf carbon content and leaf C/N‐ratio. Specific decomposition rates were assessed in a litter bag experiment. Decomposition was positively correlated with nitrogen content of litter, specific leaf area and water content of living leaves and negatively correlated with leaf C/N‐ratio. When using phylogentically independent contrasts the correlations between palatability and decomposition versus leaf and litter traits remained significant (except for specific leaf area) and may therefore reflect functional relationships. As palatability and decomposition show similar correlations to leaf and litter traits, the correlation between leaf palatability and litter decomposition rate was also significant, and this held even in a phylogenetically controlled analysis. This correlation highlights the possible effects of invertebrate herbivory on resource dynamics. In a two‐year experiment we reduced the density of above‐ground and below‐ground insect herbivores in an early successional old‐field community in a two‐factorial design by insecticide application. The palatability ranking of plants showed no relationship with the specific change of cover abundance of plants due to the reduction of above‐ or below‐ground herbivory. Thus, changes in the dominance structure as well as potentially associated changes in the resource dynamics are not the result of differences in palatability between plant species. This highlights fundamental differences between the effects of insect herbivory on ecosystems and published results from vertebrate‐grazing systems.     

Effects of plant diversity and structural complexity on parasitoid behaviour in a field experiment

1. In natural ecosystems, plants containing hosts for parasitoids are often embedded within heterogeneous plant communities. These plant communities surrounding host‐infested plants may influence the host‐finding ability of parasitoids. 2. A release‐recapture‐approach was used to examine whether the diversity and structural complexity of the community surrounding a host‐infested plant influences the aggregation behaviour of the leaf‐miner parasitoid Dacnusa sibirica Telenga and naturally occurring local leaf‐miner parasitoids. Released and locally present parasitoids were collected on potted Jacobaea vulgaris Gaertn.plants infested with the generalist leaf‐miner Chromatomyia syngenesiae Hardy. The plants were placed in experimentally established plant communities differing in plant diversity (1–9 species) and habitat complexity (bare ground, mown vegetation, and tall vegetation). Additionally, parasitoids were reared out from host mines on the trap plants. 3. Plant diversity did not influence the mean number of recaptured D. sibirica or captures of other locally present parasitoids but the number of recaptured parasitoids was influenced by habitat complexity. No D. sibirica parasitoids were recaptured in the bare ground plots or plots with mown vegetation. The mean number of recaptured D. sibirica generally increased with increasing complexity of the plant community, whereas locally present parasitoids were captured more frequently in communities with more bare ground. There was a unimodal relationship between the number of reared out parasitoids and diversity of the surrounding vegetation with the highest density of emerged parasitoids at intermediate diversity levels. 4. The present study adds to the thus far limited body of literature examining the aggregation behaviour of parasitoids in the field and suggests that the preference of parasitoids to aggregate in complex versus simple vegetation is association specific and thus depends on the parasitoid species as well as the identity of the plant community.     

Effects of tomato spotted wilt virus (TSWV) isolates, host plants, and temperature on survival, size, and development time of Frankliniella fusca

The effects of different isolates of the tomato spotted wilt tospovirus (TSWV), host plants, and temperatures on Frankliniella fusca (Hinds) (Thysanoptera: Thripidae), the most important vector of TSWV in North Carolina, were measured in the laboratory. Thrips were reared at either 18.3, 23.9, or 29.4 °C until adult eclosion on excised leaves of Datura stramonium L. or Emilia sonchifolia (L.). Plants were either infected with the TSWV isolates CFL or RG2, or left uninfected (control). The results revealed a positive relationship between larval survival and temperature, regardless of host plant or TSWV isolate. Both survival to adult and percentage transmission of TSWV by F. fusca were significantly affected by the interaction between host plant and TSWV isolate. The consequence of this interaction was that the cohort‐based percentage transmission from infected E. sonchifolia plants for CFL was 1.3‐fold greater than that of RG2, whereas the percentage transmission from infected D. stramonium plants for RG2 was twice that of CFL. Both host plant and TSWV isolates showed significant effects on thrips development time to adult and head capsule width of adult thrips, as well as on the incidence of thrips infection with TSWV. The infection status of these thrips was determined by ELISA for the NSs viral protein. Infected thrips reared on infected host foliage took longer to develop to adult and were smaller than non‐infected thrips which had also been reared on infected host foliage, demonstrating a direct effect of the TSWV on thrips. However, non‐infected thrips reared on non‐infected leaves took longer to develop than non‐infected thrips reared on infected leaves, suggesting an effect of the plant tissue on thrips. In addition, adult thrips reared on TSWV‐infected D. stramonium at 29.4 °C developed smaller head capsules than thrips developing on infected foliage at lower temperatures and on non‐infected leaves of D. stramonium or E. sonchifolia. Both TSWV isolates and host plants differentially affected females more than males. In conclusion, both the infection of thrips by TSWV and TSWV‐mediated changes in host plant quality were found to have significant biological effects on F. fusca.     

The nocturnal larvae of a specialist folivore perform better on Chromolaena odorata leaves from a shaded environment

Increasing evidence suggests that individuals of the same plant species occurring in different microhabitats often show a degree of phenotypic and phytochemical variation. Consequently, insect herbivores associated with such plant species must deal with environment‐mediated changes or variability in the traits of their host plants. In this study, we examined the effects of habitat condition (shaded vs. full‐sun habitats) on plant traits and leaf characteristics of the invasive alien plant, Chromolaena odorata (L.) King & Robinson (Asteraceae). In addition, the performance was evaluated in two generations of a specialist folivore, Pareuchaetes insulata (Walker) (Lepidoptera: Erebidae: Arctiinae), on leaves obtained from both shaded and full‐sun habitats. The study was done in an area where the insect was introduced as a biological control agent. Leaves growing in shade were less tough, had higher water and nitrogen content, and lower total non‐structural carbohydrate, compared with leaves growing in full sun. Plants growing in shade had longer leaves and were taller, but above‐ground biomass was significantly reduced compared with plants growing in full sun. In both generations (parents and offspring), P. insulata developed faster and had larger pupal mass, increased growth rate, and higher fecundity when reared on shaded foliage compared with full‐sun foliage. Although immature survival and adult longevity did not differ between habitats, Maw's host suitability index indicated that shaded leaves were more suitable for the growth and reproduction of P. insulata. We suggest that the benefits obtained by P. insulata feeding on shaded foliage are associated with reduced toughness and enhanced nitrogen and water content of leaves. These results demonstrate that light‐mediated changes in plant traits and leaf characteristics can affect insect folivore performance.     

The interaction between host and host plant influences the oviposition and performance of a generalist ectoparasitoid

The successful development of parasitoids of herbivores depends on the quality of their host, which is often affected by the host plant. Therefore, a parasitoid’s oviposition decisions will directly depend on the host, but also on plant quality. Here, we investigated the direct effects of host species and the indirect effects of the host’s food plant on the oviposition decisions and performance of the gregarious ectoparasitoid Euplectrus platyhypenae Howard (Hymenoptera: Eulophidae). With a series of no‐choice experiments, we determined the oviposition and performance of the parasitoid on: (1) two caterpillar species, fall armyworm, Spodoptera frugiperda JE Smith (Lepidoptera: Noctuidae), and velvet armyworm, Spodoptera latifascia Walker, reared on maize (Zea mays L., Poaceae), (2) the same caterpillars reared on maize, bean (Phaseolus vulgaris L., Fabaceae), or squash (Cucurbita pepo L., Cucurbitaceae) leaves, and (3) S. latifascia caterpillars reared on leaves of wild and cultivated lima bean, Phaseolus lunatus L. All these insects and plants originate from Mesoamerica where they have coexisted for thousands of years in the traditional agricultural system known as Milpa in which maize, beans, and squash are planted together. We found that the preferred and best combination of host and host plant for parasitoid performance was S. frugiperda on maize. Parasitoids laid larger clutches, had higher survival, and more females and larger adults emerged from S. frugiperda reared on maize. However, when both caterpillar species were reared on squash, S. latifascia was the better host. Contrary to the literature, S. frugiperda was not able to develop on bean plants. Results from the lima bean experiment showed that parasitoid performance was best when S. latifascia was reared on leaves of cultivated compared to wild lima bean. These findings are discussed in the context of mixed cropping in which the ability of generalist parasitoids to switch among hosts and host plant species could be advantageous for pest management.     

Time of the season: the effect of host photoperiodism on diapause induction in an insect herbivore,Leptinotarsa decemlineata

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Responses of insect herbivores to sharing a host plant with a hemiparasite: impacts on preference and performance differ with feeding guild

1. Root hemiparasites are common components of many ecosystems and can affect both the biomass and the nutritional quality of the plants they infect. The consequences of these modifications for the preference and performance of three herbivore feeding guilds sharing a host with the hemi‐parasite were examined. 2. It was predicted that as the hemiparasite increased in biomass its impact on the host would increase, as would the indirect impacts on the herbivores. It was also predicted that herbivores from different feeding guilds would respond differently to the presence of the hemiparasite, reflecting the extent to which they utilise resources disrupted by the parasite and hence are in competition with it. 3. The preference and performance of phloem‐feeding aphids, xylem‐feeding spittle bugs, and leaf‐feeding grasshoppers were measured on the host grass species, Holcus lanatus L. (Poaceae), with and without attachment from the hemi‐parasite, Rhinanthus minor L. (Orobanchaceae). 4. The effects of R. minor on the host were dependent on the hemiparasite's stage of growth, being most pronounced when it was at peak biomass. At this stage it caused a significant reduction in the biomass, water content, and total nitrogen content of the host plants. 5. Overall, herbivores benefited from, or preferred, shared host plants more than uninfected plants. The aphid benefited from sharing a host with R. minor, showing increased population growth on, and preference for, parasitised plants. The spittle bug also showed a preference for parasitised plants. The grasshopper, Chorthippus brunneus Thunberg (Orthoptera: Acrididae), did not show a preference for, or a performance response to, parasitised hosts, but it consumed significantly more plant material when caged on parasitised plants. 6. These data support the prediction that invertebrate herbivores responded to changes in host plant traits driven by the hemiparasite, and strongly suggest that these indirect interactions could impact on population and community processes within natural communities.     

Host‐plant quality adaptively affects the diapause threshold: evidence from leaf beetles in willow plantations

1. Voltinism of herbivorous insects can vary depending on environmental conditions. The leaf beetle Phratora vulgatissima L. is univoltine in Sweden but will sometimes initiate a second generation in short‐rotation coppice (SRC) willow plantations. 2. The study investigated whether increased voltinism by P. vulgatissima in plantations can be explained by (i) rapid life‐cycle development allowing two generations, or (ii) postponed diapause induction on coppiced willows. 3. In the field, no difference was found in the phenology or development of first‐generation broods between plantations (S. viminalis) and natural willow habitats (S. cinerea). However, the induction of diapause occurred 1–2 weeks later in SRC willow plantations. 4. Laboratory experiments indicated no genetic difference in the critical day‐length for diapause induction between beetles originating from plantations and natural habitats. Development time was unaffected by host‐plant quality but critical day‐length was prolonged by almost an hour when the beetles were reared on a non‐preferred willow species (S. phylicifolia). When reared on new leaves from re‐sprouting shoots of recently coppiced willow plants, diapause incidence was significantly less than when the beetles were reared on mature leaves from uncoppiced plants. 5. The study suggests that P. vulgatissima has a plastic diapause threshold influenced by host‐plant quality. The use of host‐plant quality as a diapause‐inducing stimulus is likely to be adaptive in cases where food resources are unpredictable, such as when new host‐plant tissue is produced after a disturbance. SRC willows may allow two beetle generations due to longer growing seasons of coppiced plants that grow vigorously.     

Nitrogen homeostasis in a willow leaf beetle, Plagiodera versicolora, is independent of host plant quality

While foliar nitrogen (N) content of host plants depends on environmental conditions, N content of herbivorous insects may remain relatively constant due to homeostasis. However, it is unknown to what extent insects can maintain their body elemental composition against natural variation in host plant quality. The present study examined the performance and N content of a willow leaf beetle, Plagiodera versicolora Laicharting (Coleoptera: Chrysomelidae), when fed leaves of host willow, Salix eriocarpa Franchet et Savatier (Salicaceae), with varying nutritional status. Water content, toughness, and N content of willow leaves varied seasonally, and they affected performance of the leaf beetle. The leaf beetle achieved high performance when fed young leaves. On the other hand, the N content of the leaf beetle changed little, and it was independent of that of willow leaves, indicating strong N homeostasis of the leaf beetle. We discussed the function of N homeostasis in herbivorous insects in tritrophic level interactions.     

Effect of leaf nitrogen content of tomato plants on preference and performance of a leafmining fly

Summary In a study on intraspecific host plant acceptability, Liriomyza trifolii females that had previously been exposed to plants of high nitrogen content, showed a feeding and oviposition preference for plants of high nitrogen (Minkenberg and Fredrix 1989). Females showed a preference to feed and oviposit on the high middle leaves within plants. It was hypothesized that the preference between plants was related to a better performance of females and offspring on high nitrogen plants compared to low nitrogen plants. Different nitrogen dosages were applied to tomato plants, resulting in plants containing 3.4, 3.9, 4.6 or 4.9% leaf nitrogen. L. trifolii females responded to increased leaf nitrogen with significantly increased feeding and fecundity, longer oviposition periods, and higher feeding and oviposition rates. Their offspring on the same plants showed reduced developmental time, lower mortality and increased pupal size. Consequently, intrinsic rate of increase was positively linearly related to leaf nitrogen. Size of L. trifolii females appeared to be independent of fecundity, longevity and developmental time. Pupal length of males increased with increasing developmental time. These results indicate that L. trifolii is well adapted in its intraspecific host plant selection, because the ability to distinguish between plants with differences in leaf nitrogen content will directly lead to an increase in their fecundity, longevity and overall fitness. The implications of leaf nitrogen as a significant factor in the behavior and population dynamics of L. trifolii are discussed.     

Freezing avoidance by supercooling in Olea europaea cultivars: the role of apoplastic water,solute content and cell wall rigidity

Plants can avoid freezing damage by preventing extracellular ice formation below the equilibrium freezing temperature (supercooling). We used Olea europaea cultivars to assess which traits contribute to avoid ice nucleation at sub‐zero temperatures. Seasonal leaf water relations, non‐structural carbohydrates, nitrogen and tissue damage and ice nucleation temperatures in different plant parts were determined in five cultivars growing in the Patagonian cold desert. Ice seeding in roots occurred at higher temperatures than in stems and leaves. Leaves of cold acclimated cultivars supercooled down to ?13 °C, substantially lower than the minimum air temperatures observed in the study site. During winter, leaf ice nucleation and leaf freezing damage (LT₅₀) occurred at similar temperatures, typical of plant tissues that supercool. Higher leaf density and cell wall rigidity were observed during winter, consistent with a substantial acclimation to sub‐zero temperatures. Larger supercooling capacity and lower LT₅₀ were observed in cold‐acclimated cultivars with higher osmotically active solute content, higher tissue elastic adjustments and lower apoplastic water. Irreversible leaf damage was only observed in laboratory experiments at very low temperatures, but not in the field. A comparative analysis of closely related plants avoids phylogenetic independence bias in a comparative study of adaptations to survive low temperatures.     

Inbreeding in horsenettle influences herbivore resistance

Abstract 1. Plant traits (e.g. nutrition, allelochemistry) are an important determinant of the feeding preferences and performance of insect herbivores. Recent evidence suggests that plant inbreeding can affect plant–insect interactions by impacting host‐plant quality and resistance to herbivory. 2. The effect of inbreeding on host‐plant quality for, and resistance against, the tobacco hornworm, Manduca sexta L., was assessed in the wild solanaceous weed horsenettle, Solanum carolinense L. Caterpillar preference, relative growth rate (RGR), total leaf consumption (TC), and per cent total nitrogen in leaves were examined using selfed and outcrossed progeny of eight maternal plants. 3. Inbreeding significantly influenced insect preference, with caterpillars preferring leaf discs from selfed versus outcrossed plants. There was also a breeding effect for RGR and TC, with both higher on selfed plants. No breeding effect for per cent total nitrogen was observed. 4. The results of this study indicate that inbreeding decreased resistance against the tobacco hornworm, but did not affect plant quality. Decreased plant resistance will likely alter interactions with the herbivore community and could also have important consequences for plant–herbivore–natural enemy interactions.     

Impact of plant architecture versus leaf quality on attack by leaf-tying caterpillars on five oak species

Because shelter-building herbivorous insect species often consider structural features of their host plants in selecting construction sites, their probability of attack is likely to be a function of some combination of plant architectural traits and leaf quality factors. We tested the hypothesis that plant architecture, in the form of the number of touching leaves, influences interspecific variation in attack by leaf-tying caterpillars in five species of sympatric Missouri oaks (Quercus). We compared colonization on control branches, in which both architecture and leaf quality were potentially important, with colonization on experimental branches for which we controlled for the effects of architecture by creating equal numbers of artificial ties. Colonization of artificial ties was highly correlated with natural colonization on neighboring control branches, suggesting that leaf quality factors and not architecture influenced interspecific variation in attack by leaf-tying caterpillars. Of the leaf quality factors measured (water, protein-binding capacity, nitrogen, specific leaf area, pubescence, and toughness), nitrogen was the most explanatory. With the exception of white oak, natural leaf tie colonization was positively correlated with nitrogen availability (ratio of nitrogen to protein-binding capacity), and negatively correlated with protein-binding capacity of leaf extracts. Both host plant species and subgenus oak influenced the community composition of leaf-tying caterpillars and the non-tying symbionts colonizing the ties. Host plant differences in leaf nitrogen content were positively correlated with pupal weight of one of two caterpillar species reared on all five host plant species. Thus, interspecific differences in nitrogen, nitrogen availability, and protein-binding capacity of leaf extracts are the best predictors at this time of interspecific differences in attack by leaf-tying caterpillars, in turn affecting their success on individual host plants in the laboratory.     

Companion planting – do aromatic plants disrupt host‐plant finding by the cabbage root fly and the onion fly more effectively than non‐aromatic plants?

Brassica and Allium host‐plants were each surrounded by four non‐host plants to determine how background plants affected host‐plant finding by the cabbage root fly (Delia radicum L.) and the onion fly [Delia antiqua (Meig.)] (Diptera: Anthomyiidae), respectively. The 24 non‐host plants tested in field‐cage experiments included garden ‘bedding’ plants, weeds, aromatic plants, companion plants, and one vegetable plant. Of the 20 non‐host plants that disrupted host‐plant finding by the cabbage root fly, fewest eggs (18% of check total) were laid on host plants surrounded by the weed Chenopodium album L., and most (64% of check total) on those surrounded by the weed Fumaria officinalis L. Of the 15 plants that disrupted host‐plant finding in the preliminary tests involving the onion fly, the most disruptive (8% of check total) was a green‐leaved variant of the bedding plant Pelargonium × hortorum L.H. Bail and the least disruptive (57% of check total) was the aromatic plant Mentha piperita × citrata (Ehrh.) Briq. Plant cultivars of Dahlia variabilis (Willd.) Desf. and Pelargonium×hortorum, selected for their reddish foliage, were less disruptive than comparable cultivars with green foliage. The only surrounding plants that did not disrupt oviposition by the cabbage root fly were the low‐growing scrambling plant Sallopia convolvulus L., the grey‐foliage plant Cineraria maritima L., and two plants, Lobularia maritima (L.) Desv. and Lobelia erinus L. which, from their profuse covering of small flowers, appeared to be white and blue, respectively. The leaf on which the fly landed had a considerable effect on subsequent behaviour. Flies that landed on a host plant searched the leaf surface in an excited manner, whereas those that landed on a non‐host plant remained more or less motionless. Before taking off again, the flies stayed 2–5 times as long on the leaf of a non‐host plant as on the leaf of a host plant. Host‐plant finding was affected by the size (weight, leaf area, height) of the surrounding non‐host plants. ‘Companion plants’ and aromatic plants were no more disruptive to either species of fly than the other plants tested. Disruption by all plants resulted from their green leaves, and not from their odours and/or tastes.     

Interacting effects of elevated CO2, nutrient availability and plant species on a generalist invertebrate herbivore

By affecting plant growth and phytochemistry elevated CO₂ may have indirect effects on the performance of herbivores. These effects show considerable variability across studies and may depend on nutrient availability, the carbon/nutrient‐balance in plant tissues and the secondary metabolism of plants. We studied the responses to elevated CO₂ and different nutrient availability of 12 herbaceous plant species differing in their investment into secondary compounds. Caterpillars of the generalist herbivore Spodoptera littoralis were reared on the leaves produced and their consumption and growth rates analysed. Elevated CO₂ resulted in a similar increase of biomass in all plant species, whereas the positive effect of fertilization varied among plant species. Specific leaf weight was influenced by elevated CO₂, but the effect depended on nutrient level and identity of plant species. Elevated CO₂ increased the C/N ratio of the leaves of most species. Caterpillars consumed more leaf material when plants were grown under elevated CO₂ and low nutrients. This indicates compensatory feeding due to lower tissue quality. However, the effects of elevated CO₂, nutrient availability and plant species identity on leaf consumption interacted. Both the effects of CO₂ and nutrient availability on the relative growth rate of the herbivore depended on the plant species. The feeding rate of S. littoralis on plant species that do not produce nitrogen‐containing secondary compounds (NCSC) was higher under low nutrient availability. In contrast, in plants producing NCSC nutrient availability had no effect on the feeding rate. This suggests that compensatory feeding in response to low nutrient contents may not be possible if plants produce NCSC. We conclude that elevated CO₂ causes species‐specific changes in the quality of plant tissues and consequently in changes in the preferences of herbivores for plant species. This could result in changes in plant community composition.     

Evaluation of induced responses, insect population growth, and host-plant fitness may change the outcome of tests of the preference-performance hypothesis: a case study

The preference‐performance hypothesis predicts that insect preference should correspond to host suitability for offspring development. We studied the pattern of within‐plant preference in the aphid Sipha flava and its consequences for offspring performance on the host‐plant Sorghum halepense, regarding the role of induced responses of plants to aphid feeding. The consequences of within‐plant preference on aphid population growth and host‐plant traits were also evaluated. Our results showed that winged and wingless aphids preferred to settle on mature rather than young leaves. In contrast, aphid individual growth rate was higher on young leaves when compared with mature leaves, suggesting that the outcome of this test rejected the preference‐performance hypothesis. However, the inclusion of the factor ‘previous aphid infestation’ changed the outcome from a maladaptive choice to a neutral one. Thus, individual growth rates of S. flava increased when aphids developed on leaves that had been previously infested. Interestingly, aphid growth rate on previously infested leaves did not differ between young and mature leaves. On the other hand, aphid population reproductive rate was higher and the percentage of winged aphids lower when infestation occurred on mature rather than young leaves. Aphid infestation reduced plant and shoot biomass, and increased leaf mortality. These negative effects on plant traits related to plant fitness were greater when aphid infestation occurred on young leaves. Likewise, whereas infestation on mature leaves did not cause a significant reduction in the number of flowering plants compared with control plants, aphid infestation on young leaves did reduce the number of plants at the flowering stage. Consequently, if both the reproductive rate of aphids in the mid‐term, and host‐plant fitness are taken into account, the results indicate that aphid preference for mature leaves may be an adaptive choice, thus supporting the preference‐performance hypothesis.     

Differential effects of host plant hybridization on herbivore community structure and grazing pressure on forest canopies

Interspecific hybridization in plants is known to have ecological effects on associated organisms. We examined the differences in insect herbivore community structure and grazing pressure on tree canopy leaves among natural hybrids and their parental oak species. We measured leaf traits, herbivore community structure, and grazing pressure on leaves of two oak species, Quercus crispula and Q. dentata, and their hybrids. The concentration of nitrogen in canopy leaves was greater in hybrids and in Q. dentata than in Q. crispula. The concentration of total phenolics was lower in hybrids than in Q. crispula. The concentration of condensed tannin was greater in hybrids than in Q. crispula. Relative herbivore abundance and species richness were greater on oak hybrids than on either parental species; herbivore species diversity and composition on hybrids were close to those on Q. crispula. Herbivore grazing pressure was lower on hybrids and Q. dentata than on Q. crispula. There was a negative correlation between herbivore grazing pressure and leaf nitrogen, suggesting that interspecific variation among oak taxa in herbivore pressure may be explained by leaf nitrogen; variation in herbivore community structure among oak taxa is likely to be controlled by polygenic leaf traits. Differing responses of (1) herbivore community structure and (2) herbivore grazing pressure to host plant hybridization may play important roles in regulating herbivore biodiversity in cool‐temperate forest canopies.     

How do extreme drought and plant community composition affect host plant metabolites and herbivore performance?

Water availability and plant community composition alter plant nutrient availability and the accumulation of plant defence compounds therefore having an impact on herbivore performance. Combined effects of drought stress and plant community composition on leaf chemicals and herbivore performance are largely unexplored. The objective of our study was, therefore, to find out the impact of extreme drought and of plant community composition on plant–herbivore interactions. Larvae of the generalist butterfly Spodoptera littoralis were reared on leaves of the grass Holcus lanatus which was grown in experimental communities, differing in species- and functional group richness. These communities were either subjected to extreme drought or remained under ambient climatic conditions. Drought decreased relative water content, soluble protein content, nitrogen and total phenol content and increased the content of carbohydrates in the grass. As a consequence, the larvae feeding on drought-exposed plants revealed a longer larval stage, increased pupal weight and higher adult eclosion rates. Plant community composition mainly caused changes to the defensive compounds of the grass, but also marginally affected protein and carbohydrate content. Larvae feeding on species-richest communities without legumes showed the highest mortality. Our findings imply that climate change that is projected to increase the frequency of severe droughts, as well as alter plant community compositions, is likely to affect arthropod–plant interactions through an alteration of leaf chemicals.     

Increased cytokinin levels in transgenic PSAG12–IPT tobacco plants have large direct and indirect effects on leaf senescence, photosynthesis and N partitioning

We studied the impact of delayed leaf senescence on the functioning of plants growing under conditions of nitrogen remobilization. Interactions between cytokinin metabolism, Rubisco and protein levels, photosynthesis and plant nitrogen partitioning were studied in transgenic tobacco (Nicotiana tabacum L.) plants showing delayed leaf senescence through a novel type of enhanced cytokinin syn‐thesis, i.e. targeted to senescing leaves and negatively auto‐regulated (P_SAG12–IPT), thus preventing developmental abnormalities. Plants were grown with growth‐limiting nitrogen supply. Compared to the wild‐type, endogenous levels of free zeatin (Z)‐ and Z riboside (ZR)‐type cytokinins were increased up to 15‐fold (total ZR up to 100‐fold) in senescing leaves, and twofold in younger leaves of P_SAG12–IPT. In these plants, the senescence‐associated declines in N, protein and Rubisco levels and photosynthesis rates were delayed. Senescing leaves accumulated more (¹⁵N‐labelled) N than younger leaves, associated with reduced shoot N accumulation (–60%) and a partially inverted canopy N profile in P_SAG12–IPT plants. While root N accumulation was not affected, N translocation to non‐senescing leaves was progressively reduced. We discuss potential consequences of these modified sink–source relations, associated with delayed leaf senescence, for plant productivity and the efficiency of utilization of light and minerals.