Adaptation to different host plant ages facilitates insect divergence without a host shift

Host shifts and subsequent adaption to novel host plants are important drivers of speciation among phytophagous insects. However, there is considerably less evidence for host plant-mediated speciation in the absence of a host shift. Here, we investigated divergence of two sympatric sister elm leaf beetles, Pyrrhalta maculicollis and P. aenescens, which feed on different age classes of the elm Ulmus pumila L. (seedling versus adult trees). Using a field survey coupled with preference and performance trials, we show that these beetle species are highly divergent in both feeding and oviposition preference and specialize on either seedling or adult stages of their host plant. An experiment using artificial leaf discs painted with leaf surface wax extracts showed that host plant chemistry is a critical element that shapes preference. Specialization appears to be driven by adaptive divergence as there was also evidence of divergent selection; beetles had significantly higher survival and fecundity when reared on their natal host plant age class. Together, the results identify the first probable example of divergence induced by host plant age, thus extending how phytophagous insects might diversify in the absence of host shifts.     

Studies on the relations of insect and host plant

Brevicoryne brassicae and Myzus persicae were caged on the mature leaves of brussels sprout plants receiving different amounts of nitrogen and potassium. An increase in N or decrease in K resulted in an increase in soluble nitrogen levels in the leaves. The fecundity and reproductive rate of M. persicae were correlated with these changes; B. brassicae showed a markedly lower response to the nutrient treatments. The possible relationships of these results to other contrasts between the two aphids are discussed.

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Zusammenfassung Brevicoryne brassicae (L.) und Myzus persicae (Sulz.) wurden in Käfigen auf mittelalten Blättern von Kohlpflanzen gehalten, die unterschiedlich mit Stickstoff und Kali gedüngt wurden. Erhöhung der Stickstoff- oder Verminderung der Kalidüngung steigerte den Gehalt löslichen Stickstoffs in den Blättern der Pflanze. Nachkommenzahl und Vermehrungsrate von M. persicae wurden durch diese Behandlungen mehr gefördert als die von B. brassicae und waren mit dem Gehalt an löslichem Stickstoff gut korreliert. Es ist möglich, daß sich dieser Unterschied zwischen B. brassicae und M. persicae teilweise auch in anderen Unterschieden zwischen den zwei Arten äußert, z.B. im Verhalten zu Blattalter und Wassermangel in der Pflanze. Eine hohe Kalidüngung hemmt die Vermehrung beider Arten und kann möglicherweise, wenn mit dem Eingriff der natürlichen Feinde integriert, eine nützliche Rolle in der Bekämpfung dieser Blattläuse spielen.

     

The relationships of plant and insect diversities in succession

The basic features of an intensive study on the various stages of a secondary succession, from fallow Held to birch woodland, are described. The α-β diversities of the green plants, and two orders of insects, Hetcroptera and adult Coleoptera, are described. For the vegetation, in addition to taxonotnic diversity, structural diversity, with both spatial and architectural components, was recognized. It was found that up to a successional age of 16 months, the taxonomic diversities of plants and insects rose; thereafter the diversity of the plant species declined far more than the insect species diversity. It was concluded that in the later successional stages the maintenance of a high level of taxonomic diversity of these orders of insects is correlated with the rising structural diversity of the green plants, which virtually compensates for their falling taxonomic diversity. The larger fungi appear to show a similar trend to the insects.     

Phytophagous insect fauna tracks host plant responses to exotic grass invasion

The high dependence of herbivorous insects on their host plants implies that plant invaders can affect these insects directly, by not providing a suitable habitat, or indirectly, by altering host plant availability. In this study, we sampled Asteraceae flower heads in cerrado remnants with varying levels of exotic grass invasion to evaluate whether invasive grasses have a direct effect on herbivore richness independent of the current disturbance level and host plant richness. By classifying herbivores according to the degree of host plant specialization, we also investigated whether invasive grasses reduce the uniqueness of the herbivorous assemblages. Herbivorous insect richness showed a unimodal relationship with invasive grass cover that was significantly explained only by way of the variation in host plant richness. The same result was found for polyphagous and oligophagous insects, but monophages showed a significant negative response to the intensity of the grass invasion that was independent of host plant richness. Our findings lend support to the hypothesis that the aggregate effect of invasive plants on herbivores tends to mirror the effects of invasive plants on host plants. In addition, exotic plants affect specialist insects differently from generalist insects; thus exotic plants affect not only the size but also the structural profile of herbivorous insect assemblages.     

Parasitism by Cuscuta pentagona attenuates host plant defenses against insect herbivores

Considerable research has examined plant responses to concurrent attack by herbivores and pathogens, but the effects of attack by parasitic plants, another important class of plant-feeding organisms, on plant defenses against other enemies has not been explored. We investigated how attack by the parasitic plant Cuscuta pentagona impacted tomato (Solanum lycopersicum) defenses against the chewing insect beet armyworm (Spodoptera exigua; BAW). In response to insect feeding, C. pentagona-infested (parasitized) tomato plants produced only one-third of the antiherbivore phytohormone jasmonic acid (JA) produced by unparasitized plants. Similarly, parasitized tomato, in contrast to unparasitized plants, failed to emit herbivore-induced volatiles after 3 d of BAW feeding. Although parasitism impaired antiherbivore defenses, BAW growth was slower on parasitized tomato leaves. Vines of C. pentagona did not translocate JA from BAW-infested plants: amounts of JA in parasite vines grown on caterpillar-fed and control plants were similar. Parasitized plants generally contained more salicylic acid (SA), which can inhibit JA in some systems. Parasitized mutant (NahG) tomato plants deficient in SA produced more JA in response to insect feeding than parasitized wild-type plants, further suggesting cross talk between the SA and JA defense signaling pathways. However, JA induction by BAW was still reduced in parasitized compared to unparasitized NahG, implying that other factors must be involved. We found that parasitized plants were capable of producing induced volatiles when experimentally treated with JA, indicating that resource depletion by the parasite does not fully explain the observed attenuation of volatile response to herbivore feeding. Collectively, these findings show that parasitic plants can have important consequences for host plant defense against herbivores.     

Constraints on host use by a parasitic plant

Consumers do not always utilize all suitable hosts. Understanding why parasitic plants do not always parasitize potentially suitable hosts requires a better understanding of the constraints that limit host use by parasitic plants. In Texas salt marshes, the parasitic plant Cuscuta indecora rarely parasitizes three hosts that support vigorous growth in the greenhouse. We identified three constraints on host use by C. indecora. First, a mismatch between the phenology of C. indecora and some suitable hosts meant that these hosts were not abundant when C. indecora was growing most vigorously, and therefore were underutilized. Second, C. indecora preferentially parasitized tall plants versus short ones, causing relatively short species to be underutilized. Third, C. indecora overwinters in some perennial hosts but has to reinfect annual hosts each year, causing annuals and perennials that do not support overwintering to be underutilized. In combination, these constraints, which reflect the general lack of mobility of parasitic plants relative to herbivores, remove half of the potential host species from the actual diet of C. indecora, and therefore likely represent a major limitation on the success of this parasite. Similar constraints are likely to limit the realized host range of many parasitic plants and select for generalized diet preferences.     

A plant pathogen reduces the enemy-free space of an insect herbivore on a shared host plant

An important mechanism in stabilizing tightly linked host-parasitoid and prey-predator interactions is the presence of refuges that protect organisms from their natural enemies. However, the presence and quality of refuges can be strongly affected by the environment. We show that infection of the host plant Silene latifolia by its specialist fungal plant pathogen Microbotryum violaceum dramatically alters the enemy-free space of a herbivore, the specialist noctuid seed predator Hadena bicruris, on their shared host plant. The pathogen arrests the development of seed capsules that serve as refuges for the herbivore's offspring against the specialist parasitoid Microplitis tristis, a major source of mortality of H. bicruris in the field. Pathogen infection resulted both in lower host-plant food quality, causing reduced adult emergence, and in twofold higher rates of parasitism of the herbivore. We interpret the strong oviposition preference of H. bicruris for uninfected plants in the field as an adaptive response, positioning offspring on refuge-rich, high-quality hosts. To our knowledge, this is the first demonstration that plant-inhabiting micro-organisms can affect higher trophic interactions through alteration of host refuge quality. We speculate that such interference can potentially destabilize tightly linked multitrophic interactions.     

Tri-trophic level interactions affect host plant development and abundance of insect herbivores

Understanding the interactions among plants, hemipterans, and ants has provided numerous insights into a range of ecological and evolutionary processes. In these systems, however, studies concerning the isolated direct and indirect effects of aphid colonies on host plant and other herbivores remain rare at best. The aphid Uroleucon erigeronensis forms dense colonies on the apical shoots of the host plant Baccharis dracunculilfolia (Asteraceae). The honeydew produced by these aphids attracts several species of ants that might interfere with other herbivores. Four hypotheses were tested in this system: (1) ants tending aphids reduce the abundance of other herbivores; (2) the effects of ants and aphids upon herbivores differ between chewing and fluid-sucking herbivores; (3) aphids alone reduce the abundance of other herbivores; and (4), the aphid presence negatively affects B. dracunculifolia shoot growth. The hypotheses were evaluated with ant and aphid exclusion experiments, on isolated plant shoots, along six consecutive months. We adjusted linear mixed-effects models for longitudinal data (repeated measures), with nested spatial random effect. The results showed that: (1) herbivore abundance was lower on shoots with aphids than on shoots without aphids, and even lower on shoots with aphids and ants; (2) both chewing and fluid-sucking insects responded similarly to the treatment, and (3) aphid presence affected negatively B. dracunculifolia shoot growth. Thus, since aphids alone changed plant growth and the abundance of insect herbivores, we suggest that the ant–aphid association is important to the organization of the system B. dracunculifolia-herbivorous insects.     

植物和刺吸式口器昆虫的诱导防御与反防御研究进展

刺吸式口器昆虫在长期的进化过程中形成特殊的口针结构,用于专门吸食植物韧皮部筛管细胞的汁液成分.以蚜虫为例,它们在取食过程中分泌的胶状唾液和水状唾液将有效的降低植物防御反应,其中水状唾液包含的大量酶类不仅可以帮助蚜虫穿刺植物韧皮部,刺探到筛管细胞,同时也是植物感受蚜虫为害的激发因子,诱导出植物防御反应和相关抗性基因的表达...     

Metapopulation-level adaptation of insect host plant preference and extinction-colonization dynamics in heterogeneous landscapes

Species living in highly fragmented landscapes typically occur as metapopulations with frequent turnover of local populations. The turnover rate depends on population sizes and connectivities, but it may also depend on the phenotypic and genotypic composition of populations. The Glanville fritillary butterfly (Melitaea cinxia) in Finland uses two host plant species, which show variation in their relative abundances at two spatial scales: locally among individual habitat patches and regionally among networks of patches. Female butterflies in turn exhibit spatial variation in genetically determined host plant preference within and among patch networks. Emigration, immigration and establishment of new populations have all been shown to be strongly influenced by the match between the host plant composition of otherwise suitable habitat patches and the host plant preference of migrating butterflies. The evolutionary consequences of such biased migration and colonization with respect to butterfly phenotypes might differ depending on spatial configuration and plant species composition of the patches in heterogeneous patch networks. Using a spatially realistic individual-based model we show that the model-predicted evolution of host plant preference due to biased migration explains a significant amount of the observed variation in host plant use among metapopulations living in dissimilar networks. This example illustrates how the ecological extinction-colonization dynamics may be linked with the evolutionary dynamics of life history traits in metapopulations.     

Use of an exotic host plant affects mate choice in an insect herbivore

The colonization of exotic plants by herbivorous insects has provided opportunities for investigating causes and consequences of the evolution of niche breadth. The butterfly Lycaeides melissa utilizes exotic alfalfa, Medicago sativa, which is a relatively poor larval resource, and previous studies have found that caterpillars that consume M. sativa develop into smaller and less fecund adults. Here we investigate the effect of smaller female body size on male mate preference, a previously unexplored consequence of novel host use. Smaller females, which developed on the exotic host, were less likely to be visited by males. This result was confirmed with a second set of choice tests involving females reared on a single plant species, thus ruling out host-specific confounding factors. We suggest that an effect on mate choice be considered part of the complex suite of factors determining persistence of herbivorous insects following colonization of new habitats or resources.     

Shaping the understanding of saliva-derived effectors towards aphid colony proliferation in host plant

‘Effectors’ are proteins and/or small molecules that originated from aphid saliva gland and its secretion is initiated due to interaction between host and insect. The effectors have the ability to manipulate the host cell structure as well as function similar to pathogen’s effectors. Like pathogen’s effectors, aphid effectors suppress the hosts’ defense responses as well as hosts’ defense induction or both. In the susceptible interaction with the host, aphid effectors alter plant processes that contribute to the establishment of compatibility that promotes aphid proliferation. In the susceptible reaction with the host, aphid effectors contribute to the successful salivation and sustainability of the sieve element sap ingestion that have promoting role in more aphid proliferation. In the resistant interaction with the host, aphid effectors are recognized by the typical plant receptors and elicit the induction the effective defense response. As a result, aphid proliferation is reduced due to reduced compatibility establishment in the resistant host. This review focuses on the exciting progress in aphid effector biology that insights new perspective in the molecular basis of plant–aphid interactions.     

A recently acquired host plant provides an oligophagous insect herbivore with enemy-free space

Enemy-free space (EFS) is a potentially important factor affecting host plant use by phytophagous insects. Yet only a few field studies have demonstrated that natural enemy activity is the sole mechanism underlying use of novel host plants by herbivorous insects. This may be due to the fact that in earlier studies, both herbivores and natural enemies had the opportunity to adapt to the new host plant. Here we studied the possibility that EFS underlies the recently recorded increase in Phthorimaea operculella densities on tomato plants in a few areas within its geographical range. Through field experiments in Ethiopia, we show that all three conditions proposed by Berdegue et al. to demonstrate EFS are fulfilled. First, a significantly higher proportion of larvae survive on caged than on exposed potato plants, showing that natural enemies are an important mortality factor on the original host, potato. Second, larval survival was significantly higher on exposed tomato than potato plants, implying greater protection for the herbivore from its natural enemies on tomato than on potato plants. Thus tomato plants provide P. operculella with an EFS. Finally, larval survival was significantly higher on caged potato than on caged tomato plants at the preblossom stage, indicating that, in the absence of natural enemies, there is a fitness cost when larvae feed on the sub-optimal tomato plants. Fulfillment of this third condition points to the importance of natural enemy activity relative to that of other unidentified factors, such as food quality and competition. An intensive field survey provides further support for this conclusion.     

The polymorphism, mimicry, and host plant relationships of Hypolimnas butterflies

The 30 species of Hypolimnas butterflies recognized are divided amongst six pattern morphism classes: monomorphic, weakly dimorphic, simple dual, unimodal polymorphic, partial 9-limited polymorphic, and multiple ♀ dual polymorphic. Eighty per cent of Hypolimnas species belong to one or other of the first three categories. About 60% of the species show evidence of mimicry, some 40% being mimetic in both sexes. In the Oriental region the great majority of mimetic species within the genus are modelled on Euptoea , whilst those of the Afrotropical region are mostly modelled on Amauris. The host plants recorded for the genus strongly suggest that the Urticaceae are primitive for the group, but eleven other families of flowering plants are recorded as hosts. The apparent non-utilization of Urticaceae by H. misippus (L.), the most widespread species of the group, is discussed in terms of its possible ecological and phylogenetic relationships with H. bolina (L.). A check-list of butterflies feeding on Ipomoea (a host plant for both H. bolina and H. misippus) , and on other Convolvulaceae, is included.     

Intraspecific competition facilitates the evolution of tolerance to insect damage in the perennial plant Solanum carolinense

Tolerance to herbivory (the degree to which plants maintain fitness after damage) is a key component of plant defense, so understanding how natural selection and evolutionary constraints act on tolerance traits is important to general theories of plant–herbivore interactions. These factors may be affected by plant competition, which often interacts with damage to influence trait expression and fitness. However, few studies have manipulated competitor density to examine the evolutionary effects of competition on tolerance. In this study, we tested whether intraspecific competition affects four aspects of the evolution of tolerance to herbivory in the perennial plant Solanum carolinense: phenotypic expression, expression of genetic variation, the adaptive value of tolerance, and costs of tolerance. We manipulated insect damage and intraspecific competition for clonal lines of S. carolinense in a greenhouse experiment, and measured tolerance in terms of sexual and asexual fitness components. Compared to plants growing at low density, plants growing at high density had greater expression of and genetic variation in tolerance, and experienced greater fitness benefits from tolerance when damaged. Tolerance was not costly for plants growing at either density, and only plants growing at low density benefited from tolerance when undamaged, perhaps due to greater intrinsic growth rates of more tolerant genotypes. These results suggest that competition is likely to facilitate the evolution of tolerance in S. carolinense, and perhaps in other plants that regularly experience competition, while spatio-temporal variation in density may maintain genetic variation in tolerance.     

Resolving insect phylogeny: The significance of cephalic structures of the Nannomecoptera in understanding endopterygote relationships

The Nannomecoptera are among the most enigmatic and controversial taxa of endopterygote insects, the phylogenetic resolution of which is crucial to understanding the evolution of neopteran insects. Once considered a subordinate lineage within the Mecoptera, renewed interest in nannochoristids has been prompted by evidence that the Nannomecoptera are not admissible to the clade of extant scorpion flies but are more likely to belong to the clade Siphonaptera + Nannomecoptera + Diptera. The overarching purpose of the present account is to provide novel and extensive morphological character traits in addition to those already existing for adult structures. The aim is to determine if these traits support molecular data sets that have been suggested elsewhere to clarify the phyletic position of Nannochoristidae. This account focuses on nannomecopteran larvae, which unlike those of other mecopterans have received little attention. Thus, the thrust of this investigation is to provide detailed anatomical data on nannochoristid larvae for a targeted inquiry into their phylogenetic affinities. The described characters are discussed and presented in a data matrix comprising representatives of all endopterygote orders. While the study is based primarily on the New Zealand species Nannochorista philpotti, it is proposed that all nannomecopteran larvae will prove to be similar to this taxon in most if not all structural features of significance to a higher-level phylogenetic context.     

Canopy insect herbivores in the Azorean Laurisilva forests: key host plant species in a highly generalist insect community

This article explores patterns of insect herbivore distribution in the canopy of the Laurisilva forests on seven islands in the Azores archipelago. To our knowledge, this is one of the first extensive study of this type in tree or shrub canopies of oceanic island ecosystems. One of the most frequently debated characteristics of such ecosystems is the likely prevalence of vague, ill‐defined niches due to taxonomic disharmony, which may have implications for insect‐plant interactions. For instance, an increase in ecological opportunities for generalist species is expected due to the lack of predator groups and reduced selection for chemical defence in host plants. The following two questions were addressed: 1) Are specialists species rare, and insect herbivore species randomly distributed among host plant species in the Azores? 2) Are the variances in insect herbivore species composition, frequency and richness explained by host plants or by regional island effects? We expect a proportional distribution of herbivore species between host plants, influenced by host frequency and distinct island effects; otherwise, deviation from expectation might suggest habitat preference for specific host tree crowns. Canopy beating tray samples were performed on seven islands, comprising 50 transects with 1 to 3 plant species each (10 replicates per species), giving 1320 samples from ten host species trees or shrubs in total. From a total of 129 insect herbivore species, a greater number of herbivore species was found on Juniperus brevifolia (s=65) and Erica azorica (s=53). However, the number of herbivore species per individual tree crown was higher for E. azorica than for any other host, on all islands, despite the fact that it was only the fourth more abundant plant. In addition, higher insect species richness and greater insect abundance were found on the trees of Santa Maria Island, the oldest in the archipelago. Insect species composition was strongly influenced by the presence of E. azorica, which was the only host plant with a characteristic fauna across the archipelago, whereas the fauna of other plant crowns was grouped by islands. The great insect occurrence on E. azorica reflects strong habitat fidelity, but only four species were clearly specialists. Our findings indicate a broadly generalist fauna. The simplicity of Azorean Laurisilva contributed to the understanding of insect‐plant mechanisms in canopy forest habitats.     

Dramatic transcriptional changes in an intracellular parasite enable host switching between plant and insect

Phytoplasmas are bacterial plant pathogens that have devastating effects on the yields of crops and plants worldwide. They are intracellular parasites of both plants and insects, and are spread among plants by insects. How phytoplasmas can adapt to two diverse environments is of considerable interest; however, the mechanisms enabling the "host switching" between plant and insect hosts are poorly understood. Here, we report that phytoplasmas dramatically alter their gene expression in response to "host switching" between plant and insect. We performed a detailed characterization of the dramatic change that occurs in the gene expression profile of Candidatus Phytoplasma asteris OY-M strain (approximately 33% of the genes change) upon host switching between plant and insect. The phytoplasma may use transporters, secreted proteins, and metabolic enzymes in a host-specific manner. As phytoplasmas reside within the host cell, the proteins secreted from phytoplasmas are thought to play crucial roles in the interplay between phytoplasmas and host cells. Our microarray analysis revealed that the expression of the gene encoding the secreted protein PAM486 was highly upregulated in the plant host, which is also observed by immunohistochemical analysis, suggesting that this protein functions mainly when the phytoplasma grows in the plant host. Additionally, phytoplasma growth in planta was partially suppressed by an inhibitor of the MscL osmotic channel that is highly expressed in the plant host, suggesting that the osmotic channel might play an important role in survival in the plant host. These results also suggest that the elucidation of "host switching" mechanism may contribute to the development of novel pest controls.