Chewing Insect Herbivores Research Articles

Genome-Wide Transcriptomic and Metabolomic Analyses Unveiling the Defence Mechanisms of Populus tremula against Sucking and Chewing Insect Herbivores.

Plants and insects coevolved as an evolutionarily successful and enduring association. The molecular arms race led to evolutionary novelties regarding unique mechanisms of defence and detoxification in plants and insects. While insects adopt mechanisms to conquer host defence, trees develop well-orchestrated and species-specific defence strategies against insect herbivory. However, current knowledge on the molecular underpinnings of fine-tuned tree defence responses against different herbivore insects is still restricted. In the current study, using a multi-omics approach, we unveiled the defence response of Populus tremula against aphids (Chaitophorus populialbae) and spongy moths (Lymantria dispar) herbivory. Comparative differential gene expression (DGE) analyses revealed that around 272 and 1203 transcripts were differentially regulated in P. tremula after moth and aphid herbivory compared to uninfested controls. Interestingly, 5716 transcripts were differentially regulated in P. tremula between aphids and moth infestation. Further investigation showed that defence-related stress hormones and their lipid precursors, transcription factors, and signalling molecules were over-expressed, whereas the growth-related counterparts were suppressed in P. tremula after aphid and moth herbivory. Metabolomics analysis documented that around 37% of all significantly abundant metabolites were associated with biochemical pathways related to tree growth and defence. However, the metabolic profiles of aphid and moth-fed trees were quite distinct, indicating species-specific response optimization. After identifying the suitable reference genes in P. tremula, the omics data were further validated using RT-qPCR. Nevertheless, our findings documented species-specific fine-tuning of the defence response of P. tremula, showing conservation on resource allocation for defence overgrowth under aphid and moth herbivory. Such findings can be exploited to enhance our current understanding of molecular orchestration of tree responses against herbivory and aid in developing insect pest resistance P. tremula varieties.

Defense strategies and associated phytohormonal regulation in Brassica plants in response to chewing and sap-sucking insects.

Plants have evolved distinct defense strategies in response to a diverse range of chewing and sucking insect herbivory. While chewing insect herbivores, exemplified by caterpillars and beetles, cause visible tissue damage and induce jasmonic acid (JA)-mediated defense responses, sucking insects, such as aphids and whiteflies, delicately tap into the phloem sap and elicit salicylic acid (SA)-mediated defense responses. This review aims to highlight the specificity of defense strategies in Brassica plants and associated underlying molecular mechanisms when challenged by herbivorous insects from different feeding guilds (i.e., chewing and sucking insects). To establish such an understanding in Brassica plants, the typical defense responses were categorized into physical, chemical, and metabolic adjustments. Further, the impact of contrasting feeding patterns on Brassica is discussed in context to unique biochemical and molecular modus operandi that governs the resistance against chewing and sucking insect pests. Grasping these interactions is crucial to developing innovative and targeted pest management approaches to ensure ecosystem sustainability and Brassica productivity.

Duplicated Copy Number Variant of the Maize 9-Lipoxygenase ZmLOX5 Improves 9,10-KODA-Mediated Resistance to Fall Armyworms.

Extensive genome structure variations, such as copy number variations (CNVs) and presence/absence variations, are the basis for the remarkable genetic diversity of maize; however, the effect of CNVs on maize herbivory defense remains largely underexplored. Here, we report that the naturally occurring duplication of the maize 9-lipoxygenase gene ZmLOX5 leads to increased resistance of maize to herbivory by fall armyworms (FAWs). Previously, we showed that ZmLOX5-derived oxylipins are required for defense against chewing insect herbivores and identified several inbred lines, including Yu796, that contained duplicated CNVs of ZmLOX5, referred to as Yu796-2×LOX5. To test whether introgression of the Yu796-2×LOX5 locus into a herbivore-susceptible B73 background that contains a single ZmLOX5 gene is a feasible approach to increase resistance, we generated a series of near-isogenic lines that contained either two, one, or zero copies of the Yu796-2×LOX5 locus in the B73 background via six backcrosses (BC6). Droplet digital PCR (ddPCR) confirmed the successful introgression of the Yu796-2×LOX5 locus in B73. The resulting B73-2×LOX5 inbred line displayed increased resistance against FAW, associated with increased expression of ZmLOX5, increased wound-induced production of its primary oxylipin product, the α-ketol, 9-hydroxy-10-oxo-12(Z),15(Z)-octadecadienoic acid (9,10-KODA), and the downstream defense hormones regulated by this molecule, 12-oxo-phytodienoic acid (12-OPDA) and abscisic acid (ABA). Surprisingly, wound-induced JA-Ile production was not increased in B73-2×LOX5, resulting from the increased JA catabolism. Furthermore, B73-2×LOX5 displayed reduced water loss in response to drought stress, likely due to increased ABA and 12-OPDA content. Taken together, this study revealed that the duplicated CNV of ZmLOX5 quantitively contributes to maize antiherbivore defense and presents proof-of-concept evidence that the introgression of naturally occurring duplicated CNVs of a defensive gene into productive but susceptible crop varieties is a feasible breeding approach for enhancing plant resistance to herbivory and tolerance to abiotic stress.

Rhizobial diversity impacts soybean resistance, but not tolerance, to herbivory during drought

Biodiversity within ecological communities has been shown to influence plant resistance and tolerance to herbivory, and may strongly interact with climate change. However, the effects of diversity within microbial symbionts on their plant hosts’ responses to herbivores is less well documented, particularly within the context of agricultural systems. Here we examine the interactive effects of rhizobial diversity and drought in promoting soybean resistance and tolerance to insect and mammalian herbivores in an outdoor pot experiment. We demonstrate that soybean plants provided with a diverse mixture of rhizobial strains experience 30% less damage by chewing insect herbivores under both drought (reduced watering and increased temperature) and ambient weather conditions, as well as half as much colonization by aphids under drought conditions. This effect was not due to the presence of any particularly beneficial rhizobial strain (i.e., not a selection effect), but rather due to an enhanced benefit provided by the rhizobial strains in combination (i.e., a complementarity effect). No effects of rhizobial diversity were observed on the rate of defoliation by rabbits. While rhizobial diversity did affect soybean resistance to insect herbivores, these differences in insect damage did not carry through to affect soybean growth or final yield. Thus, soybean plants exhibited high tolerance to herbivory regardless of our experimentally imposed rhizobial diversity or drought treatments. With future projections for increased pest outbreaks and drought, understanding the factors that can sustainably enhance agricultural yield is critical. Our results suggest that promoting rhizobial diversity within soybean agriculture may be one nature-based solution to promote the resistance of this critical crop to insect pests.

Plant silicon defences reduce the performance of a chewing insect herbivore which benefits a contemporaneous sap‐feeding insect

Abstract Plant‐mediated interactions between phytophagous insects are ubiquitous. Silicon (Si) accumulation can defend plants against chewing insect herbivores, although sap‐feeders are comparatively less affected. It remains unknown how Si impacts interspecific interactions between chewing and sap‐feeding insect herbivores when sharing a host plant. We grew the model grass Brachypodium distachyon with (+Si) or without (−Si) Si and assessed the impacts of Si supplementation on the contemporaneous performance and interguild interactions between a chewing (Helicoverpa armigera) and a sap‐feeding (Rhopalosiphum padi) insect herbivore. We further performed dual‐choice tests to assess insect preferences for +Si or −Si plants with or without prior insect attack. Si reduced the relative growth rate (RGR) of both separately and contemporaneously fed caterpillars (H. armigera). Conversely, aphid abundance was higher on +Si plants compared to −Si plants. Caterpillar RGR and aphid abundance were negatively correlated on shared host plants. Furthermore, decreased caterpillar RGR on +Si plants benefitted aphid colonisation, indicating the plant‐mediated effects of Si on interspecific competition between the two insects. Attack by caterpillars induced leaf Si accumulation, regardless of aphid presence. In dual‐choice tests, caterpillars preferred aphid‐attacked −Si plants to aphid‐attacked +Si plants, whereas aphids preferred caterpillar‐attacked +Si plants to caterpillar‐attacked −Si plants. Our results provide the first evidence for plant‐mediated effects of Si on interspecific competition between two insect herbivores. We suggest that the dissimilar effects of Si against different feeding guilds of herbivores may promote asymmetry in interspecific competition when sharing host plants, potentially impacting insect abundance and distribution.

Herbivory on the pedunculate oak along an urbanization gradient in Europe: Effects of impervious surface, local tree cover, and insect feeding guild.

Urbanization is an important driver of the diversity and abundance of tree‐associated insect herbivores, but its consequences for insect herbivory are poorly understood. A likely source of variability among studies is the insufficient consideration of intra‐urban variability in forest cover. With the help of citizen scientists, we investigated the independent and interactive effects of local canopy cover and percentage of impervious surface on insect herbivory in the pedunculate oak (Quercus robur L.) throughout most of its geographic range in Europe. We found that the damage caused by chewing insect herbivores as well as the incidence of leaf‐mining and gall‐inducing herbivores consistently decreased with increasing impervious surface around focal oaks. Herbivory by chewing herbivores increased with increasing forest cover, regardless of impervious surface. In contrast, an increase in local canopy cover buffered the negative effect of impervious surface on leaf miners and strengthened its effect on gall inducers. These results show that—just like in non‐urban areas—plant–herbivore interactions in cities are structured by a complex set of interacting factors. This highlights that local habitat characteristics within cities have the potential to attenuate or modify the effect of impervious surfaces on biotic interactions.

Effect of gene mutation of plants on their mechano-sensibility: the mutant of EXO70H4 influences the buckling of Arabidopsis trichomes.

With the development of molecular biology, more and more mutants of plants have been constructed, where gene mutants have been found to influence not only the biological processes but also biophysical behaviors of plant cells. Trichomes are an important appendage, which has been found to act as an active mechanosensory switch transducing mechanical signals into physiology changes, where the mechanical property of trichomes is vital for such functions. Up to now, over 40 different genes have been found with the function of regulating trichome cell morphogenesis; however, the effect of gene mutants on trichome mechanosensory function remains elusive. In this study, we found that EXO70H4, one of the most up-regulated genes in the mature trichome, not only affects the thickness of the trichome cell wall but also the mechanical property (i.e., the Young's modulus) of trichomes. Finite element method simulation results show that the buckling instability and stress concentration (e.g., exerted by insects) cannot occur on the base of the mutant exo70H4 trichome, which might further interrupt the mechanical signal transduction from branches to the base of trichomes. These results indicated that the mutant exo70H4 trichome might lack the ability to act as an active mechanosensory switch against chewing insect herbivores. Our findings provide new information about the effect of gene mutation (like crop mutants) on the mechano-sensibility and capability to resist the agricultural pests or lodging, which could be of great significance to the development of agriculture.

Differential effects of the rhizobacterium Pseudomonas simiae on above‐ and belowground chewing insect herbivores

AbstractPlant growth‐promoting rhizobacteria (PGPR) can enhance plant growth and defence. Via plant‐mediated effects, PGPR have been reported to impact the performance of generalist leaf‐chewing insects either negatively or positively. However, only a few insect species, mainly feeding on aboveground tissues, have thus far been investigated. Here, we investigated how addition of rhizobacteria to the soil in which cabbage plants are growing affects the performance of three chewing insect herbivores, two leaf chewers and one root feeder. In a greenhouse experiment, we grew white cabbage (Brassica oleracea) plants in soil supplemented with and without the rhizobacterium Pseudomonas simiae WCS417r. We investigated the consequences for three important insect pests of Brassica species, larvae of the cabbage moth Mamestra brassicae, the diamondback moth Plutella xylostella, or the cabbage root fly Delia radicum after 5 weeks of plant growth. We recorded aboveground plant biomass, insect biomass, plant defence marker gene expression levels and plant defence‐related hormone concentrations. Rhizobacterial inoculation increased aboveground plant biomass in non‐infested plants but not in infested plants. Rhizobacterial inoculation affected insect performance differently: on PGPR‐inoculated plants biomass of Plutella xylostella was lower, while biomass of Delia radicum was higher than on control plants. However, no effect was found on Mamestra brassicae biomass. Rhizobacterial inoculation increased the expression of the defence marker gene LOX2 in P. xylostella‐infested plants and MYC2 in M. brassicae‐infested plants. Transcription levels of the plant defence marker gene PAL1 showed upregulation between inoculated and non‐inoculated insect‐free plants. Levels of the phytohormones jasmonic acid, salicylic acid and abscisic acid were similar in inoculated and non‐inoculated plants. We conclude that rhizobacterial inoculation has potential to be applied in the protection of cabbage crops against the diamondback moth whereas this does not apply to reducing damage caused by the cabbage moth or cabbage root fly.

Microbes in Helicoverpa armigera oral secretions contribute to increased senescence around plant wounds

1. Plants have long been exposed to insect herbivore attack. Crucial to the plant's ability to defend itself is its ability to identify specific signals associated with attacking insects. Distinctive chemical cues, such as those associated with chewing insect oral secretions (OS), activate targeted defence responses to chewing insect herbivores.2. Herbivore‐associated cues can be complicated by the fact that many herbivores form associations with microbes that produce their own specific signals, which may induce alternative defence processes.3. Here we report that OS of the global pest, the cotton bollworm (Helicoverpa armigera), induce senescence around wounds in Brachypodium distachyon leaves. Crude OS activate greater levels of senescence than OS with reduced microbial abundance or mechanical wounding alone. Nonetheless, plants closed mechanical wounds more rapidly when treated with H. armigera OS regardless of the microbial component.4. This study concludes that H. armigera OS can activate senescence and wound closure in plant tissues and that microbes within OS have an important role in shaping plant‐herbivore interactions through additional increases in senescence.

Nonglandular silicified trichomes are essential for rice defense against chewing herbivores.

Interspecific New Rice for Africa (NERICA) varieties have been recently developed and used in Sub-Saharan Africa but herbivore resistance properties of these plants remain poorly understood. Here we report that, compared to a local Japanese cultivar Nipponbare, NERICA 1, 4 and 10 are significantly more damaged by insect herbivores in the paddy fields. In contrast to high levels of leaf damage from rice skippers and grasshoppers, constitutive and induced volatile organic compounds for indirect plant defense were higher or similar in NERICAs and Nipponbare. Accumulation of direct defense secondary metabolites, momilactones A and B, and p-coumaroylputrescine (CoP) was reduced in NERICAs, while feruloylputrescine accumulated at similar levels in all varieties. Finally, we found that Nipponbare leaves were covered with sharp nonglandular trichomes impregnated with silicon but comparable defense structures were virtually absent in herbivory-prone NERICA plants. As damage to the larval gut membranes by Nipponbare silicified trichomes that pass intact through the insect digestive system, occurs, and larval performance is enhanced by trichome removal from otherwise chemically defended Nipponbare plants, we propose that silicified trichomes work as an important defense mechanism of rice against chewing insect herbivores.

Fire mediated herbivory and plant defense of a neotropical shrub

Changes in morphological and physiological features of plants after fire events can modify their attractiveness to herbivorous insects. Our goal was to assess the effects of fire on plant–insect interactions and the development of induced defenses in the shrub Bauhinia brevipes (Fabaceae). We hypothesized that resprouting plants of B. brevipes after a fire occurrence would be more susceptible to herbivory by leaf-chewing and the gall-maker Schizomyia macrocapillata Maia (Diptera: Cecidomyiidae) than non-resprouting plants. We analyzed leaf area, leaf damage caused by chewing insect herbivores and attacks by the gall-midge S. macrocapillata, hypersensitivity response, chlorophyll content and silicon content after a fire event. Resprouting plants presented greater leaf area; higher chlorophyll concentration; hypersensitivity response and greater leaf silicon content. Although these plants experienced a greater number of attacks by S. macrocapillata the greater investment in growth and photosynthetic capacity, and possible leaf nitrogen content, can be determining attractiveness to a range of herbivores specifically galling insects. Leaf silicon content can be related with the lower damage by chewing insects, even with the greater vigor of resprouting plants. Thus, our results are contrary to some previous studies that report increases in leaf herbivory by chewer insects after fire events. Increases in hypersensitive response and silicon content seem to alter herbivorous insect preference and performance after fire occurrence.

Glucosinolate Desulfation by the Phloem-Feeding Insect Bemisia tabaci

Glucosinolates are plant secondary defense metabolites confined nearly exclusively to the order Brassicales. Upon tissue rupture, glucosinolates are hydrolyzed to various bioactive breakdown products by the endogenous plant enzyme myrosinase. As the feeding of chewing insect herbivores is associated with plant tissue damage, these insects have developed several independent strategies for coping with the glucosinolate-myrosinase defense system. On the other hand, our knowledge of how phloem-feeding insects interact with the glucosinolate-myrosinase system is much more limited. In fact, phloem feeders might avoid contact with myrosinase altogether so their susceptibility to intoxication by glucosinolate hydrolysis products is unclear. Previous studies utilizing Arabidopsis thaliana plants accumulating high levels of aliphatic- or indolic-glucosinolates indicated that both glucosinolate groups have moderate negative effects on the reproductive performance of Bemisia tabaci, a generalist phloem-feeding insect. To get a deeper understanding of the interaction between B. tabaci and glucosinolate-defended plants, adults were allowed to feed on artificial diet containing intact glucosinolates or on Brussels sprout and A. thaliana plants, and their honeydew was analyzed for the presence of possible metabolites. We found that B. tabaci is capable of cleaving off the sulfate group of intact glucosinolates, producing desulfoglucosinolates that cannot be activated by myrosinases, a mechanism described to date only in several chewing insect herbivores. The presence of desulfated glucosinolates in the honeydew of a generalist phloem-feeder may indicate the necessity to detoxify glucosinolates, likely due to some level of cellular damage during feeding, which results in glucosinolate activation, or as a mechanism to circumvent the non-enzymatic breakdown of indolic glucosinolates.

Foliar quality of co-occurring mallee eucalypts: balance of primary and secondary metabolites reflects past growing conditions

Foliar quality for herbivores is determined by the balance of primary and secondary metabolites which is dependent on leaf age which in turn is determined by the periodicity of flushing and the rate of abscission. We conducted a 10-month longitudinal study in northeastern Victoria, Australia, of the quality of fully expanded leaves of Eucalyptus gracilis, E. socialis, E. dumosa and E. incrassata. We measured N and available N (AvailN) as well as sideroxylonals and other formylated phloroglucinol compounds (FPCs) and related changes in concentration to tree phenology (flushing and flowering) and climatic conditions (temperature and rainfall). Concentrations of N, sideroxylonals and other FPCs differed significantly between species and with time as well as among trees within a species. AvailN also differed significantly between species and among trees within a species and with time for E. gracilis. Our analyses indicated that tree phenology affected N concentrations in E. gracilis only while climatic conditions affected N concentrations in E. gracilis, E. socialis and E. dumosa. Nitrogen concentrations in E. incrassata were unaffected by phenological or climatic factors. Tree phenology affected concentrations of sideroxylonals in E. gracilis and E. dumosa while climatic conditions affected concentrations of the same in E. socialis, E. dumosa and E. incrassata. The concentration of primary metabolites in expanded leaves of these eucalypts was relatively consistent compared to the concentration of quality reducing secondary metabolites which varied with the conditions experienced when leaves were expanding. Our results show that the foliar quality of leaves can be highly variable, with variations mediated by phenology and climate. These variations are likely to partially explain fluctuations in the abundance of chewing insect herbivores reliant on expanded leaves.

Cytokinin levels and signaling respond to wounding and the perception of herbivore elicitors in Nicotiana attenuata.

Nearly half a century ago insect herbivores were found to induce the formation of green islands by manipulating cytokinin (CK) levels. However, the response of the CK pathway to attack by chewing insect herbivores remains unclear. Here, we characterize the CK pathway of Nicotiana attenuata (Torr. ex S. Wats.) and its response to wounding and perception of herbivore-associated molecular patterns (HAMPs). We identified 44 genes involved in CK biosynthesis, inactivation, degradation, and signaling. Leaf wounding rapidly induced transcriptional changes in multiple genes throughout the pathway, as well as in the levels of CKs, including isopentenyladenosine and cis-zeatin riboside; perception of HAMPs present in the oral secretions (OS) of the specialist herbivore Manduca sexta amplified these responses. The jasmonate pathway, which triggers many herbivore-induced processes, was not required for these HAMP-triggered changes, but rather suppressed the CK responses. Interestingly CK pathway changes were observed also in systemic leaves in response to wounding and OS application indicating a role of CKs in mediating long distance systemic processes in response to herbivory. Since wounding and grasshopper OS elicited similar accumulations of CKs in Arabidopsis thaliana L., we propose that CKs are integral components of wounding and HAMP-triggered responses in many plant species.

Inducibility of chemical defences by two chewing insect herbivores in pine trees is specific to targeted plant tissue, particular herbivore and defensive trait

There is increasing evidence that plants can react to biotic aggressions with highly specific responses. However, few studies have attempted to jointly investigate whether the induction of plant defences is specific to a targeted plant tissue, plant species, herbivore identity, and defensive trait. Here we studied those factors contributing to the specificity of induced defensive responses in two economically important pine species against two chewing insect pest herbivores. Juvenile trees of Pinus pinaster and P. radiata were exposed to herbivory by two major pest threats, the large pine weevil Hylobius abietis (a bark-feeder) and the pine processionary caterpillar Thaumetopoea pityocampa (a folivore). We quantified in two tissues (stem and needles) the constitutive (control plants) and herbivore-induced concentrations of total polyphenolics, volatile and non-volatile resin, as well as the profile of mono- and sesquiterpenes. Stem chewing by the pine weevil increased concentrations of non-volatile resin, volatile monoterpenes, and (marginally) polyphenolics in stem tissues. Weevil feeding also increased the concentration of non-volatile resin and decreased polyphenolics in the needle tissues. Folivory by the caterpillar had no major effects on needle defensive chemistry, but a strong increase in the concentration of polyphenolics in the stem. Interestingly, we found similar patterns for all these above-reported effects in both pine species. These results offer convincing evidence that induced defences are highly specific and may vary depending on the targeted plant tissue, the insect herbivore causing the damage and the considered defensive compound.

Narboh D, a Respiratory Burst Oxidase Homolog in Nicotiana attenuata, is Required for Late Defense Responses After Herbivore AttackF

The superoxide (O₂⁻)-generating NADPH oxidases are crucial for the defense of plants against attack from pathogens; however, it remains unknown whether they also mediate responses against chewing insect herbivores. The transcripts of the respiratory burst NADPH oxidase homolog Narboh D in Nicotiana attenuate are rapidly and transiently elicited by wounding, and are amplified when Manduca sexta oral secretions (OS) are added to the wounds. The fatty-acid-amino-acid-conjugates (FACs), demonstrably the major elicitors in M. sexta OS, are responsible for the increase in Narboh D transcripts. Silencing Narboh D significantly reduced reactive oxygen species (ROS) levels after OS elicitation, but neither OS-elicited jasmonic acid (JA) or JA-isoleucine (JA-Ile) bursts, pivotal hormones that regulates plant resistance to herbivores, nor early transcripts of herbivore defense-related genes (NaJAR4 and NaPAL1), were influenced. However, late OS-elicited increases in trypsin proteinase inhibitors (TPIs), as well as the transcript levels of defense genes such as polyphenol oxidase, TPI and Thionin were significantly reduced. In addition, Narboh D-silenced plants were more vulnerable to insect herbivores, especially the larvae of the generalist Spodoptera littoralis. We thus conclude that Narboh D-based defenses play an important role in late herbivore-elicited responses.

The predictive science of community ecology

Body mass measures provide a tantalizing tool for explaining both variation in emergent community-level patterns and as a mechanistic basis for fundamental processes such as metabolism, consumption and competition. The unification of body mass, abundance and food web (ecological network) structure in community ecology is an effective way to explore future scenarios of environmental change. However, constraints over the availability of data against which to validate model predictions limit the application of size-based approaches. Here, I explore issues over the use of body size for predicting interaction strengths and hence the dynamics of natural ecosystems. The advantages, disadvantages, opportunities and limitations of such approaches are explored.

Plant sterols and host plant suitability for a phloem-feeding insect

1. Arthropods, including insects, are unique among animals in that they cannot synthesize sterols, including cholesterol, de novo. Some phytophagous insects (e.g. caterpillars, grasshoppers) generate tissue cholesterol by metabolizing plant phytosterols. Currently, little is known about sterols in plant phloem sap, and their significance to the nutritional physiology and ecology of phloem-feeding insects. 2. The sterol profiles of leaves from two plant species, Chinese cabbage and tobacco, were dominated by the phytosterols sitosterol and stigmasterol, respectively. In contrast, the principle sterol in the phloem sap of both plants was cholesterol, which is traditionally considered an animal sterol. Cholesterol was also the most abundant sterol in the carcass and honeydew of Myzus persicae aphids feeding on these plants. 3. The effect of sterol structure on M. persicae was investigated using modified tobacco plants that contained high levels of atypical steroids, specifically ketone-steroids. Aphids reared on the modified tobacco plants had a high atypical steroid content, severely reduced reproduction and high mortality. 4. Our data indicate that the total sterol composition of plants is not necessarily representative of the sterol profile available to phloem-feeding insects, and that the sterol utilization patterns of phloem-feeding insects may differ from chewing insect herbivores utilizing the same plant. Atypical steroids are naturally at insufficient concentrations for significant deleterious effects on insect herbivores, and possible reasons why plants apparently do not use them as defensive compounds are considered.

Reduced resistance of invasive varieties of the alien tree Sapium sebiferum to a generalist herbivore.

Invasive plants are often larger in their introduced range compared to their native range. This may reflect an evolved reduction in defense and increase in growth in response to low herbivory in their introduced range. Key elements of this scenario include genetic differences in defense and growth yet uniformly low rates of herbivory in the field that dissociate defense and herbivore damage for alien species. We conducted a laboratory experiment with Melanoplus angustipennis grasshoppers and Chinese Tallow Tree seedlings ( Sapium sebiferum) from its native range (China) and its introduced range (Texas, USA) where it is invasive. We caged grasshoppers with pairs of Sapium seedlings from the same continent or different continents. The amounts of leaf area removed from Texas and China seedlings, and their height growth rates, were indistinguishable when both seedlings in the pair were from the same continent. However, when grasshoppers had a choice between seedlings from different continents, they removed more Texas Sapium foliage than China Sapium foliage and height growth rates were higher for China Sapium seedlings compared to Texas seedlings. Grasshopper growth rates increased with greater Sapium foliage consumption. In a common garden in Texas, Sapium seedlings from Texas grew 40% faster than those from China. Chewing insect herbivores removed little Sapium foliage in the field experiment. Although grasshoppers preferred to feed on Texas Sapium when offered a choice in the laboratory, extremely low herbivory levels in the field may have allowed the Texas seedlings to outperform the China seedlings in the common garden. These results demonstrate post-invasion genetic differences in herbivore resistance and growth of an invasive plant species together with a decoupling of defense and herbivore choice in the introduced range.