Microsite availability, not floral herbivory, limits recruitment in peripheral native thistle populations

Plants live in environments where they are constantly, and often simultaneously, exposed to different types of biotic and abiotic stress, such as insect herbivory and water availability. How plants are adapted to abiotic conditions may determine how a surplus or shortage of water affects plant resistance to insect herbivory. Moreover, this effect may vary depending on the feeding mode of the herbivore. We explored how three closely related Rorippa plant species that vary in adaptations to different water levels, resist herbivory by four different insects (aphids: Myzus persicae , Lipaphis erysimi , and caterpillars: Pieris brassicae , Plutella xylostella ) under waterlogging or drought conditions. We hypothesized that plants that are differently adapted to water availability will be disparately affected by water availability in their resistance to insect herbivory. On the semi‐aquatic plant species Rorippa amphibia , both aphid species reached a larger colony size under drought conditions. This indicates that R. amphibia was compromised in resistance to aphid feeding when under drought conditions, to which it is less well adapted. Water conditions did not affect aphid performance on the flood‐plain species Rorippa palustris . On the terrestrial plant species Rorippa sylvestris , aphids performed worse on waterlogged than drought‐treated plants. Neither caterpillar species was significantly affected by the water availability of their food plant. Our findings suggest that water availability can have distinct effects on plant–insect interactions. We propose that plant adaptations to water conditions can be a major predictor towards explaining the variation of effects that water availability can have on plant–insect interactions.

Exotic plant species very often comprise a large proportion of urban floras. Because herbivorous insects depend on the presence of suitable host plants to maintain their populations, it is imperative to elucidate the relative importance of native and exotic hosts to understand the response of herbivorous guilds to urbanization. By using a plant-herbivore system composed of Asteraceae hosts and flower-head endophagous insects, we investigated whether the diversity and composition of herbivorous insects differs between native and exotic host-plant species in an urban environment. Although we found only seven exotic Asteraceae among the 30 species recorded, the overall abundance of exotics was considerably greater than that of native host plants. Overall, the exotic host species supported a small subset of the herbivore assemblage found on the native ones. The number of herbivore species per host species was significantly higher among the native plants, but we did not find a difference in herbivore abundance. Moreover, the higher taxonomic composition of herbivores on exotic Asteraceae was reduced, being composed of only three genera and two families from a total of 16 genera and six families of herbivores. These results provide support for the idea that plants outside of their original geographic distribution have lower loads of enemies than phylogenetically related native species. Our findings indicate that native host plants in urban areas play a critical role in supporting the native herbivorous insect fauna.

We studied the effects of elevated ultraviolet-B radiation on interactions between insect herbivores and their host plants by exposing two species of phytochemically different willows, Salix myrsinifolia and S. phylicifolia, to a modulated increase in ultraviolet radiation in an outdoor experiment and monitoring the colonisation of insect herbivores on these willows. We examined the effect of increased ultraviolet-B (UV-B) radiation on (1) the quality of willow leaves, (2) the distribution and abundance of insect herbivores feeding on these willows, (3) the resulting amount of damage, and (4) the performance of insect larvae feeding on the exposed plant tissue. Six clones of each of the two willow species were grown in eight blocks for 12 weeks in the UV-B irradiation field. The clones were exposed to a constant 50% increase in UV-B radiation (simulating 20-25% ozone depletion), to a small increase in UV-A radiation or to ambient solar irradiation. We allowed colonisation on the willows by naturally occurring insects, but also introduced adults of a leaf beetle, Phratora vitellinae, a specialist herbivore on S. myrsinifolia. Increased UV-B radiation did not affect any of the measured indices of plant quality. However, numbers of P. vitellinae on S. myrsinifolia were higher in plants with UV-B treatment compared with UV-A and shade controls. In laboratory tests, growth of the second-instar larva of P. vitellinae was not affected by UV-B treatment of S. myrsinifolia, but was retarded on UV-B treated leaves of S. phylicifolia. In addition, naturally occurring insect herbivores were more abundant on willows exposed to elevated UV-B radiation compared to those grown under control treatments. In spite of the increased abundance of insect herbivores, willows treated with elevated UV-B did not suffer more herbivore damage than willows exposed to ambient solar radiation (shade control). The observed effects of UV-B on herbivore abundance, feeding and growth varied significantly due to spatial variation in environment quality, as indicated by the UV-treatment x block interaction. The results suggest that (1) environmental variation modifies the effects of UV-B radiation on plant-insect interactions and (2) specialist herbivores might be more sensitive to chemical changes in their secondary host plants (S. phylicifolia) than to changes in their primary hosts (S. myrsinifolia).

Plant growth and productivity are strongly constrained by herbivorous insects, which reduce both yield and quality. Over the past two centuries, extensive efforts have been devoted to identifying natural insect‐resistant traits and genes that have evolved in plants. Here, we first review key milestones and advances, including empirical observation, mechanism onset, theoretical emergence, gene identification, and breeding application. Then, we categorize research on plant defenses for insect resistance into four major types: physical, chemical, ecological modulation, and behavioral defenses, and integrate them to highlight the coordination and synergy among different defenses. Finally, we propose priority areas and future directions for research to advance studies on plant–insect interactions and support effective and sustainable pest management.

All species of plants and animals occur over a finite area of the Earth’s surface. This is referred to as the species range, and many species ranges have shifted or are predicted to shift with climate change. Scientific models have predicted how these shifts are expected to change and what proportion of the implicated species will go extinct in the process. Most models assume that climatic variables such as temperature and rainfall are solely responsible for these range shifts. However, we know that the success of a species is strongly influenced by both their positive and negative interactions with other species, such as competition, mutualism, predation and herbivory. But how these biotic factors affect species ranges is poorly understood. I am using a field experiment on a species in its native habitat to better understand these interactions. My study took place in the Canadian Rocky Mountains on populations of the plant Yellow Rattle (Rhinanthus minor). I studied two transects, each with plant populations at low, mid and high elevations. Insect herbivory on plant populations was observed, as well as manipulated, via a pesticide treatment to reduce insect herbivory, and a clipping treatment to simulate natural insect herbivory. Understanding herbivory and herbivore-plant interactions over an elevational gradient may help give us a clearer idea of the complex relationship between the climatic and biotic factors that affect plant species ranges.

Plants play an important role in the interactions between insect herbivores and their pathogens. Since the seminal review by Cory and Hoover (2006) on plant-mediated effects on insect-pathogen interactions, considerable progress has been made in understanding the complexity of these tritrophic interactions. Increasing interest in the areas of nutritional and ecological immunology over the last decade have revealed that plant primary and secondary metabolites can influence the outcomes of insect-pathogen interactions by altering insect immune functioning and physical barriers to pathogen entry. Some insects use plant secondary chemicals and nutrients to prevent infections (prophylactic medication) and medicate to limit the severity of infections (therapeutic medication). Recent findings suggest that there may be selectable plant traits that enhance entomopathogen efficacy, suggesting that entomopathogens could potentially impose selection pressure on plant traits that improve both pathogen and plant fitness. Moreover, plants in nature are inhabited by diverse communities of microbes, in addition to entomopathogens, some of which can trigger immune responses in insect herbivores. Plants are also shared by numerous other herbivorous arthropods with different modes of feeding that can trigger different defensive responses in plants. Some insect symbionts and gut microbes can degrade ingested defensive phytochemicals and be orally secreted onto wounded plant tissue during herbivory to alter plant defenses. Since non-entomopathogenic microbes and other arthropods are likely to influence the outcomes of plant-insect-entomopathogen interactions, I discuss a need to consider these multitrophic interactions within the greater web of species interactions.

Herbivores are pervasive, yet their effects on plant fecundity are often variable. One potential source of variation in herbivore impacts results from differing feeding modes of herbivores. We examined the relative importance of inflorescence-feeding insects versus large ungulates in affecting the fecundity of Balsamorhiza sagittata (Asteraceae), a dominant native perennial forb in western Montana, USA. We quantified these effects across four sites that varied in elevation to determine how consistent herbivore impacts were across these divergent sites. Insect herbivores were present in flower heads at all sites but they significantly depressed plant fecundity at two of the four sites. At the two sites where herbivore suppression had significant effects, this treatment let to 1.6 and 3.4-fold increases in seed production, respectively. In contrast, across all sites ungulates had minimal impacts on balsamroot flower and seed production. Seed addition experiments revealed that at some sites and in some years B. sagittata is seed-limited, as there was a positive relationship between seed input and seedling recruitment. Thus, reductions in seed production from heavy insect herbivory may limit balsamroot recruitment in some locations. Overall, results show that inconspicuous insects have stronger effects on balsamroot fecundity than do mammalian herbivores, but the magnitude of negative impacts at both the individual and population-level vary by site.

Plants and herbivorous insects as well as their natural enemies, such as predatory and parasitoid insects, are united by intricate relationships. During the long period of co-evolution with insects, plants developed a wide diversity of features to defence against herbivores and to attract pollinators and herbivores’ natural enemies. The chemical basis of insect-plant interactions is established and many examples are studied, where feeding and oviposition site selection of phytophagous insects are dependent on the plant’s secondary chemistry. However, often overlooked mechanical interactions between insects and plants can be rather crucial. In the context of mechanoecology, the evolution of plant surfaces and insect adhesive pads is an interesting example of competition between insect attachment systems and plant anti-attachment surfaces. The present review is focused on mechanical insect-plant interactions of some important pest species, such as the polyphagous Southern Green Stinkbug Nezara viridula and two frugivorous pest species, the polyphagous Mediterranean fruit fly Ceratitis capitata and the monophagous olive fruit fly Bactrocera oleae. Their ability to attach to plant surfaces characterised by different features such as waxes and trichomes is discussed. Some attention is paid also to Coccinellidae, whose interaction with plant leaf surfaces is substantial across all developmental stages in both phytophagous and predatory species that feed on herbivorous insects. Finally, the role of different kinds of anti-adhesive nanomaterials is discussed. They can reduce the attachment ability of insect pests to natural and artificial surfaces, potentially representing environmental friendly alternative methods to reduce insect pest impact in agriculture.

5 - Climate change alters host tree physiology and drives plant-insect interactions in forests of the southwestern United States of America

The biotic resistance hypothesis is a dominant paradigm for why some introduced species fail to become invasive in novel environments. However, predictions of this hypothesis require further empirical field tests. Here, we focus on evaluating two biotic factors known to severely limit plants, interspecific competition and insect herbivory, as mechanisms of biotic resistance. We experimentally evaluated the independent and combined effects of three levels of competition by tallgrass prairie vegetation and two levels of herbivory by native insects on seedling regeneration, size, and subsequent flowering of the Eurasian Cirsium vulgare, a known invasive species elsewhere, and compared its responses to those of the ecologically similar and co-occurring native congener C. altissimum. Seedling emergence of C. vulgare was greater than that of C. altissimum, and that emergence was reduced by the highest level of interspecific competition. Insect leaf herbivory was also greater on C. vulgare than on C. altissimum at all levels of competition. Herbivory on seedlings dramatically decreased the proportion of C. vulgare producing flower heads at all competition levels, but especially at the high competition level. Competition and herbivory interacted to significantly decrease plant survival and biomass, especially for C. vulgare. Thus, both competition and herbivory limited regeneration of both thistles, but their effects on seedling emergence, survival, size and subsequent reproduction were greater for C. vulgare than for C. altissimum. These results help explain the unexpectedly low abundance recorded for C. vulgare in western tallgrass prairie, and also provide strong support for the biotic resistance hypothesis.

Merging molecular and ecological approaches in plant–insect interactions

Predicting interactions between drought and plant–insect interactions has been a challenge. Currently, we are unable to accurately predict herbivore abundance on stressed plants despite over 500 publications and half a dozen formal hypotheses. With drought predicted to increase in severity with climate change, determining herbivore abundance on stressed plants is critical for continued agricultural and natural system management. During drought, plants increase concentrations of nutrients and also suffer from water loss. Many empirical studies test drought on plants using severe continuous stress, but studies suggest drought may benefit herbivores when it is intermittent (pulsed). In our study, we tested intermittent and severe stress on herbivore and arthropod abundance in a cotton agro-ecosystem. Our goal was to determine how these types of drought influence herbivore abundance on stressed plants and the relationship between herbivore abundance and stress-related nutrients. We found that intermittent and severe water-deficit stress had different effects on insect herbivores, signifying that drought severity influences herbivore abundance. Piercing–sucking herbivores such as thrips, stink bugs, and leafhoppers were more abundant on intermittently stressed plants than on severely stressed plants. Drought did not significantly affect chewing herbivores, and their abundance was inconsistent on stressed and well-watered plants. Furthermore, nutrient concentrations were similar between stressed and unstressed plants, but herbivore abundance was greater on intermittently stressed plants, suggesting that other physiological characteristics of stressed plants and herbivore feeding ecology must be considered. Our study suggests that the drought severity must be considered when predicting herbivore abundance on stressed plants.

Plant–insect interactions, which are strongly mediated by chemical defenses, have the potential to shape invasion dynamics. Despite this, few studies have quantified natural variation in key defensive compounds of invasive plant populations, or how those defenses relate to levels of herbivory. Here, we evaluated variation in the iridoid glycosides aucubin and catalpol in rosette plants of naturally occurring, introduced populations of the North American invader, Verbascum thapsus L. (common mullein; Scrophulariaceae). We examined two scales that are likely to structure interactions with insect herbivores—among populations and within plant tissues (i.e., between young and old leaves). We additionally estimated the severity of damage incurred at these scales due to insect chewing herbivores (predominantly grasshoppers and caterpillars), and evaluated the relationship between iridoid glycoside content and leaf damage. We found significant variation in iridoid glycoside concentrations among populations and between young and old leaves, with levels of herbivory strongly tracking leaf-level investment in defense. Specifically, across populations, young leaves were highly defended by iridoids (averaging 6.5× the concentration present in old leaves, and containing higher proportions of the potentially more toxic iridoid, catalpol) and suffered only minimal damage from generalist herbivores. In contrast, old leaves were significantly less defended and accordingly more substantially utilized. These findings reveal that quantitative variation in iridoid glycosides is a key feature explaining patterns of herbivory in an introduced plant. In particular, these data support the hypothesis that defenses limit the ability of generalists to feed on mullein’s well-defended young leaves, resulting in minimal losses of high-quality tissue, and increasing performance of this introduced species.

Coevolution between plants and insects is a relatively frequent phenomenon, in part because of the enormous species diversity of both groups. It is usually diffuse, resulting in variability and radiation, and is presumed to result in unstable relationships. Coevolutionary relationships mainly occur between plants and herbivorous insects and between plants and pollinating insects. The mechanisms by which plants and insects interact and perceive each other are diverse but can be broadly split into two categories: chemical and physical. Insects use a combination of chemical and physical cues to detect and discriminate between plants, whereas plants manipulate both of these to either deter or attract insect visitors. These cues can be used either honestly (as warning of potential defences or to advertise offered rewards) or deceptively. Key Concepts Plants and insects have coexisted and interacted for more than 350 million years. There is a huge diversity of plant–insect interactions; the equally vast diversity of plant and animal species is thought to be due in part to the selective pressures arising from these interactions. Mechanisms by which plant–insect interactions occur include both physical and chemical. Multiple physical and chemical mechanisms may be involved in an individual interaction. Selection has therefore occurred on multiple aspects of both plants and insects, including their morphology, biochemistry, development, sensory systems and life cycles. Plant–insect relationships are not stable; mutualisms can shift to the detriment of either of the partners.

Global warming not only alters phenology but also nutritional quality and defense compounds in plants, which consequently hinders their defense against herbivorous insects. In this study, the performance of Spodoptera litura was analyzed to observe the effects of high temperatures on chemical-based defense in plants in the context of insect–plant interaction. Results show that high temperature reduced the nutritional value and content of defense compounds in the foliage of yellow cress (Rorippa dubia). These alterations negatively affected the performance of second instar S. litura larvae feeding on plants grown at high temperature. Low quality of the food source is likely the key cause of slow development of larvae. Adaptation of herbivorous insects known as compensatory feeding is projected resulting in more crop losses under global warming. Our data reveal temperature-induced reduction in the content of defensive compounds (in constitutive resistance) along with lower response capability against herbivore attacks (in induced resistance), which indicate a decrease in plant fitness. High temperatures caused by global warming negatively affect crop production and are expected to increase the burden on plant protection practices.