Jeffrey Harvey

Cultivated plants are often much more susceptible to insect herbivores than wild-type plants. In addressing this observation, much attention has focused on a trait-based approach, and especially on how artificial selection via domestication has modified morphological and chemical traits, in particular levels of defensive secondary metabolites. However, larger scale ecological processes, such as the spatial distribution and diversity of species in a plant community, also determine how insects locate and exploit their food plants, and these differ profoundly between natural and agricultural ecosystems. In this paper we discuss these two approaches to better understand differences in levels of insect herbivory between agricultural and natural ecosystems. We argue that studies investigating the effects of secondary metabolites on insect herbivory are compromised by the methodological approach that is often used. Insect feeding assays testing the effect of reduced concentrations of secondary metabolites in domesticated plants rely on testing a limited subset of insect species, usually those that can easily be reared in the laboratory and often are agricultural pest species. The responses of these insects do not reflect the full range of responses of the species present in the plant's natural habitat. This may explain why reduced levels of secondary metabolites in crop plants may only partially explain increased susceptibility to herbivory. Hypotheses explaining larger scale patterns of insect herbivore abundance are often based on studies in agricultural settings. In our opinion, developing broad ecological hypotheses based on studies in agricultural systems do not necessarily apply to natural systems and vice versa. To fully understand how susceptibility or resistance to insect herbivory is affected by plant traits and habitat heterogeneity, these have to be studied together in both natural and agricultural settings.

Temperature is a major driver of species interactions as it determines many physiological and behavioural parameters of ectothermic organisms such as insects. Examining the effects of elevated temperature and extreme temperature events within and between different trophic levels is crucial for understanding their broader implications for community and ecosystem level processes. We compared parasitism success of two hymenopteran parasitoid species, Diadegma semiclausum and Cotesia vestalis, under different temperature regimes when foraging intra- and interspecifically. Both parasitoid species can be found in the same habitat and are important biological control agents of the cosmopolitan lepidopteran pest Plutella xylostella, the host species in this study. Because parasitoid density may influence parasitism success through interference competition, we first investigated the effect of parasitoid density (one to four females of the same species) on parasitism success at 22 °C. In all assays, parasitoid females were released in cages with a single plant infested with 30 hosts placed in a greenhouse or climate cabinets set at 22, 27 or 33 °C and removed after 3 h. All cages were returned to 22 °C until pupation of the parasitoids or hosts, which were then counted. When females of the same species foraged together, parasitism success increased with parasitoid density. However, when both species were foraging together, parasitism success of D. semiclausum decreased with increasing temperature at both tested densities, whereas the opposite was found for C. vestalis. Nevertheless, parasitism success of D. semiclausum was always higher than that of C. vestalis, irrespective of parasitoid density or temperature, but competitive superiority of D. semiclausum decreased with increasing temperature. Increases in the magnitude and frequency of extreme temperature events under climate change are likely to have differential effects on species involved in intimate interactions, depending on community species composition, as species may differ in thermal resilience.

Parasitoid wasps are important components of insect food chains and have played a central role in biological control programs for over a century. Although the vast majority of parasitoids exploit insect herbivores as hosts, others parasitize predatory insects and arthropods, such as ladybird beetles, hoverflies, lacewings, ground beetles, and spiders, or are hyperparasitoids. Much of the research on the biology and ecology of parasitoids of predators has focused on ladybird beetles, whose parasitoids may interfere with the control of insect pests like aphids by reducing ladybird abundance. Alternatively, parasitoids of the invasive ladybird Harmonia axyridis may reduce its harmful impact on native ladybird populations. Different life stages of predatory insects and spiders are susceptible to parasitism to different degrees. Many parasitoids of predators exhibit intricate physiological interrelationships with their hosts, adaptively manipulating host behavior, biology, and ecology in ways that increase parasitoid survival and fitness.

Soils contain biotic and abiotic legacies of previous conditions that may influence plant community biomass and associated aboveground biodiversity. However, little is known about the relative strengths and interactions of the various belowground legacies on aboveground plant–insect interactions. We used an outdoor mesocosm experiment to investigate the belowground legacy effects of range-expanding versus native plants, extreme drought and their interactions on plants, aphids and pollinators. We show that plant biomass was influenced more strongly by the previous plant community than by the previous summer drought. Plant communities consisted of four congeneric pairs of natives and range expanders, and their responses were not unanimous. Legacy effects affected the abundance of aphids more strongly than pollinators. We conclude that legacies can be contained as soil ‘memories’ that influence aboveground plant community interactions in the next growing season. These soil-borne ‘memories’ can be altered by climate warming-induced plant range shifts and extreme drought.

Beneficial soil microbes can enhance plant growth and defense, but the extent to which this occurs depends on the availability of resources, such as water and nutrients. However, relatively little is known about the role of light quality, which is altered during shading, resulting a low red: far-red ratio (R:FR) of light. We examined how low R:FR light influences arbuscular mycorrhizal fungus (AMF)-mediated changes in plant growth and defense using Solanum lycopersicum (tomato) and the insect herbivore Chrysodeixis chalcites. We also examined effects on third trophic level interactions with the parasitoid Cotesia marginiventris. Under low R:FR light, non-mycorrhizal plants activated the shade avoidance syndrome (SAS), resulting in enhanced biomass production. However, mycorrhizal inoculation decreased stem elongation in shaded plants, thus counteracting the plant’s SAS response to shading. Unexpectedly, activation of SAS under low R:FR light did not increase plant susceptibility to the herbivore in either non-mycorrhizal or mycorrhizal plants. AMF did not significantly affect survival or growth of caterpillars and parasitoids but suppressed herbivore-induced expression of jasmonic acid-signaled defenses genes under low R:FR light. These results highlight the context-dependency of AMF effects on plant growth and defense and the potentially adverse effects of AMF under shading.

Ecosystem engineers are species that manipulate the physical state of ecosystems and thereby affect the behaviour and ecology of other species. Mature larvae of the parsnip webworm, Depressaria radiella Goeze, chew holes in the hollow stems of Heracleum sphondylium L. into which they pupate. The stems are separated into several compartments that are separated by filamentous membranes. Holes excavated by webworm larvae attract several beneficial species of arthropods that use them for shelter in autumn, including the common earwig (Forficula auricularia L.) and the common rough woodlouse (Porcellio scaber Latreille). If artificially made holes mimic the engineering effect of D. radiella, they could be made to attract (local) communities of beneficial arthropods and perhaps facilitate overwintering habitat. We conducted a field experiment to determine whether artificial holes perforated into stem compartments of H. sphondylium mimic the natural situation with D. radiella. At five sites near the city of Leiden, the Netherlands, H. sphondylium plants were exposed to different treatments: a single hole perforated in the first, second or third stem compartment, or in all three compartments. After 3 weeks, arthropod numbers were counted inside and around hogweed stems. The arthropod community in the stems differed from that surrounding the stems; the latter consisted mainly of woodlice and wolf spiders, whereas in the stems, in addition to woodlice, many earwigs were found and no wolf spiders. Both artificial and webworm-excavated holes that were present at one site were used by woodlice and earwigs. The position of the holes along the stem did not affect the number of arthropods found in that segment, although the arthropods exhibited a tendency to move up the stems. The results show that artificial holes mimic webworm-excavated holes in that both attract the same species of beneficial arthropods.

The larvae of insect herbivores feed on plants that may vary nutritionally (qualitatively and/or quantitatively) over the course of insect development. Plant quality may change in response to interactions with the biotic environment that in turn may affect development and biomass of the insects feeding on these plants. However, the larvae of many gregariously feeding herbivores feed on comparatively small plants with limited biomass and may also experience variation in the quantity of plant food available. Pieris brassicae L. (Lepidoptera: Pieridae) is a gregarious butterfly species laying clutches of 10–150 eggs that are often laid on small brassicaceous food plants, including the plant used in this study, Brassica nigra L. (Brassicaceae). A single B. nigra plant provides insufficient resources for the development of an entire brood of P. brassicae. In this study, we investigated the effect of both plant quality and quantity on the performance of P. brassicae when feeding on B. nigra plants. When we compared the effects of changes in plant quality induced by (1) aphid infestation, (2) exposure to pathogenic and non-pathogenic bacteria, and (3) inbreeding depression, which are all biotic stresses known to change plant quality, pupal mass and larval development time of P. brassicae were fairly similar. We then examined the effects of quantitative food constraints during immature development on pupal mass, which correlated strongly with adult size, longevity, and fecundity. Female pupal mass, longevity and fecundity were negatively correlated with the duration of starvation during larval development. No significant effect of male starvation was found on female reproduction and longevity. Thus, P. brassicae larvae were more affected by quantitative than by qualitative constraints in terms of pupal mass, which strongly correlated with female reproduction.

How a society relates to nature is shaped by the dominant social paradigm (DSP): a society’scollective view on social, economic, political, and environmental issues. The characteristics of the DSP have important consequences for natural systems and their conservation. Based on a synthesis of academic literature, we provide a new gradient of 12 types of human-nature relationships synthesized from scientific literature, and an analysis of where the DSP of industrialized, and more specifically, neoliberal societies fit on that gradient. We aim to answer how the industrialized DSP relates to nature, i.e., what types of human-nature relationships this DSP incorporates, and what the consequences of these relationships are for nature conservation and a sustainable future. The gradient of human-nature relationships is based on three defining characteristics: (1) a nature-culture divide, (2) core values, and (3) being anthropocentric or ecocentric. We argue that the industrialized DSP includes elements of the anthropocentric relationships of mastery, utilization, detachment, and stewardship. It therefore regards nature and culture as separate, is mainly driven by instrumental values, and drives detachment from and commodification of nature. Consequently, most green initiatives and policies driven by an industrialized and neoliberal DSP are based on economic incentives and economic growth, without recognition of the needs and limits of natural systems. This leads to environmental degradation and social inequality, obstructing the path to a truly sustainable society. To reach a more ecocentric DSP, systemic changes, in addition to individual changes, in the political and economic structures of the industrialized DSP are needed, along with a change in values and approach toward nature, long-term sustainability, and conservation. Key Words: conservation; dominant social paradigm; environmental degradation; human-nature relationships; industrialized society; Sustainability

Interactions with soil microbes can strongly affect plant growth and defense against aboveground herbivores. Plant species often accumulate specific soil pathogens in their rhizosphere, leading to reduced growth of plants in soils originating from stands of conspecific plants compared to soils from heterospecific plants. However, whereas effects of such conspecific vs. heterospecific soil biota on plant growth have been well documented, their effects on plant resistance and tolerance to aboveground insect herbivores have not. We compared growth and defense of Triadica sebifera plants from populations where the species is native (China), when grown in sterilized soils, or in soils harbouring belowground biota from conspecific (native Triadica) or heterospecific (native grass) soils. In each of these soils, plants were exposed to a 15-day period of foliar herbivory by a specialist weevil (Heterapoderopsis bicallosicollis), a specialist caterpillar (Gadirtha inexacta), or no herbivory (cage), followed by a 60-day recovery period. Soil biota from conspecific and hetetospecific soils differed in their effects on plant growth and defense. First, in the absence of herbivory, soil biota from heterospecific soils slightly enhanced plant growth, whereas those from conspecific soils strongly reduced plant growth. Second, soil biota from conspecific soils strongly affected plant resistance and tolerance to foliar herbivory, whereas soil biota from heterospecific plants did not. The effects of soil biota on plant defense were herbivore-specific. In particular, conspecific soil biota reduced resistance to caterpillar but not to weevil feeding, whereas they enhanced tolerance to weevil but not to caterpillar feeding. Conspecific soil biota also mitigated induction of root flavonoids by herbivores and led to reduced root phenolics in response to herbivory. Conversely, caterpillar feeding increased AMF colonization, but under these conditions, AMF colonization was negatively associated with plant biomass. In addition to testing effects on native plants, we also tested effects of native soil biota on growth and resistance of plants from the introduced range (North America). Plants from the introduced range had higher shoot production, shoot-to-root ratio, and leaf phenolic and flavonoid production than plants from the native range, but their interactions with soil biota showed only minor differences compared to plants from the native range. Our results suggest that incorporating the effects of soil biota in interactions between plants and foliar herbivores is critical for understanding variations in growth, defense, and performance among plant populations at local and broader geographic scales.

Embedded in longer term warming are extreme climatic events such as heatwaves and droughts that are increasing in frequency, duration and intensity. Changes in climate attributes such as temperature are often measured over larger spatial scales, whereas environmental conditions to which many small ectothermic arthropods are exposed are largely determined by small-scale local conditions. Exposed edges of plant patches often exhibit significant short-term (daily) variation to abiotic factors due to wind exposure and sun radiation. By contrast, within plant patches, abiotic conditions are generally much more stable and thus less variable. Over an eight-week period in the summer of 2020, including an actual heatwave, we measured small-scale (1 m²) temperature variation in patches of forbs in experimental mesocosms. We found that soil surface temperatures at the edge of the mesocosms were more variable than those within mesocosms. Drought treatment two years earlier, amplified this effect but only at the edges of the mesocosms. Within a plant patch both at the soil surface and within the canopy, the temperature was always lower than the ambient air temperature. The temperature of the soil surface at the edge of a patch may exceed the ambient air temperature when ambient air temperatures rise above 23 °C. This effect progressively increased with ambient temperature. We discuss how microscale-variation in temperature may affect small ectotherms such as insects that have limited ability to thermoregulate, in particular under conditions of extreme heat.

Species responding differently to climate change form ‘transient communities’, communities with constantly changing species composition due to colonization and extinction events. Our goal is to disentangle the mechanisms of response to climate change for terrestrial species in these transient communities and explore the consequences for biodiversity conservation. We review spatial escape and local adaptation of species dealing with climate change from evolutionary and ecological perspectives. From these we derive species vulnerability and management options to mitigate effects of climate change. From the perspective of transient communities, conservation management should scale up static single species approaches and focus on community dynamics and species interdependency, while considering species vulnerability and their importance for the community. Spatially explicit and frequent monitoring is vital for assessing the change in communities and distribution of species. We review management options such as: increasing connectivity and landscape resilience, assisted colonization, and species protection priority in the context of transient communities.

Aphids are serious pests of many crops in agroecosystems and their biological control is focused on enhancing the performance of specialized natural enemies of aphids such as parasitoid wasps and predators like ladybirds. However, ladybirds are often attacked by their own parasitoids in the fourth trophic level that can negatively affect ladybird performance and, hence, their effectiveness as control agents. The biology and ecology of these parasitoids has been less well explored. This study compared various life-history traits in two closely related parasitoids of the seven-spot ladybird, Coccinella septempunctata (Coleoptera: Coccinellidae): Oomyzus scaposus and O. spiraculus (Hymenoptera: Eulophidae), which naturally co-occur in eastern China and are facultatively gregarious koinobiont larval-pupal endoparasitoids of ladybird beetles. They can oviposit and develop in all four larval instars of their hosts but kill and emerge from the pupae. Both parasitoids did not have a clear oviposition preference for any host instar, but oviposition duration tended to increase in first to third instar hosts. Moreover, oviposition time was significantly longer for O. spiraculus than for O. scaposus. Adult eclosion from parasitized hosts (ranging between 35 and 45%) and sex ratios (85–90% female) were similar in both species and did not differ among host instars. Brood sizes were similar in both species but tended to increase in first to third instar hosts. Egg-to-adult development time was shorter and the eclosing adults of O. scaposus were heavier than those of O. spiraculus. In both species, development time decreased with host instar at parasitism, but instar-specific effects on biomass differed between the two species: heavier O. scaposus adults developed from second and third instar hosts, whereas biomass of for O. spiraculus tended to decrease with instar at parasitism. In both species, females were significantly larger than males. Our results show that expression of some of the life-history traits vary depending on which instar is parasitized, but do not point at a specific instar optimal for each of the parasitoids. Moreover, despite being closely related, there is some variability in the expression of life-history traits in both parasitoids.

Insects are among the most diverse and widespread animals across the biosphere and are well-known for their contributions to ecosystem functioning and services. Recent increases in the frequency and magnitude of climatic extremes (CE), in particular temperature extremes (TE) owing to anthropogenic climate change, are exposing insect populations and communities to unprecedented stresses. However, a major problem in understanding insect responses to TE is that they are still highly unpredictable both spatially and temporally, which reduces frequency- or direction-dependent selective responses by insects. Moreover, how species interactions and community structure may change in response to stresses imposed by TE is still poorly understood. Here we provide an overview of how terrestrial insects respond to TE by integrating their organismal physiology, multitrophic, and community-level interactions, and building that up to explore scenarios for population explosions and crashes that have ecosystem-level consequences. We argue that TE can push insect herbivores and their natural enemies to and even beyond their adaptive limits, which may differ among species intimately involved in trophic interactions, leading to phenological disruptions and the structural reorganization of food webs. TE may ultimately lead to outbreak–breakdown cycles in insect communities with detrimental consequences for ecosystem functioning and resilience. Lastly, we suggest new research lines that will help achieve a better understanding of insect and community responses to a wide range of CE.

BACKGROUND: Anthropogenic climate change (ACC) may have significant impacts on insect herbivore communities including pests. Two of the most important climate-change related factors are increased atmospheric concentrations of carbon dioxide (CO ₂), and increasing mean global temperature. Although increasing attention is being paid to the biological and ecological effects of ACC, important processes such as interspecific interaction between insect herbivores have been little explored. Here, in a field experiment using the FACE (free-air CO ₂ enrichment) system, we investigated the effect of elevated CO ₂ and temperature on survival and wing dimorphism of two species of rice planthoppers, Laodelphax striatellus and Nilaparvata lugens under interaction. RESULTS: The two species were grouped into five treatments of relative density (0/50, 13/37, 25/25, and 37/13, 50/0), each of which was allocated to one of a factorial combination of two CO ₂ concentrations and two temperature treatments (elevated and ambient levels). Our results revealed that climatic treatment has no effects on survivorship of interspecific competing planthoppers. However, climatic treatment affected wing-form of planthoppers under interspecific interaction. For females of N. lugens, in the 37/13 ratio, proportion macropterours form was lower under elevated CO ₂ + temperature than under the ambient environment or than under elevated temperature. For females of L. striatellus, proportion macropterous form did not differ among climatic treatments at each ratio treatment. CONCLUSION: These findings illustrate that climate change-related factors, by affecting the macropetry of interspecific competing planthoppers, may influence planthopper fitness. We provide new information that could assist with forecasting outbreaks of these migratory pests.

Climate extremes are expected to become more commonplace and more severe, putting species and ecosystems at unprecedented risks. We recommend that rewilding programs can create conditions for ecosystems to endure and recover rapidly from climate extremes by incorporating ecosystem engineers of various body sizes and life forms.

Among parasitoids that develop inside the bodies of feeding, growing hosts (so-called 'koinobiont' endoparasitoids), two strategies have evolved to dispose of host resources. The larvae of one group consumes most host tissues before pupation, whereas in the other the parasitoid larvae consume only host hemolymph and fat body and at maturity emerge through the host cuticle to pupate externally. Here we compared development and survival (to adult emergence) of two related larval endoparasitoids (Braconidae: Microgastrinae) of the diamondback moth, Plutella xylostella. Larvae of Dolichogenidea sicaria are tissue feeders whereas larvae of Cotesia vestalis are hemolymph feeders. Here, development of P. xylostella and the two parasitoids was compared on three populations (one cultivar [Cyrus], two wild, [Winspit and Kimmeridge]) of cabbage that have been shown to vary in direct defense and hence quality. Survival of P. xylostella and C. vestalis (to adult eclosion) did not vary with cabbage population, but did so in D. sicaria, where survival was lower when reared on the wild populations than on the cultivar. Furthermore, adult herbivore mass was significantly higher and development was significantly shorter in moths reared on the cultivar. The tissue-feeing D. sicaria was larger but took longer to develop than the hemolymph-feeder C. vestalis. The performance of both parasitoids was better on the cabbage cultivar than on the wild populations, although the effects were less apparent than in the host. Our results show that (1) differences in plant quality are diffused up the food chain, and (2) the effects of host quality are reflected on the development of both parasitoids.