Climate Change and Tritrophic Interactions: Will Modifications to Greenhouse Gas Emissions Increase the Vulnerability of Herbivorous Insects to Natural Enemies?

Air pollution disproportionately impairs beneficial invertebrates: a meta-analysis

Air pollution has the potential to disrupt ecologically- and economically-beneficial services provided by invertebrates, including pollination and natural pest regulation. To effectively predict and mitigate this disruption requires an understanding of how the impacts of air pollution vary between invertebrate groups. Here we conduct a global meta-analysis of 120 publications comparing the performance of different invertebrate functional groups in unpolluted and polluted atmospheres. We focus on the pollutants ozone, nitrogen oxides, sulfur dioxide and particulate matter. We show that beneficial invertebrate performance is reduced by air pollution, whereas the performance of plant pest invertebrates is not significantly affected. Ozone pollution has the most detrimental impacts, and these occur at concentrations below national and international air quality standards. Changes in invertebrate performance are not dependent on air pollutant concentrations, indicating that even low levels of pollution are damaging. Predicted increases in tropospheric ozone could result in unintended consequences to global invertebrate populations and their valuable ecological services.

Becoming nose-blind-Climate change impacts on chemical communication

Chemical communication via infochemicals plays a pivotal role in ecological interactions, allowing organisms to sense their environment, locate predators, food, habitats, or mates. A growing number of studies suggest that climate change-associated stressors can modify these chemically mediated interactions, causing info-disruption that scales up to the ecosystem level. However, our understanding of the underlying mechanisms is scarce. Evidenced by a range of examples, we illustrate in this opinion piece that climate change affects different realms in similar patterns, from molecular to ecosystem-wide levels. We assess the importance of different stressors for terrestrial, freshwater, and marine ecosystems and propose a systematic approach to address highlighted knowledge gaps and cross-disciplinary research avenues.

Climate Change and Major Pests of Mediterranean Olive Orchards: Are We Ready to Face the Global Heating?

Simple Summary

The phenomenon of climate change affects the entire world, especially the most vulnerable areas such as the Mediterranean. Since the olive growing is one of the main economic sources for the Mediterranean countries, investigations on olive pests under global heating are necessary. Nowadays, knowledge on the topic is scarce, and nothing is known about the effects of climate change on olive pest parasitoids and predators. This information could be fundamental to understand the phenomena of pest outbreaks that are spreading in the Mediterranean olive orchards. The use of prevention tools (e.g., monitoring, prediction models) may help in controlling olive pests under a climate change scenario.

Abstract

Evidence of the impact of climate change on natural and agroecosystems is nowadays established worldwide, especially in the Mediterranean Basin, an area known to be very susceptible to heatwaves and drought. Olea europaea is one of the main income sources for the Mediterranean agroeconomy, and it is considered a sensitive indicator of the climate change degree because of the tight relationship between its biology and temperature trend. Despite the economic importance of the olive, few studies are nowadays available concerning the consequences that global heating may have on its major pests. Among the climatic parameters, temperature is the key one influencing the relation between the olive tree and its most threatening parasites, including Bactrocera oleae and Prays oleae. Therefore, several prediction models are based on this climatic parameter (e.g., cumulative degree day models). Even if the use of models could be a promising tool to improve pest control strategies and to safeguard the Mediterranean olive patrimony, they are not currently available for most O. europaea pests, and they have to be used considering their limits. This work stresses the lack of knowledge about the biology and the ethology of olive pests under a climate change scenario, inviting the scientific community to focus on the topic.

The Impact of Climate Change on Agricultural Insect Pests

Climate change and global warming are of great concern to agriculture worldwide and are among the most discussed issues in today's society. Climate parameters such as increased temperatures, rising atmospheric CO2 levels, and changing precipitation patterns have significant impacts on agricultural production and on agricultural insect pests. Changes in climate can affect insect pests in several ways. They can result in an expansion of their geographic distribution, increased survival during overwintering, increased number of generations, altered synchrony between plants and pests, altered interspecific interaction, increased risk of invasion by migratory pests, increased incidence of insect-transmitted plant diseases, and reduced effectiveness of biological control, especially natural enemies. As a result, there is a serious risk of crop economic losses, as well as a challenge to human food security. As a major driver of pest population dynamics, climate change will require adaptive management strategies to deal with the changing status of pests. Several priorities can be identified for future research on the effects of climatic changes on agricultural insect pests. These include modified integrated pest management tactics, monitoring climate and pest populations, and the use of modelling prediction tools.

Responses of forest insect pests to climate change: not so simple

Climate change is a multi-faceted phenomenon, including elevated CO₂, warmer temperatures, more severe droughts and more frequent storms. All these components can affect forest pests directly, or indirectly through interactions with host trees and natural enemies. Most of the responses of forest insect herbivores to climate change are expected to be positive, with shorter generation time, higher fecundity and survival, leading to increased range expansion and outbreaks. Forest insect pest can also benefit from synergistic effects of several climate change pressures, such as hotter droughts or warmer storms. However, lesser known negative effects are also likely, such as lethal effects of heat waves or thermal shocks, less palatable host tissues or more abundant parasitoids and predators. The complex interplay between abiotic stressors, host trees, insect herbivores and their natural enemies makes it very difficult to predict overall consequences of climate change on forest health. This calls for the development of process-based models to simulate pest population dynamics under climate change scenarios.

Effects of Host Plants Reared under Elevated CO2 Concentrations on the Foraging Behavior of Different Stages of Corn Leaf Aphids Rhopalosiphum maidis

Climate change is a major environmental concern and is directly related to the increasing concentrations of greenhouse gases. The increase in concentrations of atmospheric carbon dioxide (CO2), not only affects plant growth and development, but also affects the emission of plant organic volatile compounds (VOCs). Changes in the plant odor profile may affect the plant-insect interactions, especially the behavior of herbivorous insects. In this study, we compared the foraging behavior of corn leaf aphid (Rhopalosiphum maidis) on barley (Hordeum vulgare L.) seedlings grown under contrasted CO2 concentrations. During the dual choice bioassays, the winged and wingless aphids were more attracted by the VOCs of barley seedlings cultivated under ambient CO2 concentrations (aCO2; 450 ppm) than barley seedlings cultivated under elevated CO2 concentrations (eCO2; 800 ppm), nymphs were not attracted by the VOCs of eCO2 barley seedlings. Then, volatile compositions from 14-d-old aCO2 and eCO2 barley seedlings were investigated by GC-MS. While 16 VOCs were identified from aCO2 barley seedlings, only 9 VOCs were found from eCO2 barley seedlings. At last, we discussed the potential role of these chemicals observed during choice bioassays. Our findings lay foundation for functional response of corn leaf aphid under climate change through host plant modifications.

Elevated CO₂ Concentrations Impact the Semiochemistry of Aphid Honeydew without Having a Cascade Effect on an Aphid Predator

Honeydew is considered a cornerstone of the interactions between aphids and their natural enemies. Bacteria activity occurring in aphid honeydew typically results in the release of volatile organic compounds (VOCs) that are used by the natural enemies of aphids to locate their prey. Because atmospheric carbon dioxide (CO₂) concentration directly impacts the physiology of plants, we raise the hypothesis that elevated CO₂ concentrations impact the quantity of honeydew produced by aphids, as well as the diversity and quantity of honeydew VOCs, leading to cascade effects on the foraging behavior of aphids' natural enemies. Using solid-phase microextraction, we analyzed the VOCs emitted by honeydew from pea aphids (Acyrthosiphon pisum Harris) reared under 450 ± 50 ppm of CO₂ (aCO₂) or 800 ± 50 ppm CO₂ (eCO₂). While the total amount of honeydew excreted was only slightly reduced by eCO₂ concentrations, we detected qualitative and quantitative differences in the semiochemistry of aphid honeydew between CO₂ conditions. Three VOCs were not found in the honeydew of eCO₂ aphids: 3-methyl-2-buten-1-ol, 2-methyl-1-butanol, and isobutanol. However, no difference was observed in the searching and oviposition behaviors of hoverfly (Episyrphus balteatus (De Geer)) females exposed to plants covered with honeydew originating from the different CO₂ conditions. The present work showed the effect of a particular aspect of atmospheric changes, and should be extended to other abiotic parameters, such as temperature.

Combined effects of environmental disturbance and climate warming on insect herbivory in mountain birch in subarctic forests: Results of 26-year monitoring

Both pollution and climate affect insect-plant interactions, but the combined effects of these two abiotic drivers of global change on insect herbivory remain almost unexplored. From 1991 to 2016, we monitored the population densities of 25 species or species groups of insects feeding on mountain birch (Betula pubescens ssp. czerepanovii) in 29 sites and recorded leaf damage by insects in 21 sites in subarctic forests around the nickel-copper smelter at Monchegorsk, north-western Russia. The leaf-eating insects demonstrated variable, and sometimes opposite, responses to pollution-induced forest disturbance and to climate variations. Consequently, we did not discover any general trend in herbivory along the disturbance gradient. Densities of eight species/species groups correlated with environmental disturbance, but these correlations weakened from 1991 to 2016, presumably due to the fivefold decrease in emissions of sulphur dioxide and heavy metals from the smelter. The densities of externally feeding defoliators decreased from 1991 to 2016 and the densities of leafminers increased, while the leaf roller densities remained unchanged. Consequently, no overall temporal trend in the abundance of birch-feeding insects emerged despite a 2-3°C elevation in spring temperatures. Damage to birch leaves by insects decreased during the observation period in heavily disturbed forests, did not change in moderately disturbed forests and tended to increase in pristine forests. The temporal stability of insect-plant interactions, quantified by the inverse of the coefficient of among-year variations of herbivore population densities and of birch foliar damage, showed a negative correlation with forest disturbance. We conclude that climate differently affects insect herbivory in heavily stressed versus pristine forests, and that herbivorous insects demonstrate diverse responses to environmental disturbance and climate variations. This diversity of responses, in combination with the decreased stability of insect-plant interactions, increases the uncertainty in predictions on the impacts of global change on forest damage by insects.

Elevated carbon dioxide reduces emission of herbivore-induced volatiles in Zea mays

Terpene volatiles produced by sweet corn (Zea mays) upon infestation with pests such as beet armyworm (Spodoptera exigua) function as part of an indirect defence mechanism by attracting parasitoid wasps; yet little is known about the impact of climate change on this form of plant defence. To investigate how a central component of climate change affects indirect defence, we measured herbivore-induced volatile emissions in plants grown under elevated carbon dioxide (CO₂ ). We found that S. exigua infested or elicitor-treated Z. mays grown at elevated CO₂ had decreased emission of its major sesquiterpene, (E)-β-caryophyllene and two homoterpenes, (3E)-4,8-dimethyl-1,3,7-nonatriene and (3E,7E)-4,8,12-trimethyl-1,3,7,11-tridecatetraene. In contrast, inside the leaves, elicitor-induced (E)-β-caryophyllene hyper-accumulated at elevated CO₂ , while levels of homoterpenes were unaffected. Furthermore, gene expression analysis revealed that the induction of terpene synthase genes following treatment was lower in plants grown at elevated CO₂ . Our data indicate that elevated CO₂ leads both to a repression of volatile synthesis at the transcriptional level and to limitation of volatile release through effects of CO₂ on stomatal conductance. These findings suggest that elevated CO₂ may alter the ability of Z. mays to utilize volatile terpenes to mediate indirect defenses.

Warming Alters Prey Density and Biological Control in Conventional and Organic Agricultural Systems

SYNOPSIS

Studies have shown that organically farmed fields promote natural predator populations and often have lower pest populations than conventional fields, due to a combination of increased predation pressure and greater plant resistance to pest damage. It is unknown how pest populations and predator efficacy may respond in these farming systems as global temperatures increase. To test these questions, we placed enclosures in eight alfalfa fields farmed using conventional (n = 4) or organic (n = 4) practices for 25 years. We stocked enclosures with pea aphids and 0, 2, or 4 predaceous ladybeetles. Half of the enclosures per field were then either left at ambient temperature or plastic-wrapped to warm them by 2 °C. Aphid abundances were similar in conventional and organic fields under ambient conditions, but were significantly more abundant in conventional than in organic fields when enclosures were warmed. Predator efficacy was reduced under low predator abundance (Hippodamia convergens = 2) in conventional fields under warming conditions; predation strength in organic fields was unaffected by warming. Alfalfa biomass increased with increased predators in all farming and temperature treatments. Our study suggests that biological control may be more easily maintained in organic than in conventional systems as global temperature increases.

Can plant-natural enemy communication withstand disruption by biotic and abiotic factors?

The attraction of natural enemies towards herbivore-induced plant volatiles is a well-documented phenomenon. However, the majority of published studies are carried under optimal water and nutrient regimes and with just one herbivore. But what happens when additional levels of ecological complexity are added? Does the presence of a second herbivore, microorganisms, and abiotic stress interfere with plant-natural enemy communication? or is communication stable enough to withstand disruption by additional biotic and abiotic factors?Investigating the effects of these additional levels of ecological complexity is key to understanding the stability of tritrophic interactions in natural ecosystems and may aid to forecast the impact of environmental disturbances on these, especially in climate change scenarios, which are often associated with modifications in plant and arthropod species distribution and increased levels of abiotic stress.This review explores the literature on natural enemy attraction to herbivore-induced volatiles when, besides herbivory, plants are challenged by additional biotic and abiotic factors.The aim of this review was to establish the impact of different biotic and abiotic factors on plant-natural enemy communication and to highlight critical aspects to guide future research efforts.

Climate Change, Nutrition, and Bottom-Up and Top-Down Food Web Processes

Climate change ecology has focused on climate effects on trophic interactions through the lenses of temperature effects on organismal physiology and phenological asynchronies. Trophic interactions are also affected by the nutrient content of resources, but this topic has received less attention. Using concepts from nutritional ecology, we propose a conceptual framework for understanding how climate affects food webs through top-down and bottom-up processes impacted by co-occurring environmental drivers. The framework integrates climate effects on consumer physiology and feeding behavior with effects on resource nutrient content. It illustrates how studying responses of simplified food webs to simplified climate change might produce erroneous predictions. We encourage greater integrative complexity of climate change research on trophic interactions to resolve patterns and enhance predictive capacities.

Will climate change affect insect pheromonal communication?

Understanding how climate change will affect species interactions is a challenge for all branches of ecology. We have only limited understanding of how increasing temperature and atmospheric CO₂ and O₃ levels will affect pheromone-mediated communication among insects. Based on the existing literature, we suggest that the entire process of pheromonal communication, from production to behavioural response, is likely to be impacted by increases in temperature and modifications to atmospheric CO₂ and O₃ levels. We argue that insect species relying on long-range chemical signals will be most impacted, because these signals will likely suffer from longer exposure to oxidative gases during dispersal. We provide future directions for research programmes investigating the consequences of climate change on insect pheromonal communication.