Heat stress and host-parasitoid interactions: lessons and opportunities in a changing climate

Ongoing climate change is increasing the frequency and magnitude of high-temperature events (HTEs), causing heat stress in parasitoids and their hosts. We argue that HTEs and heat stress should be viewed in terms of the intersecting life cycles of host and parasitoid. Recent studies illustrate how the biological consequences of a given HTE may vary dramatically depending on its timing within these lifecycles. The temperature sensitivity of host manipulation by parasitoids, and by viral endosymbionts of many parasitoids, can contribute to differing responses of hosts and parasitoids to HTEs. In some cases, these effects can result in reduced parasitoid success and increased host herbivory and may disrupt the ecological interactions between hosts and parasitoids. Because most studies to date involve endoparasitoids of aphid or lepidopteran hosts in agricultural systems, our understanding of heat responses of host-parasitoid interactions in natural systems is quite limited.

Dispersal stabilizes coupled ecological and evolutionary dynamics in a host-parasitoid system

When ecological and evolutionary dynamics occur on comparable timescales, persistence of the ensuing eco-evolutionary dynamics requires both ecological and evolutionary stability. This unites key questions in ecology and evolution: How do species coexist, and what maintains genetic variation in a population? In this work, we investigated a host-parasitoid system in which pea aphid hosts rapidly evolve resistance to Aphidius ervi parasitoids. Field data and mathematical simulations showed that heterogeneity in parasitoid dispersal can generate variation in parasitism-mediated selection on hosts through time and space. Experiments showed how evolutionary trade-offs plus moderate host dispersal across this selection mosaic cause host-parasitoid coexistence and maintenance of genetic variation in host resistance. Our results show how dispersal can stabilize both the ecological and evolutionary components of eco-evolutionary dynamics.

Multidecadal, continent-level analysis indicates agricultural practices impact wheat aphid loads more than climate change

Temperature has a large influence on insect abundances, thus under climate change, identifying major drivers affecting pest insect populations is critical to world food security and agricultural ecosystem health. Here, we conducted a meta-analysis with data obtained from 120 studies across China and Europe from 1970 to 2017 to reveal how climate and agricultural practices affect populations of wheat aphids. Here we showed that aphid loads on wheat had distinct patterns between these two regions, with a significant increase in China but a decrease in Europe over this time period. Although temperature increased over this period in both regions, we found no evidence showing climate warming affected aphid loads. Rather, differences in pesticide use, fertilization, land use, and natural enemies between China and Europe may be key factors accounting for differences in aphid pest populations. These long-term data suggest that agricultural practices impact wheat aphid loads more than climate warming.

Hormesis and insects: Effects and interactions in agroecosystems

Insects in agroecosystems contend with many stressors - e.g., chemicals, heat, nutrient deprivation - that are often encountered at low levels. Exposure to mild stress is now well known to induce hormetic (stimulatory) effects in insects, with implications for insect management, and ecological structure and function in agroecosystems. In this review, we examine the major ecological niches insects occupy or guilds to which they belong in agroecosystems and how hormesis can manifest within and across these groups. The mechanistic underpinnings of hormesis in insects are starting to become established, explaining the many phenotypic hormetic responses observed in insect reproduction, development, and behavior. Whereas potential effects on insect populations are well supported in laboratory experiments, field-based hypothesis-driven research on hormesis is greatly lacking. Furthermore, because most ecological paradigms are founded within the context of communities, entomological agroecologists interested in hormesis need to 'level up' and test hypotheses that explore effects on species interactions, and community structure and functioning. Embedded in this charge is to continue experimentation on herbivorous pest species while shifting more focus towards insect natural enemies, pollinators, and detritivores - guilds that play crucial roles in highly functioning agroecosystems that have been understudied in hormesis research. Important areas for future insect agroecology research on hormesis are discussed.

Towards Predictions of Interaction Dynamics between Cereal Aphids and Their Natural Enemies: A Review

Simple Summary

Understanding how pests and their natural enemies interact dynamically during the growing season and what drivers act on those interactions will help to develop efficient pest control strategies. We reviewed empirical and modeling publications on the drivers influencing the aphids–natural enemy dynamics. We found disparities between what is known empirically and what is used as main drivers in the models. Predation and parasitism are rarely measured empirically but are often represented in models, while plant phenology is supposed to be a strong driver of aphids’ dynamics while it is rarely used in models. Since modelers and empirical scientists do not share a lot of publications, we incite more crossover works between both communities to elaborate (i) new empirical settings based on simulation results and (ii) build more accurate and robust models integrating more key drivers of the aphid dynamics. These models could be integrated into decision support systems to help advisors and farmers to design more effective integrated pest management systems.

Abstract

(1) Although most past studies are based on static analyses of the pest regulation drivers, evidence shows that a greater focus on the temporal dynamics of these interactions is urgently required to develop more efficient strategies. (2) Focusing on aphids, we systematically reviewed (i) empirical knowledge on the drivers influencing the dynamics of aphid–natural enemy interactions and (ii) models developed to simulate temporal or spatio-temporal aphid dynamics. (3) Reviewed studies mainly focus on the abundance dynamics of aphids and their natural enemies, and on aphid population growth rates. The dynamics of parasitism and predation are rarely measured empirically, although it is often represented in models. Temperature is mostly positively correlated with aphid population growth rates. Plant phenology and landscape effects are poorly represented in models. (4) We propose a research agenda to progress towards models and empirical knowledge usable to design effective CBC strategies. We claim that crossover works between empirical and modeling community will help design new empirical settings based on simulation results and build more accurate and robust models integrating more key drivers of aphid dynamics. Such models, turned into decision support systems, are urgently needed by farmers and advisors in order to design effective integrated pest management.

Oscillation, synchrony, and multi-factor patterns between cereal aphids and parasitoid populations in southern Brazil

In different parts of the world, aphid populations and their natural enemies are influenced by landscapes and climate. In the Neotropical region, few long-term studies have been conducted, maintaining a gap for comprehension of the effect of meteorological variables on aphid population patterns and their parasitoids in field conditions. This study describes the general patterns of oscillation in cereal winged aphids and their parasitoids, selecting meteorological variables and evaluating their effects on these insects. Aphids exhibit two annual peaks, one in summer-fall transition and the other in winter-spring transition. For parasitoids, the highest annual peak takes place during winter and a second peak occurs in winter-spring transition. Temperature was the principal meteorological regulator of population fluctuation in winged aphids and parasitoids during the year. The favorable temperature range is not the same for aphids and parasitoids. For aphids, temperature increase resulted in population growth, with maximum positive effect at 25°C. Temperature also positively influenced parasitoid populations, but the growth was asymptotic around 20°C. Although rainfall showed no regulatory function on aphid seasonality, it influenced the final number of insects over the year. The response of aphids and parasitoids to temperature has implications for trophic compatibility and regulation of their populations. Such functions should be taken into account in predictive models.

The abundance and diversity of fruit flies and their parasitoids change with elevation in guava orchards in a tropical Andean forest of Peru, independent of seasonality

Lower elevations are generally thought to contain a greater abundance and diversity of insect communities and their natural enemies than higher elevations. It is less clear, however, how changes in seasons influence this pattern. We conducted a 2-year study (2013‒2014) in guava orchards located in a tropical Andean forest of Peru to investigate differences in fruit flies (Diptera: Tephritidae) and their parasitoid communities at two elevations and over two seasons. Fruit fly traps were installed, monitored, and guava fruits were sampled from eight orchards at low (800–950 m above sea level) and high (1,700–1,900 m above sea level) elevations and during the dry and rainy seasons. At each orchard, adult fruit fly trap captures and emergence of fruit flies and their parasitoids from guava fruit were quantified to determine their abundance and species composition. There was a greater abundance and species richness of fruit flies captured in traps at lower elevations, as well as higher abundance and species evenness of fruit flies that emerged from fruit, indicating that lower elevations are associated with larger fruit fly populations. The abundance, species richness and diversity of parasitoids were also greater at lower elevations. Consequently, guava fruit infestation and fruit fly parasitism rates were also greater at lower elevations. Seasonality also influenced fruit fly populations with a greater number of flies emerging from guava fruit and more fruit infested in the rainy season. However, seasonality had no effect on parasitoid population parameters or rate of parasitism, nor did it interact with elevation as an influence of populations of fruit flies or their parasitoids in guava orchards. This study highlights the importance of examining both elevation and seasonality for a better understanding of the population dynamics of fruit flies and their parasitoids in tropical agroecosystems.

Warm temperatures increase population growth of a nonnative defoliator and inhibit demographic responses by parasitoids

Changes in thermal regimes that disparately affect hosts and parasitoids could release hosts from biological control. When multiple natural enemy species share a host, shifts in host-parasitoid dynamics could depend on whether natural enemies interact antagonistically vs. synergistically. We investigated how biotic and abiotic factors influence the population ecology of larch casebearer (Coleophora laricella), a nonnative pest, and two imported parasitoids, Agathis pumila and Chrysocharis laricinellae, by analyzing (1) temporal dynamics in defoliation from 1962 to 2018, and (2) historical, branch-level data on densities of larch casebearer and parasitism rates by the two imported natural enemies from 1972 to 1995. Analyses of defoliation indicated that, prior to the widespread establishment of parasitoids (1962 to ~1980), larch casebearer outbreaks occurred in 2-6 yr cycles. This pattern was followed by a >15-yr period during which populations were at low, apparently stable densities undetectable via aerial surveys, presumably under control from parasitoids. However, since the late 1990s and despite the persistence of both parasitoids, outbreaks exhibiting unstable dynamics have occurred. Analyses of branch-level data indicated that growth of casebearer populations, A. pumila populations, and within-casebearer densities of C. laricinellae-a generalist whose population dynamics are likely also influenced by use of alternative hosts-were inhibited by density dependence, with high intraspecific densities in one year slowing growth into the next. Casebearer population growth was also inhibited by parasitism from A. pumila, but not C. laricinellae, and increased with warmer autumnal temperatures. Growth of A. pumila populations and within-casebearer densities of C. laricinellae increased with casebearer densities but decreased with warmer annual maximum temperatures. Moreover, parasitism by A. pumila was associated with increased growth of within-casebearer densities of C. laricinellae without adverse effects on its own demographics, indicating a synergistic interaction between these parasitoids. Our results indicate that warming can be associated with opposing effects between trophic levels, with deleterious effects of warming on one natural enemy species potentially being exacerbated by similar impacts on another. Coupling of such parasitoid responses with positive responses of hosts to warming might have contributed to the return of casebearer outbreaks to North America.

Temperature alters the shape of predator-prey cycles through effects on underlying mechanisms

Background

Predicting the effects of climate warming on the dynamics of ecological systems requires understanding how temperature influences birth rates, death rates and the strength of species interactions. The temperature dependance of these processes—which are the underlying mechanisms of ecological dynamics—is often thought to be exponential or unimodal, generally supported by short-term experiments. However, ecological dynamics unfold over many generations. Our goal was to empirically document shifts in predator–prey cycles over the full range of temperatures that can possibly support a predator–prey system and then to uncover the effect of temperature on the underlying mechanisms driving those changes.

Methods

We measured the population dynamics of the Didinium-Paramecium predator–prey system across a wide range of temperatures to reveal systematic changes in the dynamics of the system. We then used ordinary differential equation fitting to estimate parameters of a model describing the dynamics, and used these estimates to assess the long-term temperature dependance of all the underlying mechanisms.

Results

We found that predator–prey cycles shrank in state space from colder to hotter temperatures and that both cycle period and amplitude varied with temperature. Model parameters showed mostly unimodal responses to temperature, with one parameter (predator mortality) increasing monotonically with temperature and one parameter (predator conversion efficiency) invariant with temperature. Our results indicate that temperature can have profound, systematic effects on ecological dynamics, and these can arise through diverse and simultaneous changes in multiple underlying mechanisms. Predicting the effects of temperature on ecological dynamics may require additional investigation into how the underlying drivers of population dynamics respond to temperature beyond a short-term, acute response.

Timing alters how a heat shock affects a host-parasitoid interaction

Abiotic factors' effects on species are now well-studied, yet they are still often difficult to predict, especially for strongly interacting species. If these altered abiotic factors and species interactions occur as discrete events in time, such complications may occur because of the events' relative timing. One such discrete abiotic factor is the short-duration, large magnitude increase in temperature called a heat shock. This study investigates how the timing of heat shocks affects the successful attack and reproduction of a parasitoid wasp (Aphidius ervi) attacking its host, the pea aphid (Acyrthosiphon pisum). We tested three relative timings: 1) heat shock before the wasp attacks hosts, 2) heat shock while the wasp is foraging, and 3) heat shock after the wasp has attacked hosts. In each scenario we compared wasp mummy production (pupal stage) with and without a heat shock. Our results showed that a heat shock had the largest effect when it occurred while wasps actively foraged, with fewer mummies produced when exposed to a heat shock compared to the no heat shock control. Follow-up behavioral tests suggest this was caused by wasps becoming inactive during heat shocks. In contrast, when heat shocks were applied three days before or after foraging, we found no difference in mummy production between the heat shock treatment and no heat shock control. These results show the potential importance of timing when considering the ramifications of an altered abiotic factor, especially with relatively discrete abiotic events and interactions.

Perceptions of Climate Change and Drivers of Insect Pest Outbreaks in Vegetable Crops in Limpopo Province of South Africa

Vegetable production is a source of income for smallholder farmers in Limpopo Province, South Africa. Vegetable production is constrained by the negative impacts of climate change and pests. This study assessed farmers’ awareness of climate change, farmers’ knowledge of insect pests and factors that influence insect pests’ prevalence. The data were collected using quantitative and qualitative methods. The data were subjected to descriptive and bivariate analysis. About 84.5% of smallholder farmers were aware of climate change. Late rainfall (24.4%), long dry spells (15%) and increased drought frequency (19.4%) were highlighted as dominant indicators of climate change by farmers. Aphids (22.2%), Bagrada hilaris (12.5%) and Spodoptera frugiperda (10.2%) were the most prevalent insect pests within the Vhembe District. Warmer winters, dry spells and high temperatures were perceived by farmers to influence insect pests’ prevalence within the district. It can be concluded that farmers are aware of climate change and climatic factors influencing pest prevalence within the district. Pest risk maps are needed to improve the preparedness of the government and farmers in controlling insect pests under changing climates.

Relative importance of long-term changes in climate and land-use on the phenology and abundance of legume crop specialist and generalist aphids

Insect populations are prone to respond to global changes through shifts in phenology, distribution and abundance. However, global changes cover several factors such as climate and land-use, the relative importance of these being largely unknown. Here, we aim at disentangling the effects of climate, land-use, and geographical drivers on aphid abundance and phenology in France, at a regional scale and over the last 40 years. We used aerial data obtained from suction traps between 1978 and 2015 on five aphid species varying in their degree of specialization to legumes, along with climate, legume crop area and geographical data. Effects of environmental and geographical variables on aphid annual abundance and spring migration dates were analyzed using generalized linear mixed models. We found that within the last four decades, aphids have advanced their spring migration by a month, mostly due to the increase in temperature early in the year, and their abundance decreased by half on average, presumably in response to a combination of factors. The influence of legume crop area decreased with the degree of specialization of the aphid species to such crops. The effect of geographical variation was high even when controlling for environmental variables, suggesting that many other spatially structured processes act on aphid population characteristics. Multifactorial analyses helped to partition the effects of different global change drivers. Climate and land-use changes have strong effects on aphid populations, with important implications for future agriculture. Additionally, trait-based response variation could have major consequences at the community scale.

A matrix model describing host-parasitoid population dynamics: The case of Aphelinus certus and soybean aphid

Integrating elements from life tables into population models within a matrix framework has been an underutilized method of describing host-parasitoid population dynamics. This type of modeling is useful in describing demographically-structured populations and in identifying points in the host developmental timeline susceptible to parasitic attack. We apply this approach to investigate the effect of parasitism by the Asian parasitoid Aphelinus certus on its host, the soybean aphid (Aphis glycines). We present a matrix population model with coupled equations that are analogous to a Nicholson-Bailey model. To parameterize the model, we conducted several bioassays outlining host and parasitoid life history and supplemented these studies with data obtained from the literature. Analysis of the model suggests that, at a parasitism rate of 0.21 d-1, A. certus is capable of maintaining aphid densities below economically damaging levels in 31.0% of simulations. Several parameters-parasitoid lifespan, colonization timeline, host developmental stage, and mean daily temperature-were also shown to markedly influence the overall dynamics of the system. These results suggest that A. certus might provide a valuable service in agroecosystems by suppressing soybean aphid populations at relatively low levels of parasitism. Our results also support the use of A. certus within a dynamic action threshold framework in order to maximize the value of biological control in pest management programs.

Abrupt Change in Ecological Systems: Inference and Diagnosis

Abrupt ecological changes are, by definition, those that occur over short periods of time relative to typical rates of change for a given ecosystem. The potential for such changes is growing due to anthropogenic pressures, which challenges the resilience of societies and ecosystems. Abrupt ecological changes are difficult to diagnose because they can arise from a variety of circumstances, including rapid changes in external drivers (e.g., climate, or resource extraction), nonlinear responses to gradual changes in drivers, and interactions among multiple drivers and disturbances. We synthesize strategies for identifying causes of abrupt ecological change and highlight instances where abrupt changes are likely. Diagnosing abrupt changes and inferring causation are increasingly important as society seek to adapt to rapid, multifaceted environmental changes.

Structural dynamics in the host-parasitoid system of the pine needle gall midge (Thecodiplosis japonensis) during invasion

The structural dynamics of host–parasitoid populations play a key role in the mechanism of natural community development with invasive species. Species invading new habitats experience coevolution with their newly acquired natural enemies, and their population dynamics are driven by a complex interaction between biological and environmental factors. We examined the biological and environmental factors which potentially influence a community of parasitoids throughout the 25-year invasion history of the pine needle gall midge (PNGM), Thecodiplosis japonensis, an important pest of pines in eastern Asia. We found that differences in establishment sequence and competitive ability among the parasitoids attacking this species determined the parasitoid community’s structure and dynamics. In particular, the timing for the initial establishment of the host–parasitoid association, incomplete superiority in competition among parasitoids, and indirect competition by a combination of the parasitoids were important factors for determining community’s structure and dynamics. Finally, the history of change in the community composition could be explained by the phenology differences in its member species, mediated by environmental factors.