Rapid increase in dispersal during range expansion in the invasive ladybird Harmonia axyridis

Phylogenetic and population genetic analyses of Thrips tabaci Lindeman (Thysanoptera: Thripidae) on Allium host in India

Background

Onion thrips (Thrips tabaci) is a complex of cryptic species with subtle morphological differences and distinct genetic backgrounds; thus, species identification using traditional methods remains challenging. The existence of different haplotypes and genotypes within a species can significantly influence various aspects of its biology, including host preference, reproductive capacity, resistance to pesticides, and vector competence for plant viruses. Understanding the genetic diversity and population structure of cryptic species within T. tabaci will not only aid in the development of more effective control strategies tailored to specific genetic variants but also in monitoring population dynamics, tracking invasive species, and implementing quarantine measures to prevent the spread of economically damaging thrips biotypes.

Methods

This study aims to explore intraspecies genetic diversity and molecular evolutionary relationships of the mitochondrial cytochrome oxidase gene subunit I (mtCOI) in T. tabaci populations from India. To capture diversity within the Indian T. tabaci populations, amplicon sequencing was performed for the thrips mtCOI gene from eight diverse localities in India. A total of 48 sequences retrieved for the mtCOI gene from the NCBI Nucleotide database were analysed.

Results

Multiple insertions and deletions were detected at various genomic positions across the populations from different localities, with the highest variation observed in the 300-400 genome position range. Molecular diversity analyses identified 30 haplotypes within the population, with certain subpopulations exhibiting higher gene flow. Analysis of single nucleotide polymorphism patterns within the mtCOI gene across diverse Indian locales revealed significant intrapopulation genetic heterogeneity and its potential repercussions on gene functionality. Elevated F statistics (Fst) values in the northern-western subpopulations suggested high genetic variability, particularly evident in haplotype networks originating mainly from the northern region, notably Delhi. While most populations displayed stable and ancient evolutionary histories, thrips populations from northern, western, and north-eastern regions indicated rapid growth.

Evaluation of the Potential Flight Ability of the Casuarina Moth, Lymantria xylina (Lepidoptera: Erebidae)

Lymantria xylina Swinhoe (Lepidoptera: Erebidae) is a potentially invasive pest, similar to Lymantria dispar asiatica Vnukovskij and Lymantria dispar japonica Motschulsky (Lepidoptera: Erebidae). To evaluate its potential for spread and flight distance related to egg deposition on vessels at ports, we employed a flight mill to assess the flight capabilities of its adults under varying conditions. Our findings revealed that females primarily flew short distances and ceased flying after 3:00 AM, whereas males covered much longer distances throughout the day. Sex, age, and flight duration significantly influenced flight ability. Females exhibited weaker flight capability than males, and their ability declined with increasing age or flight duration. Notably, 1-day-old moths displayed the strongest flight ability, with average flight distances of up to 3.975 km for females and 8.441 km for males. By the fifth day, females no longer flew, and males experienced reduced flight ability. After continuous hanging for 16 h, females lost most of their flight capacity, while males remained capable of flight even after 32 h. Additionally, female flight ability decreased significantly after mating, possibly due to factors such as egg-carrying capacity, weight, and load ratio. This study provides a foundation for assessing the risk of long-distance dispersal of L. xylina via ocean-going freighters, considering female moths' phototactic flight and oviposition.

What makes a great invader? Anatomical traits as predictors of locomotor performance and metabolic rate in an invasive frog

Invasive species are characterized by their ability to establish and spread in a new environment. In alien populations of anurans, dispersal and fitness-related traits such as endurance, burst performance and metabolism are key to their success. However, few studies have investigated inter-individual variation in these traits and more specifically have attempted to understand the drivers of variation in these traits. Associations of anatomical features may be excellent predictors of variation in performance and could be targets for selection or subject to trade-offs during invasions. In this study, we used marsh frogs (Pelophylax ridibundus), a species that has been introduced in many places outside its native range and which is now colonizing large areas of Western Europe. We first measured the inter-individual variation in resting metabolism, the time and distance they were able to jump until exhaustion, and their peak jump force, and then measured the mass of specific organs and lengths of body parts suspected to play a role in locomotion and metabolism. Among the 5000 bootstrap replicates on body size-corrected variables, our statistical models most often selected the stomach (75.42%), gonads (71.46%) and the kidneys (67.26%) as predictors of inter-individual variation in metabolism, and the gluteus maximus muscle (97.24%) mass was the most frequently selected predictor of jump force. However, endurance was poorly associated with the anatomical traits (R2distance=0.42, R2time=0.37). These findings suggest that selection on these predictors may lead to physiological changes that may affect the colonization, establishment and dispersal of these frogs.

Decoupling pioneering traits from latitudinal patterns in a north American bird experiencing a southward range shift

Ecogeographic rules describe spatial patterns in biological trait variation and shed light on the drivers of such variation. In animals, a consensus is emerging that 'pioneering' traits may facilitate range shifts via a set of bold, aggressive and stress-resilient traits. Many of these same traits are associated with more northern latitudes, and most range shifts in the northern hemisphere indicate northward movement. As a consequence, it is unclear whether pioneering traits are simply corollaries of existing latitudinal variation, or whether they override other well-trodden latitudinal patterning as a unique ecogeographic rule of phenotypic variation. The tree swallow Tachycineta bicolor is a songbird undergoing a southward range shift in the eastern United States, in direct opposition of the poleward movement seen in most other native species' range shifts. Because this organic range shift countervails the typical direction of movement, this case study provides for unique ecological insights on organisms and their ability to thrive in our changing world. We sampled female birds across seven populations, quantifying behavioural, physiological and morphological traits. We also used GIS and field data to quantify a core set of ecological factors with strong ties to these traits as well as female performance. Females at more southern expansion sites displayed higher maternal aggression, higher baseline corticosterone and more pronounced elevation of corticosterone following a standardized stressor, contrary to otherwise largely conserved latitudinal patterning in these traits. Microhabitat variation explained some quantitative phenotypic variation, but the expansion and historic ranges did not differ in openness, distance to water or breeding density. This countervailing range shift therefore suggests that pioneering traits are not simply corollaries of existing latitudinal variation, but rather, they may override other well-trodden latitudinal patterning as a unique ecogeographic rule of phenotypic variation.

Global invasion history and native decline of the common starling: insights through genetics

Few invasive birds are as globally successful as the Common or European Starling (Sturnus vulgaris). Native to the Palearctic, the starling has been intentionally introduced to North and South America, South Africa, Australia, and the Pacific Islands, enabling us to explore species traits that may contribute to its invasion success. Coupling the rich studies of life history and more recent explorations of genomic variation among invasions, we illustrate how eco-evolutionary dynamics shape the invasion success of this long-studied and widely distributed species. Especially informative is the comparison between Australian and North American invasions, because these populations colonized novel ranges concurrently and exhibit shared signals of selection despite distinct population histories. In this review, we describe population dynamics across the native and invasive ranges, identify putatively selected traits that may influence the starling’s spread, and suggest possible determinants of starling success world-wide. We also identify future opportunities to utilize this species as a model for avian invasion research, which will inform our understanding of species’ rapid evolution in response to environmental change.

Travelling with a parasite: the evolution of resistance and dispersal syndromes during experimental range expansion

Rapid evolutionary change during range expansions can lead to diverging range core and front populations, with the emergence of dispersal syndromes (coupled responses in dispersal and life-history traits). Besides intraspecific effects, range expansions may be impacted by interspecific interactions such as parasitism. Yet, despite the potentially large impact of parasites imposing additional selective pressures on the host, their role on range expansions remains largely unexplored. Using microcosm populations of the ciliate Paramecium caudatum and its bacterial parasite Holospora undulata, we studied experimental range expansions under parasite presence or absence. We found that the interaction of range expansion and parasite treatments affected the evolution of host dispersal syndromes. Namely, front populations showed different associations of population growth parameters and swimming behaviours than core populations, indicating divergent evolution. Parasitism reshaped trait associations, with hosts evolved in the presence of the parasite exhibiting overall increased resistance and reduced dispersal. Nonetheless, when comparing infected range core and front populations, we found a positive association, suggesting joint evolution of resistance and dispersal at the front. We conclude that host-parasite interactions during range expansions can change evolutionary trajectories; this in turn may feedback on the ecological dynamics of the range expansion and parasite epidemics.

Morphological Variation in Bumblebees (Bombus terrestris) (Hymenoptera: Apidae) After Three Decades of an Island Invasion

Introduced social insects can be highly invasive outside of their native range. Around the world, the introduction and establishment of the eusocial bumblebee Bombus terrestris (L. 1758) (Hymenoptera: Apidae) has negatively impacted native pollinators and ecosystems. Understanding how morphological variation is linked to environmental variation across invasive ranges can indicate how rapidly species may be diverging or adapting across novel ranges and may assist with predicting future establishment and spread. Here we investigate whether B. terrestris shows morphological variation related to environmental variation across the island of Tasmania (Australia) where it was introduced three decades ago. We collected 169 workers from 16 sites across Tasmania and related relative abundance and morphology to landscape-wide climate, land use, and vegetation structure. We found weak morphological divergence related to environmental conditions across Tasmania. Body size of B. terrestris was positively associated with the percentage of urban land cover, a relationship largely driven by a single site, possibly reflecting high resource availability in urban areas. Proboscis length showed a significant negative relationship with the percentage of pasture. Wing loading and local abundance were not related to the environmental conditions within sites. Our results reflect the highly adaptable nature of B. terrestris and its ability to thrive in different environments, which may have facilitated the bumblebee's successful invasion across Tasmania.

Genetics reveals shifts in reproductive behaviour of the invasive bird parasite Philornis downsi collected from Darwin’s finch nests

Due to novel or dynamic fluctuations in environmental conditions and resources, host and parasite relationships can be subject to diverse selection pressures that may lead to significant changes during and after invasion of a parasite. Genomic analyses are useful for elucidating evolutionary processes in invasive parasites following their arrival to a new area and host. Philornis downsi (Diptera: Muscidae), the avian vampire fly, was introduced to the Galápagos Islands circa 1964 and has since spread across the archipelago, feeding on the blood of developing nestlings of endemic land birds. Since its discovery, there have been significant changes to the dynamics of P. downsi and its novel hosts, such as shifting mortality rates and changing oviposition behaviour, however no temporal genetic studies have been conducted. We collected P. downsi from nests and traps from a single island population over a 14-year period, and genotyped flies at 469 single nucleotide polymorphisms (SNPs) using restriction-site associated DNA sequencing (RADSeq). Despite significant genetic differentiation (FST) between years, there was no evidence for genetic clustering within or across four sampling years between 2006 and 2020, suggesting a lack of population isolation. Sibship reconstructions from P. downsi collected from 10 Darwin’s finch nests sampled in 2020 showed evidence for shifts in reproductive behaviour compared to a similar genetic analysis conducted in 2004–2006. Compared with this previous study, females mated with fewer males, individual females oviposited fewer offspring per nest, but more unique females oviposited per nest. These findings are important to consider within reproductive control techniques, and have fitness implications for both parasite evolution and host fitness.

The fast invasion of Europe by the box tree moth: an additional example coupling multiple introduction events, bridgehead effects and admixture events

Identifying the invasion routes of non-native species is crucial to understanding invasions and customizing management strategies. The box tree moth, Cydalima perspectalis, is native to Asia and was recently accidentally introduced into Europe as a result of the ornamental plant trade. Over the last 15 years, it has spread across the continent and has reached the Caucasus and Iran. It is threatening Buxus trees in both urban areas and forests. To investigate the species’ invasion routes, native and invasive box tree moth populations were sampled, and moth’s genetic diversity and structure were compared using microsatellite markers. Our approximate Bayesian computation analyses strongly suggest that invasion pathways were complex. Primary introductions originating from eastern China probably occurred independently twice in Germany and once in the Netherlands. There were also possibly bridgehead effects, where at least three invasive populations may have served as sources for other invasive populations within Europe, with indication of admixture between the two primary invasive populations. The bridgehead populations were likely those in the countries that play a major role in the ornamental plant trade in Europe, notably Germany, the Netherlands, and Italy. All these invasion processes likely facilitated its fast expansion across Europe and illustrate the role played by the ornamental plant trade not only in the moth’s introduction from China but also in the species’ spread across Europe, leading to an invasion with a complex pattern.

The evolutionary dynamics of biological invasions: A multi-approach perspective

Biological invasions, the establishment and spread of non-native species in new regions, can have extensive economic and environmental consequences. Increased global connectivity accelerates introduction rates, while climate and land-cover changes may decrease the barriers to invasive populations spread. A detailed knowledge of the invasion history, including assessing source populations, routes of spread, number of independent introductions, and the effects of genetic bottlenecks and admixture on the establishment success, adaptive potential, and further spread, is crucial from an applied perspective to mitigate socioeconomic impacts of invasive species, as well as for addressing fundamental questions on the evolutionary dynamics of the invasion process. Recent advances in genomics together with the development of geographic information systems provide unprecedented large genetic and environmental datasets at global and local scales to link population genomics, landscape ecology, and species distribution modeling into a common framework to study the invasion process. Although the factors underlying population invasiveness have been extensively reviewed, analytical methods currently available to optimally combine molecular and environmental data for inferring invasive population demographic parameters and predicting further spreading are still under development. In this review, we focus on the few recent insect invasion studies that combine different datasets and approaches to show how integrating genetic, observational, ecological, and environmental data pave the way to a more integrative biological invasion science. We provide guidelines to study the evolutionary dynamics of invasions at each step of the invasion process, and conclude on the benefits of including all types of information and up-to-date analytical tools from different research areas into a single framework.

A review: Learning from the flight of beetles

Some Coleoptera (popularly referred to as beetles) can fly at a low Reynolds number with their deployable hind wings, which directly enables a low body weight-a good bioinspiration strategy for miniaturization of micro-air vehicles (MAVs). The hind wing is a significant part of the body and has a folding/unfolding mechanism whose unique function benefits from different structures and materials. This review summarizes the actions, factors, and mechanisms of beetle flight and bioinspired MAVs with deployable wings. The elytron controlled by muscles is the protected part for the folded hind wing and influences flight performance. The resilin, the storage material for elasticity, is located in the folding parts. The hind wings' folding/unfolding mechanism and flight performance can be influenced by vein structures of hollow, solid and wrinkled veins, the hemolymph that flows in hollow veins and its hydraulic mechanism, and various mechanical properties of veins. The action of beetle flight includes flapping flight, hovering, gliding, and landing. The hind wing is passively deformed through force and hemolymph, and the attack angle of the hind wing and the nanomechanics of the veins, muscles and mass body determine the flight performance. Based these factors, bioinspired MAVs with a new deployable wing structure and new materials will be designed to be much more effective and miniaturized. The new fuels and energy supply are significant aspects of MAVs.

Interactions Among Native and Non-Native Predatory Coccinellidae Influence Biological Control and Biodiversity

Over the past 30 yr, multiple species of predatory Coccinellidae, prominently Coccinella septempunctata L. and Harmonia axyridis (Pallas) (Coleoptera: Coccinellidae) have spread to new continents, influencing biodiversity and biological control. Here we review the mechanisms underlying these ecological interactions, focusing on multi-year field studies of native and non-native coccinellids and those using molecular and quantitative ecological methods. Field data from Asia show that H. axyridis, C. septempunctata, and Propylea japonica (Thunberg) (Coleoptera: Coccinellidae) are regularly among the most abundant predatory species but their rank varies by habitat. Studies of these species in their native Asian range, primarily related to their range in mainland China, document different patterns of seasonal abundance, species specific associations with prey, and habitat separation. Intraguild predation is well documented both in Asia and in newly invaded areas, and H. axyridis benefits most from this interaction. Harmonia axyridis also seems to rely more on cannibalism in times of prey scarcity than other species, and relatively sparse data indicate a lower predation pressure on it from natural enemies of coccinellids. Declines in the abundance of native coccinellids following the spread and increase of non-native species, documented in several multi-year studies on several continents, is a major concern for native biodiversity and the persistence of native coccinellid species. We suggest that future studies focus more attention on the community ecology of these invasive species in their native habitats.

Environmental correlates of genetic variation in the invasive European starling in North America

Populations of invasive species that colonize and spread in novel environments may differentiate both through demographic processes and local selection. European starlings (Sturnus vulgaris) were introduced to New York in 1890 and subsequently spread throughout North America, becoming one of the most widespread and numerous bird species on the continent. Genome-wide comparisons across starling individuals and populations can identify demographic and/or selective factors that facilitated this rapid and successful expansion. We investigated patterns of genomic diversity and differentiation using reduced-representation genome sequencing of 17 winter-season sampling sites. Consistent with this species' high dispersal rate and rapid expansion history, we found low geographical differentiation and few F_ST outliers even at a continental scale. Despite starting from a founding population of ~180 individuals, North American starlings show only a moderate genetic bottleneck, and models suggest a dramatic increase in effective population size since introduction. In genotype-environment associations we found that ~200 single-nucleotide polymorphisms are correlated with temperature and/or precipitation against a background of negligible genome- and range-wide divergence. Given this evidence, we suggest that local adaptation in North American starlings may have evolved rapidly even in this wide-ranging and evolutionarily young system. This survey of genomic signatures of expansion in North American starlings is the most comprehensive to date and complements ongoing studies of world-wide local adaptation in these highly dispersive and invasive birds.

Philopatry at the frontier: A demographically driven scenario for the evolution of multilevel societies in baboons (Papio)

The baboons (Papio sp.) exhibit marked interspecies variation in social behavior. The thesis presented here argues, first, that male philopatry is a crucial factor, arguably the crucial factor, underlying the other distinctive features (one-male units, multilevel society) shared by hamadryas and Guinea baboons, but not other species of Papio. The second suggestion is that male philopatry as a population norm was not an adaptation to a particular habitat or set of ecological circumstances but evolved in the common ancestor of hamadryas and Guinea baboons as a response to natural selection in the demographic context peculiar to the frontier of a rapidly expanding population. Other derived features of social structure (male-male tolerance, some facultative female dispersal) subsequently evolved to accommodate male philopatry. The mitochondrial genetic population structure of extant baboons preserves a footprint of the initial expansion of 'modern' Papio. Immediately after the expansion, male-philopatric, multilevel populations with a general physical and behavioral resemblance to Guinea baboons occupied the whole northern hemisphere range of the genus. Behavioral and physical autapomorphies of hamadryas baboons evolved in a subpopulation of this ancestral northern base, in response to a less productive habitat of the Horn of Africa. Subsequently, ancestral olive baboons 'reinvented' male dispersal. They and yellow baboons, another male-dispersing species, then replaced most of the male-philopatric northern populations, by male-driven introgression and nuclear swamping.

Insights from Population Genomics to Enhance and Sustain Biological Control of Insect Pests

Biological control-the use of organisms (e.g., nematodes, arthropods, bacteria, fungi, viruses) for the suppression of insect pest species-is a well-established, ecologically sound and economically profitable tactic for crop protection. This approach has served as a sustainable solution for many insect pest problems for over a century in North America. However, all pest management tactics have associated risks. Specifically, the ecological non-target effects of biological control have been examined in numerous systems. In contrast, the need to understand the short- and long-term evolutionary consequences of human-mediated manipulation of biological control organisms for importation, augmentation and conservation biological control has only recently been acknowledged. Particularly, population genomics presents exceptional opportunities to study adaptive evolution and invasiveness of pests and biological control organisms. Population genomics also provides insights into (1) long-term biological consequences of releases, (2) the ecological success and sustainability of this pest management tactic and (3) non-target effects on native species, populations and ecosystems. Recent advances in genomic sequencing technology and model-based statistical methods to analyze population-scale genomic data provide a much needed impetus for biological control programs to benefit by incorporating a consideration of evolutionary consequences. Here, we review current technology and methods in population genomics and their applications to biological control and include basic guidelines for biological control researchers for implementing genomic technology and statistical modeling.

Local dispersal pathways during the invasion of the cactus moth, Cactoblastis cactorum, within North America and the Caribbean

Cactoblastis cactorum, a species of moth native to Argentina, feeds on several prickly pear cactus species (Opuntia) and has been successfully used as a biological control of invading Opuntia species in Australia, South Africa and native ruderal Opuntia species in some Caribbean islands. Since its introduction to the Caribbean its spread was uncontrolled, invading successfully Florida, Texas and Louisiana. Despite this long history of invasion, we are still far from understanding the factors determining the patterns of invasion of Cactoblastis in North America. Here, we explored three non-mutually exclusive explanations: a) a stepping stone model of colonization, b) long distance colonization due to hurricanes, and/or c) hitchhiking through previously reported commercial routes. Genetic diversity, genetic structure and the patterns of migration among populations were obtained by analyzing 10 nuclear microsatellite loci. Results revealed the presence of genetic structure among populations of C. cactorum in the invaded region and suggest that both marine commercial trade between the Caribbean islands and continental USA, as well as recurrent transport by hurricanes, explain the observed patterns of colonization. Provided that sanitary regulations avoiding human-mediated dispersal are enforced, hurricanes probably represent the most important agent of dispersal and future invasion to continental areas.

A Review of the Phenotypic Traits Associated with Insect Dispersal Polymorphism, and Experimental Designs for Sorting out Resident and Disperser Phenotypes

Dispersal represents a key life-history trait with several implications for the fitness of organisms, population dynamics and resilience, local adaptation, meta-population dynamics, range shifting, and biological invasions. Plastic and evolutionary changes of dispersal traits have been intensively studied over the past decades in entomology, in particular in wing-dimorphic insects for which literature reviews are available. Importantly, dispersal polymorphism also exists in wing-monomorphic and wingless insects, and except for butterflies, fewer syntheses are available. In this perspective, by integrating the very latest research in the fast moving field of insect dispersal ecology, this review article provides an overview of our current knowledge of dispersal polymorphism in insects. In a first part, some of the most often used experimental methodologies for the separation of dispersers and residents in wing-monomorphic and wingless insects are presented. Then, the existing knowledge on the morphological and life-history trait differences between resident and disperser phenotypes is synthetized. In a last part, the effects of range expansion on dispersal traits and performance is examined, in particular for insects from range edges and invasion fronts. Finally, some research perspectives are proposed in the last part of the review.

Dispersibility of the Pale Grass Blue Butterfly Zizeeria maha (Lepidoptera: Lycaenidae) Revealed by One-Individual Tracking in the Field: Quantitative Comparisons between Subspecies and between Sexes

The pale grass blue butterfly Zizeeria maha (Lepidoptera: Lycaenidae) has been used as an environmental indicator species for radioactive pollution after the Fukushima nuclear accident. Here, based on the one-individual tracking method in the field, we examined dispersal-associated and other behavioral traits of this butterfly, focusing on two subspecies, Z. maha argia in mainland Japan and Z. maha okinawana in Okinawa. The accumulated distances in the adult lifespan were 18.9 km and 38.2 km in mainland and Okinawa males, respectively, and 15.0 km and 7.8 km in mainland and Okinawa females, respectively. However, the mean distance from the starting point was only 24.2 m and 21.1 m in the mainland and Okinawa males, respectively, and 13.7 m and 7.4 m in the mainland and Okinawa females, respectively. Some quantitative differences in resting and feeding were found between subspecies and between sexes. The ARIMA (autoregressive integrated moving average) model indicated that the dispersal distance was 52.3 m (99% confidence interval value of 706.6 m) from the starting point in mainland males. These results support the idea that despite some behavioral differences, both subspecies of this butterfly are suitable as an environmental indicator because of the small dispersal ranges.

Tradeoffs between dispersal and reproduction at an invasion front of cane toads in tropical Australia

Individuals at the leading edge of a biological invasion experience novel evolutionary pressures on mating systems, due to low population densities coupled with tradeoffs between reproduction and dispersal. Our dissections of >1,200 field-collected cane toads (Rhinella marina) at a site in tropical Australia reveal rapid changes in morphological and reproductive traits over a three-year period after the invaders first arrived. As predicted, individuals with dispersal-enhancing traits (longer legs, narrower heads) had reduced reproductive investment (lower gonad mass). Post-invasion, the population was increasingly dominated by individuals with less dispersive phenotypes and a higher investment into reproduction (including, increased expression of sexually dimorphic traits in males). These rapid shifts in morphology and reproductive biology emphasise the impacts of the invasion process on multiple, interlinked aspects of organismal biology.

Moving On from the Insect Apocalypse Narrative: Engaging with Evidence-Based Insect Conservation

Recent studies showing temporal changes in local and regional insect populations received exaggerated global media coverage. Confusing and inaccurate science communication on this important issue could have counterproductive effects on public support for insect conservation. The insect apocalypse narrative is fuelled by a limited number of studies that are restricted geographically (predominantly the United Kingdom, Europe, the United States) and taxonomically (predominantly some bees, macrolepidoptera, and ground beetles). Biases in sampling and analytical methods (e.g., categorical versus continuous time series, different diversity metrics) limit the relevance of these studies as evidence of generalized global insect decline. Rather, the value of this research lies in highlighting important areas for priority investment. We summarize research, communication, and policy priorities for evidence-based insect conservation, including key areas of knowledge to increase understanding of insect population dynamics. Importantly, we advocate for a balanced perspective in science communication to better serve both public and scientific interests.

Flight capacity increases then declines from the core to the margins of an invasive species' range

Individuals that disperse farther than other individuals are more likely to be on the frontlines of spreading populations and may be more likely to mate with one another as a consequence of their spatial proximity. Over generations, this process-known as spatial sorting-can produce patterns of increasing dispersal ability from a population's core towards the spreading front. By contrast, when the spread of a population is limited by the availability of suitable habitat, theory predicts that range boundaries can select against more dispersive phenotypes and produce patterns of decreasing dispersal capacity towards population margins. In a common garden study of invasive kudzu bugs (Megacopta cribraria)-which are limited by the availability of hostplants in their southern and western margins-I show that midrange individuals fly 49% farther than individuals in the core and 37% farther than individuals at margins. This result highlights that other processes, such as maternal effects or selection at range boundaries, may create more complicated patterns of dispersal ability across landscapes than predicted by models of spatial sorting alone.

The role of dispersal, selection, and timing of sampling on the false discovery rate of loci under selection during geographic range expansion

Identifying adaptive loci is important to understand the evolutionary potential of species undergoing range expansion. However, in expanding populations, spatial demographic processes such as allele surfing can create spatial patterns of neutral genetic variation that appear similar to those generated through adaptive processes. As a result, the false discovery rate of adaptive loci may be inflated in landscape genomic analyses. Here, we take a simulation modelling approach to investigate how range expansion affects our ability to correctly distinguish between neutral and adaptive genetic variation, using the mountain pine beetle outbreak system as a motivating example. We simulated the demographic and population genetic dynamics of populations undergoing range expansion using an individual-based genetic model CDMetaPOP. We investigated how the false discovery rate of adaptive loci is affected by (i) dispersal capacity, (ii) timing of sampling, and (iii) the strength of selection on an adaptive reference locus. We found that a combination of weak dispersal, weak selection, and early sampling presents the greatest risk of misidentifying loci under selection. Expanding populations present unique challenges to the reliable identification of adaptive loci. We demonstrate that there is a need for further methodological development to account for directional demographic processes in landscape genomics.

Pathogens associated with invasive or introduced insects threaten the health and diversity of native species

Insect populations are declining even in protected areas, but the underlying causes are unclear. Here, I consider whether the factors driving the loss of insect diversity include invasive and/or introduced insects transmitting pathogens to less-resistant native species. The introduction of insects into new areas for biocontrol, to promote pollination, or for mass rearing in insect farms, threatens the health and diversity of indigenous insects by the co-introduction of entomopathogens whose spillover is difficult to control. Even less virulent pathogens or covert infections can become lethal if environmental stressors weaken the resistance of indigenous host species in an additive, potentiating or synergistic manner. More research is needed to develop effective strategies that protect the health and diversity of native insects.

Local adaptation, dispersal evolution, and the spatial eco-evolutionary dynamics of invasion

Local adaptation and dispersal evolution are key evolutionary processes shaping the invasion dynamics of populations colonizing new environments. Yet their interaction is largely unresolved. Using a single-species population model along a one-dimensional environmental gradient, we show how local competition and dispersal jointly shape the eco-evolutionary dynamics and speed of invasion. From a focal introduction site, the generic pattern predicted by our model features a temporal transition from wave-like to pulsed invasion. Each regime is driven primarily by local adaptation, while the transition is caused by eco-evolutionary feedbacks mediated by dispersal. The interaction range and cost of dispersal arise as key factors of the duration and speed of each phase. Our results demonstrate that spatial eco-evolutionary feedbacks along environmental gradients can drive strong temporal variation in the rate and structure of population spread, and must be considered to better understand and forecast invasion rates and range dynamics.

Differences in the Phenology of Harmonia axyridis (Coleoptera: Coccinellidae) and Native Coccinellids in Central Europe

Harmonia axyridis (Pallas), an invasive non-native species in central Europe, can outcompete other aphidophagous species. The distribution and abundance of H. axyridis vary depending on different host plants, and its effects on native coccinellid communities may change accordingly. The distribution and abundance of coccinellids in central Europe (50°N, 14°E) were investigated from 2010 to 2016. Coccinellids were counted at regular intervals on cereals (Avena, Hordeum, and Triticum), herbaceous plants (Matricaria and Urtica) and trees (Acer, Betula, and Tilia). Additionally, the occurrence over time of each species on these plants was assessed and used as an index of persistence. Across all years, the adults and larvae of H. axyridis were the dominant species of coccinellid on trees. However, H. axyridis was less abundant on herbaceous plants and cereals than on trees. Populations of native coccinellids and H. axyridis co-occurred on trees and persisted for the same length of time, while native coccinellids persisted longer than H. axyridis on herbaceous plants and cereals. Compared to 1976-1986, in the 2010s, the abundance of native species decreased on all plants by 50-70%. The presence of H. axyridis could be considered as a factor driving changes in the assemblages of native coccinellids.

Propagule pressure and hunting pressure jointly determine genetic evolution in insular populations of a global frog invader

Islands are often considered to be more susceptible to biological invasions and to suffer greater impacts from invaders than mainland areas, and this difference is generally attributed to differences in species introductions, ecological factors or human activities between islands and mainland areas. Genetic variation, as a good estimate of evolutionary potential, can influence the invasion process and impacts of alien species. However, few studies have compared the genetic diversity of alien species between islands and a corresponding mainland. Here, we examined the genetic variation and differentiation in feral populations (30 sampled individuals/population) of a globally invasive species (the American bullfrog, Lithobates catesbeianus) that was extensively farmed on 14 islands in the Zhoushan Archipelago of China and in three nearby regions on the mainland. We quantified the relative importance of propagule pressure and hunting pressures on the genetic variation of bullfrog populations and found that insular populations have greater genetic variation than their mainland counterparts. Although genetic differentiation between the populations was observed, no evidence of recent bottlenecks or population expansion in any of the tested population was found. Our results suggest that the propagule pressures of bullfrogs escaping from farms, multiple releases and hunting pressure influence the genetic variation among bullfrog populations. These results might have important implications for understanding the establishment and evolution of alien species on islands and for the management of invasive species.

The tethered flight technique as a tool for studying life-history strategies associated with migration in insects

1. Every year billions of insects engage in long-distance, seasonal mass migrations which have major consequences for agriculture, ecosystem services and insect-vectored diseases. Tracking this movement in the field is difficult, with mass migrations often occurring at high altitudes and over large spatial scales. 2. As such, tethered flight provides a valuable tool for studying the flight behaviour of insects, giving insights into flight propensity (e.g. distance, duration and velocity) and orientation under controlled laboratory settings. By experimentally manipulating a variety of environmental and physiological traits, numerous studies have used this technology to study the flight behaviour of migratory insects ranging in size from aphids to butterflies. Advances in functional genomics promise to extend this to the identification of genetic factors associated with flight. Tethered flight techniques have been used to study migratory flight characteristics in insects for more than 50 years, but have never been reviewed. 3. This study summarises the key findings of this technology, which has been employed in studies of species from six Orders. By providing detailed descriptions of the tethered flight systems, the present study also aims to further the understanding of how tethered flight studies support field observations, the situations under which the technology is useful and how it might be used in future studies. 4. The aim is to contextualise the available tethered flight studies within the broader knowledge of insect migration and to describe the significant contribution these systems have made to the literature.

Geographic clines in wing morphology relate to colonization history in New World but not Old World populations of yellow dung flies

Geographic clines offer insights about putative targets and agents of natural selection as well as tempo and mode of adaptation. However, demographic processes can lead to clines that are indistinguishable from adaptive divergence. Using the widespread yellow dung fly Scathophaga stercoraria (Diptera: Scathophagidae), we examine quantitative genetic differentiation (QST ) of wing shape across North America, Europe, and Japan, and compare this differentiation with that of ten microsatellites (FST ). Morphometric analyses of 28 populations reared at three temperatures revealed significant thermal plasticity, sexual dimorphism, and geographic differentiation in wing shape. In North America morphological differentiation followed the decline in microsatellite variability along the presumed route of recent colonization from the southeast to the northwest. Across Europe, where S. stercoraria presumably existed for much longer time and where no molecular pattern of isolation by distance was evident, clinal variation was less pronounced despite significant morphological differentiation (QST >FST ). Shape vector comparisons further indicate that thermal plasticity (hot-to-cold) does not mirror patterns of latitudinal divergence (south-to-north), as might have been expected under a scenario with temperature as the major agent of selection. Our findings illustrate the importance of detailed phylogeographic information when interpreting geographic clines of dispersal traits in an adaptive evolutionary framework.

Population-specific expression of antimicrobial peptides conferring pathogen resistance in the invasive ladybird Harmonia axyridis

The harlequin ladybird Harmonia axyridis has emerged as a model species in the context of invasion biology and possesses an expanded repertoire of antimicrobial peptides (AMPs). Here we measured the expression of 22 AMP genes in adult beetles from native and introduced populations, and from a biocontrol population, allowing us to compare populations differing in terms of invasive performance. Strikingly, we observed population-specific expression profiles for particular AMPs. Following an immune challenge, the genes for Coleoptericin1 (Col1), Coleoptericin-likeB and Defensin1 were induced up to several thousand times more strongly in the invasive populations compared to the native and biocontrol populations. To determine the role of Col1 in pathogen resistance, the corresponding gene was silenced by RNA interference (RNAi), causing higher mortality in beetles subsequently infected with the entomopathogen Pseudomonas entomophila. The RNAi-triggered susceptibility to this pathogen was reversed by the injection of a synthetic Col1 peptide. We show that a native population is more susceptible to P. entomomophila infection than an invasive population. This is the first study demonstrating population-specific differences in the immune system of an invasive species and suggests that rapid gene expression changes and a highly adaptive immune system could promote pathogen resistance and thereby invasive performance.

Rapid adaptive evolution in novel environments acts as an architect of population range expansion

Colonization and expansion into novel landscapes determine the distribution and abundance of species in our rapidly changing ecosystems worldwide. Colonization events are crucibles for rapid evolution, but it is not known whether evolutionary changes arise mainly after successful colonization has occurred, or if evolution plays an immediate role, governing the growth and expansion speed of colonizing populations. There is evidence that spatial evolutionary processes can speed range expansion within a few generations because dispersal tendencies may evolve upwards at range edges. Additionally, rapid adaptation to a novel environment can increase population growth rates, which also promotes spread. However, the role of adaptive evolution and the relative contributions of spatial evolution and adaptation to expansion are unclear. Using a model system, red flour beetles (Tribolium castaneum), we either allowed or constrained evolution of populations colonizing a novel environment and measured population growth and spread. At the end of the experiment we assessed the fitness and dispersal tendency of individuals originating either from the core or edge of evolving populations or from nonevolving populations in a common garden. Within six generations, evolving populations grew three times larger and spread 46% faster than populations in which evolution was constrained. Increased size and expansion speed were strongly driven by adaptation, whereas spatial evolutionary processes acting on edge subpopulations contributed less. This experimental evidence demonstrates that rapid evolution drives both population growth and expansion speed and is thus crucial to consider for managing biological invasions and successfully introducing or reintroducing species for management and conservation.

Evolution of density-dependent movement during experimental range expansions

Range expansions and biological invasions are prime examples of transient processes that are likely impacted by rapid evolutionary changes. As a spatial process, range expansions are driven by dispersal and movement behaviour. Although it is widely accepted that dispersal and movement may be context-dependent, for instance density-dependent, and best represented by reaction norms, the evolution of density-dependent movement during range expansions has received little experimental attention. We therefore tested current theory predicting the evolution of increased movement at low densities at range margins using highly replicated and controlled range expansion experiments across multiple genotypes of the protist model system Tetrahymena thermophila. Although rare, we found evolutionary changes during range expansions even in the absence of initial standing genetic variation. Range expansions led to the evolution of negatively density-dependent movement at range margins. In addition, we report the evolution of increased intrastrain competitive ability and concurrently decreased population growth rates in range cores. Our findings highlight the importance of understanding movement and dispersal as evolving reaction norms and plastic life-history traits of central relevance for range expansions, biological invasions and the dynamics of spatially structured systems in general.

Genetics of dispersal

Dispersal is a process of central importance for the ecological and evolutionary dynamics of populations and communities, because of its diverse consequences for gene flow and demography. It is subject to evolutionary change, which begs the question, what is the genetic basis of this potentially complex trait? To address this question, we (i) review the empirical literature on the genetic basis of dispersal, (ii) explore how theoretical investigations of the evolution of dispersal have represented the genetics of dispersal, and (iii) discuss how the genetic basis of dispersal influences theoretical predictions of the evolution of dispersal and potential consequences.

Dispersal has a detectable genetic basis in many organisms, from bacteria to plants and animals. Generally, there is evidence for significant genetic variation for dispersal or dispersal‐related phenotypes or evidence for the micro‐evolution of dispersal in natural populations. Dispersal is typically the outcome of several interacting traits, and this complexity is reflected in its genetic architecture: while some genes of moderate to large effect can influence certain aspects of dispersal, dispersal traits are typically polygenic. Correlations among dispersal traits as well as between dispersal traits and other traits under selection are common, and the genetic basis of dispersal can be highly environment‐dependent.

By contrast, models have historically considered a highly simplified genetic architecture of dispersal. It is only recently that models have started to consider multiple loci influencing dispersal, as well as non‐additive effects such as dominance and epistasis, showing that the genetic basis of dispersal can influence evolutionary rates and outcomes, especially under non‐equilibrium conditions. For example, the number of loci controlling dispersal can influence projected rates of dispersal evolution during range shifts and corresponding demographic impacts. Incorporating more realism in the genetic architecture of dispersal is thus necessary to enable models to move beyond the purely theoretical towards making more useful predictions of evolutionary and ecological dynamics under current and future environmental conditions. To inform these advances, empirical studies need to answer outstanding questions concerning whether specific genes underlie dispersal variation, the genetic architecture of context‐dependent dispersal phenotypes and behaviours, and correlations among dispersal and other traits.

Comparative transcriptomics in three ladybird species supports a role for immunity in invasion biology

The spread of the invasive harlequin ladybird (Harmonia axyridis) in Europe is accompanied by the decline of the native and non-invasive two-spotted ladybird (Adalia bipunctata). Here we show that microsporidia carried by H. axyridis can kill A. bipunctata following the oral uptake of spores, suggesting that their horizontal transmission via intraguild predation may help the invader to outcompete its native competitor. The native seven-spotted ladybird (Coccinella septempunctata) is thought to be less susceptible both to the spread of H. axyridis and to its microsporidia. To investigate whether the distinct levels of pathogen susceptibility in these three ladybird species are determined by their immune systems, we compared the immunity-related transcriptomes of untreated beetles and beetles challenged with suspensions of bacteria and yeast. We found that H. axyridis carries three and four times as many genes encoding antimicrobial peptides representing the attacin, coleoptericin and defensin families than C. septempunctata and A. bipunctata, respectively. Gene expression studies following the injection of bacteria and yeasts into beetles revealed that members of these three antimicrobial peptide families are also induced more strongly in H. axyridis than C. septempunctata or A. bipunctata. Our results therefore support the hypothesis that a superior immune system promotes the performance of invasive species.

Rapid trait evolution drives increased speed and variance in experimental range expansions

Range expansions are central to two ecological issues reshaping patterns of global biodiversity: biological invasions and climate change. Traditional theory considers range expansion as the outcome of the demographic processes of birth, death and dispersal, while ignoring the evolutionary implications of such processes. Recent research suggests evolution could also play a critical role in determining expansion speed but controlled experiments are lacking. Here we use flour beetles (Tribolium castaneum) to show experimentally that mean expansion speed and stochastic variation in speed are both increased by rapid evolution of traits at the expansion edge. We find that higher dispersal ability and lower intrinsic growth rates evolve at the expansion edge compared with spatially nonevolving controls. Furthermore, evolution of these traits is variable, leading to enhanced variance in speed among replicate population expansions. Our results demonstrate that evolutionary processes must be considered alongside demographic ones to better understand and predict range expansions.

Spatial structure provides unique opportunities for evolution during range expansions. Here, the authors show experimentally using the red flour beetle, Tribolium castaneum, that dispersal and growth can evolve through spatial processes, increasing expansion speed and its variance.

Rapid evolution of dispersal ability makes biological invasions faster and more variable

Genetic variation in dispersal ability may result in the spatial sorting of alleles during range expansion. Recent theory suggests that spatial sorting can favour the rapid evolution of life history traits at expanding fronts, and therefore modify the ecological dynamics of range expansion. Here we test this prediction by disrupting spatial sorting in replicated invasions of the bean beetle Callosobruchus maculatus across homogeneous experimental landscapes. We show that spatial sorting promotes rapid evolution of dispersal distance, which increases the speed and variability of replicated invasions: after 10 generations of range expansion, invasions subject to spatial sorting spread 8.9% farther and exhibit 41-fold more variable spread dynamics relative to invasions in which spatial sorting is suppressed. Correspondingly, descendants from spatially evolving invasions exhibit greater mean and variance in dispersal distance. Our results reveal an important role for rapid evolution during invasion, even in the absence of environmental filters, and argue for evolutionarily informed forecasts of invasive spread by exotic species or climate change migration by native species.

Theory suggests that spatial sorting by dispersal ability can generate evolutionarily accelerated range expansions. Using the bean beetle Callosobruchus maculatus, this study shows that evolution not only increases the speed of range expansion, as predicted, but also increases variability.

After the games are over: life-history trade-offs drive dispersal attenuation following range expansion

Increased dispersal propensity often evolves on expanding range edges due to the Olympic Village effect, which involves the fastest and fittest finding themselves together in the same place at the same time, mating, and giving rise to like individuals. But what happens after the range's leading edge has passed and the games are over? Although empirical studies indicate that dispersal propensity attenuates following range expansion, hypotheses about the mechanisms driving this attenuation have not been clearly articulated or tested. Here, we used a simple model of the spatiotemporal dynamics of two phenotypes, one fast and the other slow, to propose that dispersal attenuation beyond preexpansion levels is only possible in the presence of trade‐offs between dispersal and life‐history traits. The Olympic Village effect ensures that fast dispersers preempt locations far from the range's previous limits. When trade‐offs are absent, this preemptive spatial advantage has a lasting impact, with highly dispersive individuals attaining equilibrium frequencies that are strictly higher than their introduction frequencies. When trade‐offs are present, dispersal propensity decays rapidly at all locations. Our model's results about the postcolonization trajectory of dispersal evolution are clear and, in principle, should be observable in field studies. We conclude that empirical observations of postcolonization dispersal attenuation offer a novel way to detect the existence of otherwise elusive trade‐offs between dispersal and life‐history traits.