Retracing the routes of introduction of invasive species: the case of the Sirex noctilio woodwasp

Population level interactions between an invasive woodwasp, an invasive nematode and a community of native parasitoids

Parasitic nematodes and hymenopteran parasitoids have been introduced and used extensively to control invasive Eurasian Sirex noctilio woodwasps in pine plantations in the Southern Hemisphere where no members of this community are native. Sirex noctilio has more recently invaded North America where Sirex-associated communities are native. Sirex noctilio and its parasitic nematode, Deladenus siricidicola, plus six native hymenopteran woodwasp parasitoids in New York and Pennsylvania, were sampled from 204 pines in 2011–2019. Sirex noctilio had become the most common woodwasp in this region and the native parasitoids associated with the native woodwasps had expanded their host ranges to use this invader. We investigated the distributions of these species among occupied trees and the interactions between S. noctilio and natural enemies as well as among the natural enemies. Sirex noctilio were strongly aggregated, with a few of the occupied trees hosting hundreds of woodwasps. Nematode parasitism was positively associated with S. noctilio density, and negatively associated with the density of rhyssine parasitoids. Parasitism by the parasitoid Ibalia leucospoides was positively associated with host (S. noctilio) density, while parasitism by the rhyssine parasitoids was negatively associated with density of S. noctilio. Thus, most S. noctilio come from a few attacked trees in a forest, and S. noctilio from those high-density trees experienced high parasitism by both the invasive nematode and the most abundant native parasitoid, I. l. ensiger. There is little evidence for direct competition between the nematodes and parasitoids. The negative association occurring between rhyssine parasitoids and I. l. ensiger suggests rhyssines may suffer from competition with I. l. ensiger which parasitize the host at an earlier life stage. In addition to direct competition with the native woodwasp Sirex nigricornis for suitable larval habitat within weakened trees, the large S. noctilio population increases the parasitoid and nematode populations, which may increase parasitism of S. nigricornis.

Forest Insect Biosecurity: Processes, Patterns, Predictions, Pitfalls

The economic and environmental threats posed by non-native forest insects are ever increasing with the continuing globalization of trade and travel; thus, the need for mitigation through effective biosecurity is greater than ever. However, despite decades of research and implementation of preborder, border, and postborder preventative measures, insect invasions continue to occur, with no evidence of saturation, and are even predicted to accelerate. In this article, we review biosecurity measures used to mitigate the arrival, establishment, spread, and impacts of non-native forest insects and possible impediments to the successful implementation of these measures. Biosecurity successes are likely under-recognized because they are difficult to detect and quantify, whereas failures are more evident in the continued establishment of additional non-native species. There are limitations in existing biosecurity systems at global and country scales (for example, inspecting all imports is impossible, no phytosanitary measures are perfect, knownunknowns cannot be regulated against, and noncompliance is an ongoing problem). Biosecurity should be a shared responsibility across countries, governments, stakeholders, and individuals.

Molecular insights into the invasion dynamics of Carcinus crabs in South Africa

Knowledge of the introduction history and spread dynamics of invasive species can provide important insights for management (Ens et al. in Environ Rev (in press), 2022), however such information is often unavailable for accidental introductions. Here we infer how the European shore crab, Carcinus maenas, and its congener, the Mediterranean shore crab, C. aestuarii, were introduced to and spread within South Africa. We do this using nuclear microsatellite data and Bayesian assignment tests and Approximate Bayesian Computation (ABC) modelling that included samples from the native and other invasive ranges of these two species. We also compared the genetic diversity and structure of one of the South African populations during and after intensive management, with that of another, unmanaged, population. South African populations had higher genetic diversity than invasive Carcinus populations from elsewhere in the world. Moreover, the ABC analyses suggest that South African populations originated from an admixture event between individuals of C. maenas from a population in the native range and an invasive population from Canada. We also identified instances of hybridisation between Carcinus maenas and C. aestuarii in South Africa. South African populations showed no genetic structure, suggesting either extensive migration between them or that populations arose from the same initial introduction. Management of Carcinus did not affect genetic diversity or structure, and we suspect that the management duration was insufficient to target a full generation of crabs. Together these results suggest multiple introductions and/or high propagule pressure to South Africa, crab (larval or adult) movement between existing populations, and some hybridisation. For eradication from South Africa to be achieved, management would need to concurrently target all known invasive populations and clearly establish that new introductions could be prevented.

Population differentiation and intraspecific genetic admixture in two Eucryptorrhynchus weevils (Coleoptera: Curculionidae) across northern China

Increasing damage of pests in agriculture and forestry can arise both as a consequence of changes in local species and through the introduction of alien species. In this study, we used population genetics approaches to examine population processes of two pests of the tree‐of‐heaven trunk weevil (TTW), Eucryptorrhynchus brandti (Harold) and the tree‐of‐heaven root weevil (TRW), E. scrobiculatus (Motschulsky) on the tree‐of‐heaven across their native range of China. We analyzed the population genetics of the two weevils based on ten highly polymorphic microsatellite markers. Population genetic diversity analysis showed strong population differentiation among populations of each species, with F_ST ranges from 0.0197 to 0.6650 and from −0.0724 to 0.6845, respectively. Populations from the same geographic areas can be divided into different genetic clusters, and the same genetic cluster contained populations from different geographic populations, pointing to dispersal of the weevils possibly being human‐mediated. Redundancy analysis showed that the independent effects of environment and geography could account for 93.94% and 29.70% of the explained genetic variance in TTW, and 41.90% and 55.73% of the explained genetic variance in TRW, respectively, indicating possible impacts of local climates on population genetic differentiation. Our study helps to uncover population genetic processes of these local pest species with relevance to control methods.

Remnants of horizontal transfers of Wolbachia genes in a Wolbachia-free woodwasp

Background

Wolbachia is a bacterial endosymbiont of many arthropod and nematode species. Due to its capacity to alter host biology, Wolbachia plays an important role in arthropod and nematode ecology and evolution. Sirex noctilio is a woodwasp causing economic loss in pine plantations of the Southern Hemisphere. An investigation into the genome of this wasp revealed the presence of Wolbachia sequences. Due to the potential impact of Wolbachia on the populations of this wasp, as well as its potential use as a biological control agent against invasive insects, this discovery warranted investigation.

Results

In this study we first investigated the presence of Wolbachia in S. noctilio and demonstrated that South African populations of the wasp are unlikely to be infected. We then screened the full genome of S. noctilio and found 12 Wolbachia pseudogenes. Most of these genes constitute building blocks of various transposable elements originating from the Wolbachia genome. Finally, we demonstrate that these genes are distributed in all South African populations of the wasp.

Conclusions

Our results provide evidence that S. noctilio might be compatible with a Wolbachia infection and that the bacteria could potentially be used in the future to regulate invasive populations of the wasp. Understanding the mechanisms that led to a loss of Wolbachia infection in S. noctilio could indicate which host species or host population should be sampled to find a Wolbachia strain that could be used as a biological control against S. noctilio.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12862-022-01995-x.

Demography of an invading forest insect reunited with hosts and parasitoids from its native range

The Sirex woodwasp Sirex noctilio Fabricius (Hymenoptera: Siricidae), a widespread invasive pest of pines in the Southern Hemisphere, was first detected in North America in 2004. This study assessed the impacts of life history traits, host resistance and species interactions on the demography of S. noctilio in New York, Pennsylvania and Vermont, then compared key metrics to those found in the native range in Galicia, Spain. Many trees naturally attacked by S. noctilio in North America produced no adult woodwasps, with 5 of 38 infested trees (13%) sampled across six sites yielding 64% of emerging insects. Reproductive success was highest in the introduced host scots pine, Pinus sylvestris, but native red pine, Pinus resinosa, produced larger insects. Sirex noctilio required one or sometimes two years to develop and sex ratios were male biased, 1:2.98 ♀:♂. Body size and fecundity were highly variable, but generally lower than observed in non-native populations in the Southern Hemisphere. Hymenopteran parasitoids killed approximately 20% of S. noctilio larvae and 63% of emerging adults were colonized by the parasitic nematode Deladenus siricidicola, although no nematodes entered eggs. Demographic models suggested that S. noctilio in the northeastern USA have a higher potential for population growth than populations in the native range: estimated finite factor of increase, λ, was 4.17–4.52 (depending on tree species colonized), compared to λ = 1.57 in Spain.

Invasive and Quarantine Risks of Cacopsylla chinensis (Hemiptera: Psyllidae) in East Asia: Hybridization or Gene Flow Between Differentiated Lineages

Pear psyllids are major pests and the causal agents of pear decline disease in orchards. In the past two decades, their outbreaks have raised issues pertaining to invasions and taxonomic identification of the dimorphic Cacopsylla chinensis (Yang and Li) in East Asia. The present study elucidated, as an aid to quarantine management, the invasive origins, differentiation history, and putative gene flow and hybridization between C. chinensis and its sibling species Cacopsylla jukyungi (Kwon). Analyses revealed that the ancestors of C. jukyungi might have diverged from C. chinensis approximately 3.5 million yr ago (Mya) and that differentiation between C. chinensis lineages I and II probably occurred 1.5 Mya. The known overlapping distribution of C. chinensis and C. jukyungi in northeastern China and the two C. chinensis lineages in the Bohai Rim region and Taiwan could be attributed to recent population expansion after the Last Glacial Maximum and/or anthropogenic activities. Analyses of the nuclear gene demonstrated that frequent gene flow between the two C. chinensis lineages and the paraphyletic relationship between C. chinensis and C. jukyungi might be caused by incomplete lineage sorting or hybridization events. On the basis of the current distribution, it is evident that C. jukyungi is not present in middle-southern China, whereas C. chinensis is not distributed in Japan and Korea. Preventing new invasions of Cacopsylla psyllids among geographic regions through the transportation of pear scions is thus pivotal in East Asia, particularly for the possible genetic exchanges among differentiated lineages after secondary invasion events.

Biotic and abiotic effects on density, body size, sex ratio, and survival in immature stages of the European woodwasp, Sirex noctilio

Resource quality can have direct or indirect effects on female oviposition choice, offspring growth and survival, and ultimately on body size and sex ratio. We examined these patterns in Sirex noctilio Fabricus, the globally invasive European pine woodwasp, in South African Pinus patula plantations. We studied how tree position as well as natural variation in biotic and abiotic factors influenced sex-specific density, larval size, tunnel length, male proportion, and survival across development. Twenty infested trees divided into top, middle, and bottom sections were sampled at three time points during larval development. We measured moisture content, bluestain fungal colonization, and co-occurring insect density and counted, measured, and sexed all immature wasps. A subset of larval tunnels was measured to assess tunnel length and resource use efficiency (tunnel length as a function of immature wasp size). Wasp density increased from the bottoms to the tops of trees for both males and females. However, the largest individuals and the longest tunnels were found in bottom sections. Male bias was strong (~10:1) and likewise differed among sections, with the highest proportion in the middle and top sections. Sex ratios became more strongly male biased due to high female mortality, especially in top and middle sections. Biotic and abiotic factors such as colonization by Diplodia sapinea, weevil (Pissodes sp.) density, and wood moisture explained modest residual variation in our primary mixed effects models (0%-22%). These findings contribute to a more comprehensive understanding of sex-specific resource quality for S. noctilio and of how variation in key biotic and abiotic factors can influence body size, sex ratio, and survival in this economically important woodwasp.

Tree health in South Africa: Retrospect and prospect

South Africa is a country with very limited natural forest cover. Consequently, the timber and fibre needs of the country cannot be provided for from indigenous forest. It is largely for this reason that South Africa initially developed a highly productive plantation forest industry, which today makes a substantial contribution to the local economy. These plantations are based on non-native species of Eucalyptus, Pinus and Australian Acacia. In the early years of establishment, South African plantations were relatively free of pest and pathogen problems. But, over time, an increasing number of insects, fungi and bacteria have emerged as serious threats to the sustainability of the forestry industry. Numerous native pests and pathogens, especially insects, have adapted to these introduced tree species to cause damage or disease. The problem is compounded by the accidental introduction of non-native pests and pathogens, and this has been at a rapidly increasing rate over the past three decades. Some of these introduced pests and pathogens also threaten the fitness and even the survival of many indigenous South African tree species. Fortunately, South Africa has developed an impressive knowledge base and range of integrated management options to deal with these problems. This development was first driven by government programmes, and in more recent years by public–private partnerships between industry, universities and government. It is clear from the pattern of emergence of pests and pathogens in recent years that South Africa will deal with an increasing number of these problems and a continuously changing tree health environment. This requires robust investment in both quarantine and mitigation mechanisms to protect the country’s biodiversity as well as to ensure the sustainability of its wood and fibre industries.

Assessing Niche Shifts and Conservatism by Comparing the Native and Post-Invasion Niches of Major Forest Invasive Species

Invasive species experience biotic and abiotic conditions that may (or may not) resemble their native environment. We explored the methodology of determining climatic niches and compared the native and post-invasion niches of four invasive forest pests to determine if these species experienced shifts or changes in their new climatic niches. We used environmental principle components analysis (PCA-env) method to quantify climatic niche shifts, expansions, and temporal changes. Furthermore, we assessed the effect of variable selection in the delineation and comparison of niche space. We found that variable selection influenced the delineation and overlap of each niche, whereas the subset of climatic variables selected from the first two PCA-env axes explained more variance in environmental conditions than the complete set of climatic variables for all four species. Most focal species showed climatic niche shifts in their invasive range and had not yet fully occupied the available niche within the invaded range. Our species varied the proportion of niche overlap between the native and invasive ranges. By comparing native and invasive niches, we can help predict a species’ potential range expansion and invasion potential. Our results can guide monitoring and help inform management of these and other invasive species.

Ancient globetrotters-connectivity and putative native ranges of two cosmopolitan biofouling amphipods

The geographic distributions of some coastal marine species have appeared as cosmopolitan ever since they were first scientifically documented. In particular, for many benthic species that are associated with anthropogenic substrata, there is much speculation as to whether or not their broad distributions can be explained by natural mechanisms of dispersal. Here, we focused on two congeneric coastal crustaceans with cosmopolitan distributions—the tube-dwelling amphipods Jassa marmorata and Jassa slatteryi. Both species are common elements of marine biofouling on nearly all kinds of artificial hard substrata in temperate to warm seas. We hypothesized that the two species’ modern occurrences across the oceans are the result of human shipping activities that started centuries ago. Mitochondrial DNA sequences of the CO1 fragment of specimens from distinct marine regions around the world were analysed, evaluating genetic structure and migration models and making inferences on putative native ranges of the two Jassa species. Populations of both species exhibited considerable genetic diversity with differing levels of geographic structure. For both species, at least two dominant haplotypes were shared among several geographic populations. Rapid demographic expansion and high migration rates between geographically distant regions support a scenario of ongoing dispersal all over the world. Our findings indicate that the likely former native range of J. marmorata is the Northwest Atlantic, whereas the likely former native range of J. slatteryi is the Northern Pacific region. As corroborated by the genetic connectivity between populations, shipping still appears to be the more successful vector of the two species’ dispersal when compared to natural mechanisms. Historical invasion events that likely started centuries ago, along with current ongoing dispersal, confirm these species’ identities as true “neocosmopolitans”.

Genetic variability among native and introduced strains of the parasitic nematode Deladenus siricidicola

Sirex noctilio is an invasive Eurasian woodwasp that can kill pine (Pinus spp.) trees and has been introduced to areas of the Southern Hemisphere where plantations of introduced pines are grown. The main method of control of this invasive pest has been introduction and augmentation of a parasitic nematode, Deladenus siricidicola. The strain of D. siricidicola used for biological control of S. noctilio in the Southern Hemisphere originated in Sopron, Hungary. The genotype of D. siricidicola used for biological control sterilizes females of the strain of S. noctilio present in Australia. However, different strains of S. noctilio have been introduced to different geographic areas that have been invaded and different combinations of D. siricidicola and S. noctilio genotypes vary in whether these nematodes sterilize female S. noctilio. Moreover, even in the event of sterilization, partial sterilization can occur, where not all woodwasp eggs are compromised. Sirex noctilio has now invaded North America accidentally, putatively accompanying D. siricidicola, but these host/parasite pairings do not result in female sterilization. More information is needed about the genetic diversity of D. siricidicola both where it is native and introduced. In addition, the host range of these nematodes is necessary to understand to evaluate their potential use in areas where pine communities are native. We collected and evaluated Deladenus parasitizing S. noctilio, S. juvencus, and associated insects in Hungary, Denmark, Spain, and Italy, as well as in the United States. Phylogenetic analyses were unable to fully provide fine resolution, although some community structure was evident. Many D. siricidicola samples from Hungary had identical COI and ITS sequences to the strain of D. siricidicola accidentally introduced to North America putatively when S. noctilio invaded. The same or similar strains of D. siricidicola parasitize two different Sirex species that utilize pines as well as a Sirex parasitoid and a pine-boring beetle, demonstrating some limited variability in host specificity of this species. These results highlight the genetic diversity of Deladenus siricidicola in its native range in Europe.

One maternal lineage leads the expansion of Thaumastocoris peregrinus (Hemiptera: Thaumastocoridae) in the New and Old Worlds

The bronze bug, Thaumastocoris peregrinus, an Australian native insect, has become a nearly worldwide invasive pest in the last 16 years and has been causing significant damage to eucalypts (Myrtaceae), including Eucalyptus spp. and Corymbia spp. Its rapid expansion leads to new questions about pathways and routes that T. peregrinus used to invade other continents and countries. We used mtDNA to characterize specimens of T. peregrinus collected from 10 countries where this species has become established, including six recently invaded countries: Chile, Israel, Mexico, Paraguay, Portugal, and the United States of America. We then combined our mtDNA data with previous data available from South Africa, Australia, and Europe to construct a world mtDNA network of haplotypes. Haplotype A was the most common present in all specimens of sites sampled in the New World, Europe, and Israel, however from Australia second more frequently. Haplotype D was the most common one from native populations in Australia. Haplotype A differs from the two major haplotypes found in South Africa (D and G), confirming that at least two independent invasions occurred, one from Australia to South Africa, and the other one from Australia to South America (A). In conclusion, Haplotype A has an invasion success over many countries in the World. Additionally, analyzing data from our work and previous reports, it is possible to suggest some invasive routes of T. peregrinus to predict such events and support preventive control measures.

Invasion History of Sirex noctilio Based on COI Sequence: The First Six Years in China

Sirex noctilio F. (Hymenoptera: Siricidae: Siricinae), a new invasive species in China, is a significant international forestry pest which, transported via logs and related wood packing materials, has led to environmental damage and substantial economic loss in many countries around the world. It was first detected in China in 2013, and since then infestations have been found in 18 additional sites. Using a 322 bp fragment of the mitochondrial barcode gene COI, we studied the genetic diversity and structure of S. noctilio populations in both native and invaded ranges, with a specific focus in China. Twelve haplotypes were found across the native and invaded distribution of the pest, of which three were dominant; among these there were only one or two mutational steps between each pair of haplotypes. No obvious genetic structure was found other than in Chinese populations. China has a unique and dominant haplotype not found elsewhere, and compared with the rest of the world, the genetic structure of Chinese populations suggested a multiple invasion scenario.

Genomic biosurveillance of forest invasive alien enemies: A story written in code

The world's forests face unprecedented threats from invasive insects and pathogens that can cause large irreversible damage to the ecosystems. This threatens the world's capacity to provide long-term fiber supply and ecosystem services that range from carbon storage, nutrient cycling, and water and air purification, to soil preservation and maintenance of wildlife habitat. Reducing the threat of forest invasive alien species requires vigilant biosurveillance, the process of gathering, integrating, interpreting, and communicating essential information about pest and pathogen threats to achieve early detection and warning and to enable better decision-making. This process is challenging due to the diversity of invasive pests and pathogens that need to be identified, the diverse pathways of introduction, and the difficulty in assessing the risk of establishment. Genomics can provide powerful new solutions to biosurveillance. The process of invasion is a story written in four chapters: transport, introduction, establishment, and spread. The series of processes that lead to a successful invasion can leave behind a DNA signature that tells the story of an invasion. This signature can help us understand the dynamic, multistep process of invasion and inform management of current and future introductions. This review describes current and future application of genomic tools and pipelines that will provide accurate identification of pests and pathogens, assign outbreak or survey samples to putative sources to identify pathways of spread, and assess risk based on traits that impact the outbreak outcome.

Population analysis reveals genetic structure of an invasive agricultural thrips pest related to invasion of greenhouses and suitable climatic space

Biological invasions of pests into climatically unsuitable areas can be facilitated by human-regulated environments, in which case there may be an impact on genetic structure through population processes and/or adaptation. Here, we investigated the population genetic structure of an invasive agricultural pest, Thrips palmi, in China, which has expanded its distribution range through using greenhouses. Early invaded populations showed a relatively higher level of genetic diversity than recently expanded greenhouse populations. Strong population genetic structure corresponded to a pattern of isolation by distance, with no recent gene flow and low historical gene flow among populations, reflecting limited ongoing dispersal. A genetic signature of population expansion was detected in early invaded populations and three northern populations from greenhouses, suggesting that the greenhouse environments facilitated expansion of this species. Redundancy analysis showed that the independent effects of environment and geography could explain 51.68% and 32.06% of the genetic variance, respectively. These findings point to climate- and greenhouse-related spatial expansion, with the potential for adaptation by T. palmi. They emphasize the contribution of human-regulated environments on the successes of this invasive species, a situation likely to apply to other invasive species that use greenhouse environments.

Mechanisms that influence sex ratio variation in the invasive hymenopteran Sirex noctilio in South Africa

Sirex noctilio is an economically important invasive pest of commercial pine forestry in the Southern Hemisphere. Newly established invasive populations of this woodwasp are characterized by highly male-biased sex ratios that subsequently revert to those seen in the native range. This trend was not observed in the population of S. noctilio from the summer rainfall regions in South Africa, which remained highly male-biased for almost a decade. The aim of this study was to determine the cause of this persistent male bias. As an explanation for this pattern, we test hypotheses related to mating success, female investment in male versus female offspring, and genetic diversity affecting diploid male production due to complementary sex determination. We found that 61% of females in a newly established S. noctilio population were mated. Microsatellite data analysis showed that populations of S. noctilio from the summer rainfall regions in South Africa are far less genetically diverse than those from the winter rainfall region, with mean Nei's unbiased gene diversity indexes of 0.056 and 0.273, respectively. These data also identified diploid males at low frequencies in both the winter (5%) and summer (2%) rainfall regions. The results suggest the presence of a complementary sex determination mechanism in S. noctilio, but imply that reduced genetic diversity is not the main driver of the male bias observed in the summer rainfall region. Among all the factors considered, selective investment in sons appears to have the most significant influence on male bias in S. noctilio populations. Why this investment remains different in frontier or early invasive populations is not clear but could be influenced by females laying unfertilized eggs to avoid diploid male production in populations with a high genetic relatedness.

Efficacy of Kamona strain Deladenus siricidicola nematodes for biological control of Sirex noctilio in North America and hybridisation with invasive conspecifics

Sirexnoctilio is an invasive woodwasp that, along with its symbiotic fungus, has killed pine trees (Pinus spp.) in North America and in numerous countries in the Southern Hemisphere. We tested a biological control agent in North America that has successfully controlled S.noctilio in Oceania, South Africa, and South America. Deladenussiricidicola nematodes feed on the symbiotic white rot fungus Amylostereumareolatum and can switch to being parasitic on S.noctilio. When parasitic, the Kamona nematode strain can sterilise the eggs of S.noctilio females. However, in North America, a different strain of D.siricidicola (NA), presumably introduced along with the woodwasp, parasitises but does not sterilise S.noctilio. We tested the sterilising Kamona biological control strain of D.siricidicola against S.noctilio in North America. Interactions between the biological control strain and the NA strain could include competitive exclusion, co-infection within hosts or hybridisation. We reared D.siricidicola Kamona on an A.areolatum strain native to North America (IGS-BE) and another strain (IGS-BDF) used commercially to mass-produce the nematode in Australia. We inoculated Kamona reared on either strain of A.areolatum into logs infested with S.noctilio larvae and compared parasitism rates against control logs. Individual nematodes were isolated from S.noctilio hemocoels and from sterilised eggs and were genotyped with eight microsatellite loci. A high rate of parasitisation of S.noctilio by D.siricidicolaNA was found for all treatments and we found evidence of both co-infection and hybridisation. Surprisingly, sterilisation rates were not related to the rates of parasitisation by D.siricidicola Kamona.

Molecular Characterization of the 2016 New World Screwworm (Diptera: Calliphoridae) Outbreak in the Florida Keys

New World screwworm (NWS), Cochliomyia hominivorax (Coquerel 1858) (Diptera: Calliphoridae), is a myiasis-causing fly that can be a serious threat to the health of livestock, wildlife, and humans. Its progressive eradication from the southern United States, Mexico, and Central America from the 1950s to 2000s is an excellent example of successful pest management using sterile insect technique (SIT). In late 2016, autochthonous NWS were detected in the Florida Keys, representing this species' first invasion in the United States in >30 yr. Rapid use of quarantine and SIT was successful in eliminating the infestation by early 2017; however, the geographic source of this infestation remains unknown. Here, we use amplicon sequencing to generate mitochondrial and nuclear sequence data representing all confirmed cases of NWS from this infestation, and compare these sequences to preexisting data sets sampling the native distribution of NWS. We ask two questions regarding the FL Keys outbreak. First, is this infestation the result of a single invasion from one source, or multiple invasions from different sources? And second, what is the geographic origin of this invasion? We found virtually no sequence variation between specimens collected from the FL Keys outbreak, which is consistent with a single source of introduction. However, we also found very little geographic resolution in any of the data sets, which precludes identification of the source of this outbreak. Our lack of success in answering our second question speaks to the need for finer-scale genetic or genomic assessments of NWS population structure, which would facilitate source determination of potential future outbreaks.

Deciphering the Routes of invasion of Drosophila suzukii by Means of ABC Random Forest

Deciphering invasion routes from molecular data is crucial to understanding biological invasions, including identifying bottlenecks in population size and admixture among distinct populations. Here, we unravel the invasion routes of the invasive pest Drosophila suzukii using a multi-locus microsatellite dataset (25 loci on 23 worldwide sampling locations). To do this, we use approximate Bayesian computation (ABC), which has improved the reconstruction of invasion routes, but can be computationally expensive. We use our study to illustrate the use of a new, more efficient, ABC method, ABC random forest (ABC-RF) and compare it to a standard ABC method (ABC-LDA). We find that Japan emerges as the most probable source of the earliest recorded invasion into Hawaii. Southeast China and Hawaii together are the most probable sources of populations in western North America, which then in turn served as sources for those in eastern North America. European populations are genetically more homogeneous than North American populations, and their most probable source is northeast China, with evidence of limited gene flow from the eastern US as well. All introduced populations passed through bottlenecks, and analyses reveal five distinct admixture events. These findings can inform hypotheses concerning how this species evolved between different and independent source and invasive populations. Methodological comparisons indicate that ABC-RF and ABC-LDA show concordant results if ABC-LDA is based on a large number of simulated datasets but that ABC-RF out-performs ABC-LDA when using a comparable and more manageable number of simulated datasets, especially when analyzing complex introduction scenarios.

The balance of trade in alien species between South Africa and the rest of Africa

Background: Alien organisms are not only introduced from one biogeographical region to another but also spread within regions. As South Africa shares land borders with six countries, multiple opportunities exist for the transfer of alien species between South Africa and other African countries; however, the direction and importance of intra-regional spread is unclear. Objectives: The aim of this study was to gain a greater understanding of the introduction of alien species into Africa and the spread of species between South Africa and other African countries. Method: We developed scenarios that describe the routes by which alien species are introduced to and spread within Africa and present case studies for each. Using data from literature sources and databases, the relative importance of each scenario for alien birds and insect pests of eucalypts was determined, and the direction and importance of intra-regional spread was assessed. Results: Alien species from many taxonomic groups have, through various routes, been introduced to and spread within Africa. For birds and eucalypt insect pests, the number of species spreading in the region has recently increased, with South Africa being a major recipient of birds (14 species received and 5 donated) and a major donor of eucalypt insect pests (1 species received and 10 donated). For both groups, many introduced species have not yet spread in the region. Conclusion: The intra-regional spread of alien species in Africa represents an important and possibly increasing threat to biosecurity. To address this threat, we propose a framework that details how African countries could cooperate and develop a coordinated response to alien species introductions.

Low genetic diversity but strong population structure reflects multiple introductions of western flower thrips (Thysanoptera: Thripidae) into China followed by human-mediated spread

Historical invasion scenarios based on observational records are usually incomplete and biased, but these can be supplemented by population genetic data. The western flower thrips (WFT), Frankliniella occidentalis, invaded China in the last 13 years and has rapidly become one of the most serious pests in the country. To assess whether this invasion involved a single event or multiple events, we examined patterns of genetic diversity and population structure of WFT across 12 Chinese populations and a native US population based on mitochondrial DNA and/or 18 microsatellite loci. The average allelic richness and haplotype diversity in Chinese populations were significantly lower than in a population from its native range. The distribution of mitochondrial haplotypes suggested multiple independent invasions of WFT into China, including two invasions into the Beijing region. Based on microsatellite data, two distinct clusters were identified, with both of them splitting further into two clusters; in the Beijing region, the microsatellite data also provided evidence for two introductions. Both the absence of isolation by distance and the fact that distant populations were similar genetically suggest patterns of WFT movement linked to human activities. Our study therefore suggests multiple introductions of WFT into China and human-assisted spread.

Genetic Analyses Suggest Separate Introductions of the Pine Pathogen Lecanosticta acicola Into Europe

Lecanosticta acicola is a heterothallic ascomycete that causes brown spot needle blight on native and nonnative Pinus spp. in many regions of the world. In this study we investigated the origin of European L. acicola populations and estimated the level of random mating of the pathogen in affected areas. Part of the elongation factor 1-α gene was sequenced, 11 microsatellite regions were screened, and the mating type idiomorphs were determined for 201 isolates of L. acicola collected from three continents and 17 host species. The isolates from Mexico and Guatemala were unique, highly diverse and could represent cryptic species of Lecanosticta. The isolates from East Asia formed a uniform and discrete group. Two distinct populations were identified in both North America and Europe. Approximate Bayesian computation analyses strongly suggest independent introductions of two populations from North America into Europe. Microsatellite data and mating type distributions indicated random recombination in the populations of North America and Europe. Its intercontinental introduction can most likely be explained as a consequence of the movement of infected plant material. In contrast, the spread of L. acicola within Europe appears to be primarily due to conidial dispersion and probably also ascospore dissemination.

Contemporary genetic structure and postglacial demographic history of the black scorpionfish, Scorpaena porcus, in the Mediterranean and the Black Seas

Understanding the distribution of genetic diversity in the light of past demographic events linked with climatic shifts will help to forecast evolutionary trajectories of ecosystems within the current context of climate change. In this study, mitochondrial sequences and microsatellite loci were analysed using traditional population genetic approaches together with Bayesian dating and the more recent approximate Bayesian computation scenario testing. The genetic structure and demographic history of a commercial fish, the black scorpionfish, Scorpaena porcus, was investigated throughout the Mediterranean and Black Seas. The results suggest that the species recently underwent population expansions, in both seas, likely concomitant with the warming period following the Last Glacial Maximum, 20 000 years ago. A weak contemporaneous genetic differentiation was identified between the Black Sea and the Mediterranean Sea. However, the genetic diversity was similar for populations of the two seas, suggesting a high number of colonizers entered the Black Sea during the interglacial period and/or the presence of a refugial population in the Black Sea during the glacial period. Finally, within seas, an east/west genetic differentiation in the Adriatic seems to prevail, whereas the Black Sea does not show any structured spatial genetic pattern of its population. Overall, these results suggest that the Black Sea is not that isolated from the Mediterranean, and both seas revealed similar evolutionary patterns related to climate change and changes in sea level.

Insights into the Introduction History and Population Genetic Dynamics of the Nile Monitor (Varanus niloticus) in Florida

Invasive species are widely recognized as important drivers of the ongoing biodiversity crisis. The US state of Florida is especially susceptible to the proliferation of invasive reptiles, and nonnative lizards currently outnumber native lizard species. At present, there are 3 documented breeding populations of the Nile monitor (Varanus niloticus) in different regions of Southern Florida, and these populations are considered potential dangers to threatened, fossorial endemics, such as burrowing owls, American crocodiles, and gopher tortoises. Nevertheless, at present, both the introduction histories of these populations and the degree to which they are connected by gene flow are not known. To address these issues, we genotyped V. niloticus from Cape Coral, Homestead Air Reserve Base, and West Palm Beach at 17 microsatellite loci and conducted a variety of analyses to assess both intrapopulation genetic diversity, the degree of gene flow between populations, and the most likely introduction scenario. The results of our analyses demonstrate that all 3 populations have limited genetic diversity (mean number of effective alleles across loci in all 3 populations ~ 2.00) and are highly differentiated from one another (G ST = 0.268; G″ST = 0.628). Our results also suggest that these populations resulted from independent introduction events that occurred within the past few decades. Consequently, we advise that wildlife managers focus management efforts on containment of existing populations and intensification of monitoring efforts on potential migration corridors.

New distribution records for the rare genus Afrotremex Pasteels (Siricidae: Hymenoptera) and provision of interactive Lucid identification keys to species

BACKGROUND

Afrotremex Pasteels, 1951 is a rare genus of wasps endemic to the Afrotropical region, containing 6 species represented by 14 specimens. Specimens were previously only recorded from central Africa: Cameroon, Congo, Democratic Republic of Congo, Gabon and Uganda.

NEW INFORMATION

Here we record two additional specimens housed in the Natural History Museum in London (BMNH), one of which is a male of A. xylophagus Goulet, 2014 collected in Ghana (previously Gold Coast). This record extends the known distribution of the genus into west Africa, and represents the second known male specimen for the genus. The other BMNH specimen is a female paratype of A. violaceus Pasteels, 1951 collected in the Democratic Republic of Congo. We provide high quality photographs of these additional two specimens. Images of all six known species are openly available online on WaspWeb. In addition we have developed interactive online Lucid Matrix and Lucid Phoenix identification keys to the species, which are openly available on WaspWeb at: http://www.waspweb.org/Siricoidea/Siricidae/Keys/index.htm.

Inferring Invasion History of Red Swamp Crayfish (Procambarus clarkii) in China from Mitochondrial Control Region and Nuclear Intron Sequences

Identifying the dispersal pathways of an invasive species is useful for adopting the appropriate strategies to prevent and control its spread. However, these processes are exceedingly complex. So, it is necessary to apply new technology and collect representative samples for analysis. This study used Approximate Bayesian Computation (ABC) in combination with traditional genetic tools to examine extensive sample data and historical records to infer the invasion history of the red swamp crayfish, Procambarus clarkii, in China. The sequences of the mitochondrial control region and the proPOx intron in the nuclear genome of samples from 37 sites (35 in China and one each in Japan and the USA) were analyzed. The results of combined scenarios testing and historical records revealed a much more complex invasion history in China than previously believed. P. clarkii was most likely originally introduced into China from Japan from an unsampled source, and the species then expanded its range primarily into the middle and lower reaches and, to a lesser extent, into the upper reaches of the Changjiang River in China. No transfer was observed from the upper reaches to the middle and lower reaches of the Changjiang River. Human-mediated jump dispersal was an important dispersal pathway for P. clarkii. The results provide a better understanding of the evolutionary scenarios involved in the rapid invasion of P. clarkii in China.

Using ABC and microsatellite data to detect multiple introductions of invasive species from a single source

The introduction of invasive species to new locations (that is, biological invasions) can have major impact on biodiversity, agriculture and public health. As such, determining the routes and modality of introductions with genetic data has become a fundamental goal in molecular ecology. To assist with this goal, new statistical methods and frameworks have been developed, such as approximate Bayesian computation (ABC) for inferring invasion history. Here, we present a model of invasion accounting for multiple introductions from a single source (MISS), a heretofore largely unexplored model. We simulate microsatellite data to evaluate the power of ABC to distinguish between single and multiple introductions from the same source, under a range of demographic parameters. We also apply ABC to microsatellite data from three invasions of bumblebee in New Zealand. In addition, we assess the performance of several methods of summary statistics selection. Our simulated results suggested good ability to distinguish between one- and two-wave models over much but not all of the parameter space tested, independent of summary statistics used. Globally, parameter estimation was good except for bottleneck timing. For one of the bumblebee species, we clearly rejected the MISS model, while for the other two we found inconclusive results. Since a second wave may provide genetic reinforcement to initial colonists, help relieve inbreeding among founders, or increase the hazard of the invasion, its detection may be crucial for managing invasions; we suggest that the MISS model could be considered as a potential model in future theoretical and empirical studies of invasions.

Classical biological control of an invasive forest pest: a world perspective of the management of Sirex noctilio using the parasitoid Ibalia leucospoides (Hymenoptera: Ibaliidae)

Classical biological control is a key method for managing populations of pests in long-lived crops such as plantation forestry. The execution of biological control programmes in general, as the evaluation of potential natural enemies remains, to a large extent, an empirical endeavour. Thus, characterizing specific cases to determine patterns that may lead to more accurate predictions of success is an important goal of the much applied ecological research. We review the history of introduction, ecology and behaviour of the parasitoid Ibalia leucospoides. The species is a natural enemy of Sirex noctilio, one of the most important pests of pine afforestation worldwide. We use an invasion ecology perspective given the analogy between the main stages involved in classical biological control and the biological invasion processes. We conclude that success in the establishment, a common reason of failure in biocontrol, is not a limiting factor of success by I. leucospoides. A mismatch between the spread capacity of the parasitoid and that of its host could nevertheless affect control at a regional scale. In addition, we suggest that given its known life history traits, this natural enemy may be a better regulator than suppressor of the host population. Moreover, spatial and temporal refuges of the host population that may favour the local persistence of the interaction probably reduce the degree to which S. noctilio population is suppressed by the parasitoid. We emphasize the fact that some of the biological attributes that promote establishment may negatively affect suppression levels achieved. Studies on established non-native pest-parasitoid interactions may contribute to defining selection criteria for classical biological control which may prove especially useful in integrated pest management IPM programmes of invasive forest insects.

Sirex woodwasp: a model for evolving management paradigms of invasive forest pests

The Sirex woodwasp, Sirex noctilio, and its fungal mutualist, Amylostereum areolatum, together constitute one of the most damaging invasive pests of pine. Despite a century of research and well-established management programs, control remains unpredictable and spread continues to new areas. Variable success in managing this pest has been influenced by complex invasion patterns, the multilayered nature of biological interactions, the varying local ecologies, and microevolutionary population processes in both the biocontrol organisms and in the wasps. Recent research findings are challenging the historical perspectives on methods to manage the Sirex woodwasp, calling for management programs to incorporate the variable local dynamics affecting this pest complex. In this regard, the Sirex woodwasp provides a superb model to illustrate the need for a different approach to develop efficient and sustainable management tools to deal with the growing and global nature of pest invasions in forests and plantations.

Genetic evidence for multiple sources of the non-native fish Cichlasoma urophthalmus (Günther; Mayan Cichlids) in southern Florida

The number and diversity of source populations may influence the genetic diversity of newly introduced populations and affect the likelihood of their establishment and spread. We used the cytochrome b mitochondrial gene and nuclear microsatellite loci to identify the sources of a successful invader in southern Florida, USA, Cichlasoma urophthalmus (Mayan cichlid). Our cytochrome b data supported an introduction from Guatemala, while our microsatellite data suggested movement of Mayan Cichlids from the upper Yucatán Peninsula to Guatemala and introductions from Guatemala and Belize to Florida. The mismatch between mitochondrial and nuclear genomes suggests admixture of a female lineage from Guatemala, where all individuals were fixed for the mitochondrial haplotype found in the introduced population, and a more diverse but also relatively small number of individuals from Belize. The Florida cytochrome b haplotype appears to be absent from Belize (0 out of 136 fish screened from Belize had this haplotype). Genetic structure within the Florida population was minimal, indicating a panmictic population, while Mexican and Central American samples displayed more genetic subdivision. Individuals from the Upper Yucatán Peninsula and the Petén region of Guatemala were more genetically similar to each other than to fish from nearby sites and movement of Mayan Cichlids between these regions occurred thousands of generations ago, suggestive of pre-Columbian human transportation of Mayan Cichlids through this region. Mayan Cichlids present a rare example of cytonuclear disequilibrium and reduced genetic diversity in the introduced population that persists more than 30 years (at least 7-8 generations) after introduction. We suggest that hybridization occurred in ornamental fish farms in Florida and may contribute their establishment in the novel habitat. Hybridization prior to release may contribute to other successful invasions.

Demographic histories and genetic diversities of Fennoscandian marine and landlocked ringed seal subspecies

Island populations are on average smaller, genetically less diverse, and at a higher risk to go extinct than mainland populations. Low genetic diversity may elevate extinction probability, but the genetic component of the risk can be affected by the mode of diversity loss, which, in turn, is connected to the demographic history of the population. Here, we examined the history of genetic erosion in three Fennoscandian ringed seal subspecies, of which one inhabits the Baltic Sea 'mainland' and two the 'aquatic islands' composed of Lake Saimaa in Finland and Lake Ladoga in Russia. Both lakes were colonized by marine seals after their formation c. 9500 years ago, but Lake Ladoga is larger and more contiguous than Lake Saimaa. All three populations suffered dramatic declines during the 20th century, but the bottleneck was particularly severe in Lake Saimaa. Data from 17 microsatellite loci and mitochondrial control-region sequences show that Saimaa ringed seals have lost most of the genetic diversity present in their Baltic ancestors, while the Ladoga population has experienced only minor reductions. Using Approximate Bayesian computing analyses, we show that the genetic uniformity of the Saimaa subspecies derives from an extended founder event and subsequent slow erosion, rather than from the recent bottleneck. This suggests that the population has persisted for nearly 10,000 years despite having low genetic variation. The relatively high diversity of the Ladoga population appears to result from a high number of initial colonizers and a high post-colonization population size, but possibly also by a shorter isolation period and/or occasional gene flow from the Baltic Sea.