Rebecca M. Turner

Invasive conifers are widespread and impactful weeds, resulting in significant ecological change and management costs. Treatments to control invasive species are expensive and require long-term commitment, yet their effectiveness under operational conditions is rarely quantified. The effectiveness of two operational control approaches – aerial application of foliar herbicides and ground-based manual methods – applied to manage invasive pines in New Zealand (NZ) in typically high-density and low-density infestations, respectively, was assessed. The objective was to quantify the effectiveness of each method in reducing Pinus contorta populations and to characterize the post-treatment population structure over time to assess reinvasion risk. Pine mortality and regeneration were surveyed in operationally treated infestations 1–4 years after control across a range of invaded environments in NZ. Mortality following aerial boom spray exceeded 90% on average across all height classes 3 years after treatment. Ground-based methods achieved high mortality among larger individuals but were significantly less effective for pines shorter than 60 cm. Both operational approaches allowed populations to persist via surviving individuals in the initial years post-treatment; however, the resulting population structures differed between treatments. Reinvasion risk also differed between treatments. Following aerial boom spray, reinvasion was driven primarily by surviving larger trees, whereas following ground-based control, it was driven by missed seedlings that persisted after treatment. Despite high mortality, aerial boom spray left an estimated 1,094 surviving pines ha ^-1 4 years after treatment, approximately half of which exceeded 2 m in height. In contrast, the large number of seedlings missed by ground treatments could lead to rapid invasion recovery. Follow-up treatment within 2–4 years is likely to be necessary for both control methods, although management should target contrasting residual population structures and reinvasion pathways.

One fundamental factor thought to influence invasion success is the relatedness of a species to the species in the invaded range. Close relatedness of invading with native species means a lot of shared evolutionary history and adaptation to local conditions, which might give the invader a pre-adaptation advantage, resulting in higher invasion success (Darwin’s pre-adaptation hypothesis). On the other hand, distant relatedness between invader and natives means less similarity and fewer overlaps in resource requirements, likely resulting in less competition and higher invasion success (Darwin’s naturalisation hypothesis). These opposing hypotheses are referred to as Darwin’s naturalisation conundrum. While the majority of empirical studies on this topic have focused on plants and vertebrate animals, these hypotheses have never been tested for insects despite their importance as invasive species. To address this shortcoming, we assembled and harmonised lists of native insect species occurring in three world regions and of non-native species exchanged between these regions: North America, Europe and New Zealand, with 87,147, 94,332 and 11,889 native species, respectively. We then tested if species belonging to shared genera are over-represented in the pool of non-native species that have become established in the invaded region relative to all species native to the source region. We found that non-native species were significantly more likely to belong to a genus shared between source and recipient regions than to a genus absent from the native pool in the recipient regions, indicating that taxonomic similarity (a proxy for phylogenetic similarity and niche overlap) promotes invasion success in insects, and thus supporting the pre-adaptation hypothesis.

Global trade has accelerated the spread of wood-boring insects. Woodwasps (Hymenoptera: Siricidae) include several species that have established outside their native ranges, but their global translocation is not well documented. Despite several biases, port interception records can serve as indirect proxies of propagule pressure and can be used to quantify invasion pathways. Here, we use historical border interception records from the USA (1914–2020), Canada (1997–2019), and New Zealand (1950–2007) to infer global transport pathways and propagule pressure of siricids. We compiled 3054 interception records, identifying dominant species, source biogeographical regions, and associated commodities. Interceptions were overwhelmingly linked to wood packaging materials, confirming this pathway as the principal vector of introduction. Most of the intercepted woodwasp species originated from the European and Asian Palearctic regions, indicating these areas as major historical sources of propagule pressure. However, increasing records from non-native regions suggest possible secondary introductions from previously established non-native populations (i.e., bridgehead regions). Despite frequent interceptions, only a subset of species has established, highlighting mismatches between propagule pressure and invasion success, likely mediated by ecological constraints on establishment such as host availability and climate. Our results demonstrate the value of long-term interception datasets for reconstructing invasion pathways, while emphasizing their limitations due to taxonomic uncertainty and sampling bias. Improving species identification and standardizing biosecurity data collection could enhance the use of interception records for invasion risk assessment and management.

The majority of non-native animal species globally are insects, though some insect species are more successful invaders than others. These differences can be attributed, in part, to differences among dominant trophic groups. Previous analyses indicate that insect herbivores are generally over-represented among non-native species while other groups, such as predators, are under-represented. Here we explore how invasion success varies among insect taxa and trophic groups. We quantify over-representation in species grouped by taxon (order or family) and larval trophic group (herbivore, detritivore, predator, parasite, brood carer); over- and under-representation is computed by comparing proportional representation of groups among non-native species in 12 world regions with total numbers in these same groups globally. Although herbivores are generally the most successful group among non-natives, we found their invasion success to vary among their taxonomic groups: herbivores are over-represented among Hemiptera, Diptera, Thysanoptera and Hymenoptera, but under-represented among Lepidoptera and Orthoptera; similar patterns are seen at the family-level within orders. Even after accounting for trophic group, some orders were still over-represented. Within trophic groups, this pattern appeared strongest for herbivores, where predominantly parthenogenetic families belonging to the Hemiptera and Thysanoptera were over-represented in non-native assemblages, while families in the Lepidoptera and Polyneoptera were under-represented. Over time (1850s to 2000s), fractions of non-native species numbers in certain orders and trophic groups have varied, such as among parasites, where a considerable turnover took place from mostly bird-lice (Psocodea) in the 19^th century to parasitic wasps (Hymenoptera) in the 20^th century. It is thus likely that factors other than trophic group, such as associations with invasion pathways (e.g., plants, wood packaging), cause the observed differences in the over-representation of families belonging to different orders.

Propagule pressure is one of the key drivers of establishment success of non-native species, including insects. However, border interception data, which have the potential to act as a proxy for true arrival rates (i.e., propagule pressure) of insects into a country, are seldom used to predict invasions. This can be due to the limited amount of interception data in some countries, difficulties accessing such data, and when these data are available, difficulties in addressing biases caused by variation in interception probability for different taxa due to policy changes, and operational influences of import inspections. The type of interception data required to reliably estimate arrival rates is rarely available. To improve the use of interception data as a proxy for propagule pressure, we investigated the fraction of established species which had interceptions and vice versa by taxonomic group and by biological characteristics (development type and feeding group), using several national datasets from five continents. We identified higher fractions of established species that were intercepted for plant feeding insect groups compared to fractions of non-plant feeding groups, even in countries with more general import inspection strategies. This is likely to reflect greater search effort for and recording of plant-feeding insect groups during inspections. To address this variation in interception probability and improve establishment predictions based on interception frequency, we developed a methodology to account for such taxonomic variation. We apply this to three hazard lists of insects, containing potential pests for an industry or potential pests for a country, to assess its effectiveness.

Aim: The Hemiptera is the fifth‐largest insect order but among non‐native insect species is approximately tied with the Coleoptera as the most species‐rich insect order (Hemiptera comprise 20% more species than in world fauna). This over‐representation may result from high propagule pressure or from high species invasiveness. Here, we assess the reasons for over‐representation in this group by analysing geographical, temporal and taxonomic variation in numbers of historical invasions.Location: Global.Method: We assembled lists of historical Hemiptera invasions in 12 world regions, countries or islands (Australia, Chile, Europe, New Zealand, North America, South Africa, South Korea, Japan and the Galapagos, Hawaiian, Okinawa and Ogasawara Islands) and border interception data from nine countries (Australia, Canada, European Union, United Kingdom, Hawaii, Japan, New Zealand, South Korea, USA mainland and South Africa). Using these data, we identified hemipteran superfamilies that are historically over‐represented among established non‐native species, and superfamilies that are over‐represented among arrivals (proxied by interceptions). We also compared temporal patterns of establishments among hemipteran suborders and among regions.Results: Across all regions, patterns of over‐ and under‐representation were similar. The Aphidoidea, Coccoidea, Aleyrodoidea, Cimicoidea and Phylloxeroida were over‐represented among non‐native species. These same superfamilies were not consistently over‐represented among intercepted species indicating that propagule pressure does not completely explain the tendency of some Hemiptera to be over‐represented among invasions. Asexual reproduction is common in most over‐represented superfamilies and this trait may be key to explaining high invasion success in these superfamilies.Conclusions: We conclude that both propagule pressure and species invasiveness are drivers of high invasion success in the Sternorrhyncha suborder (aphids, scales, whiteflies) and this group plays a major role in the exceptional invasion success of Hemiptera in general. The high historical rates of invasion by Sternorrhyncha species provide justification for biosecurity measure focusing on exclusion of this group.

Aim: Invasions of non‐native insects can have substantial impacts on agriculture, forestry, human health and biodiversity with considerable economic and environmental consequences. To understand the causes of these invasions, it is important to quantify the relative influence of principal drivers such as international imports and climatic effects.Location: North America, Chile, Europe, Australia, New Zealand and Japan.Time Period: 1881–2020.Methods: To evaluate the relative contributions of various factors in explaining global variation in numbers of non‐native insect establishments in different world regions, we conducted two multivariate regression analyses to quantify temporal changes in family‐level composition and native ranges of established non‐native species in several world regions.Results: There were temporal changes in the family‐level composition of non‐native species assemblages. Prior to 1900, invasions were dominated by scale insects, subsequently shifting to a more diverse set of species, except in North America, which had relatively small compositional change over time compared to other regions. Spatial and temporal variation in the composition of established species was associated with differences in the origin of imports and climatic factors, each explaining 26.3% and 27.4% of the total variation, respectively. The analysis of native ranges of non‐native species indicated that there was no consistent temporal variation across all regions. Established species in New Zealand were predominantly native to Australasia and species in North America and Chile were mainly from Europe. Non‐native species in Europe mainly originated from the Nearctic region while those in Japan and Australia generally originated from multiple regions. Climatic factors in the destination regions had a primary effect (66.3%) on variation in the native range of established species, although imports also had substantial effects (45.4%).Main Conclusions: Geographical variation in climate and imports act together as drivers of establishment success for non‐native insects in all six regions.

The geographical exchange of non-native species can be highly asymmetrical, with some world regions donating or receiving more species than others. Several hypotheses have been proposed to explain such asymmetries, including differences in propagule pressure, source species (invader) pools, environmental features in recipient regions, or biological traits of invaders. We quantified spatiotemporal patterns in the exchange of non-native insects between Europe, North America, and Australasia, and then tested possible explanations for these patterns based on regional trade (import values) and model estimates of invader pool sizes. Europe was the dominant donor of non-native insect species between the three regions, with most of this asymmetry arising prior to 1950. This could not be explained by differences in import values (1827–2014), nor were there substantial differences in the sizes of modelled invader pools. Based on additional evidence from literature, we propose that patterns of historical plant introductions may explain these asymmetries, but this possibility requires further study.