European Insect Research Articles

Scientific Opinion on the risks to plant health posed byXylella fastidiosain the EU territory, with the identification and evaluation of risk reduction options

The EFSA Panel on Plant Health conducted a pest risk assessment and an evaluation of risk reduction options for Xylella fastidiosa. X. fastidiosa has been detected in olive in the EU with a distribution restricted to the region of Apulia in Italy and is under official control. X. fastidiosa has a very broad host range, including many common cultivated and wild plants. All xylem fluid-feeding insects in Europe are considered to be potential vectors. Philaenus spumarius (Hemiptera: Aphrophoridae), a polyphagous spittlebug widespread in the whole risk assessment area, has been identified as a vector in Apulia. The probability of entry of X. fastidiosa from countries where X. fastidiosa is reported is very high with plants for planting and moderate with infectious insect vectors carried with plant commodities or travelling as stowaways. Establishment and spread in the EU is very likely. The consequences are considered to be major because yield losses and other damage would be high and require costly control measures. The systematic use of insecticides for vector control may create environmental impacts. With regard to risk reduction options, strategies for the prevention of introduction and for the containment of outbreaks should focus on the two main pathways (plants for planting and infectious insect vectors) and combine the most effective options in an integrated approach. For plants for planting, these could be pest-free production areas, surveillance, certification, screened greenhouse production, vector control and testing for infection and, for some plant species, treatments (e.g. thermotherapy). To prevent entry of the infectious vectors, insecticide treatments and inspection of consignments and production sites are required. The Panel has also reviewed the effectiveness of risk reduction options for X. fastidiosa and its vectors listed in Directive 2000/29/EC and in the EU emergency measures. The Panel recommends the continuation and intensification of research on the host range, epidemiology and control of the Apulian outbreak.

Host range expansion of native insects to exotic trees increases with area of introduction and the presence of congeneric native trees

Summary Exotic tree species are widely used in forest plantations for their often high productivity and performance compared to native trees. However, these advantages may be compromised by herbivore damage. A list of European insect species that have expanded their host range to one of 28 exotic tree species introduced to Europe was compiled from a systematic literature review. The number of successful expansions was analysed using three predictors: (i) phylogenetic relatedness between exotic and European tree species; (ii) area covered by exotic tree species in Europe; and (iii) time since their introduction into Europe. In total, 590 host expansions of native insects to exotic trees were found, mainly of polyphagous species (43%); 25% of the cases reported some type of damage. Bark and wood borers, and defoliators were the dominant guilds. The number of recruited native insect species and cases where major damage occurred was positively correlated with the geographical extent of exotic trees in Europe and the presence of congeneric native trees. Synthesis and applications. The use of exotic tree species creates opportunities for native insect herbivores to expand their host range and increase their damage if they are widely planted next to native congeners. Risk assessment studies are recommended when introducing new tree species for forestry plantations. Risk assessments should include trials on susceptibility to any potential damaging organisms in the introduced range.

Evolution and island endemism of morphologically cryptic Baetis and Cloeon species (Ephemeroptera, Baetidae) on the Canary Islands and Madeira

Summary The Canary Islands and Madeira are reportedly home to seven recognised species of baetid mayflies (Ephemeroptera, Baetidae), two of which also occur on the European mainland. Their species status remains unsure, and loss of habitat suggests they are of conservation concern. We applied morphological characters and a general mixed Yule‐coalescent (gmyc) model analysis of the cytochrome c oxidase subunit 1 (cox1) gene to delineate putative species within morphologically cryptic species groups Baetis (Rhodobaetis) and Cloeon dipterum s.l. We used a three‐gene mitochondrial data set (1450 base pairs) to infer phylogenetic relationships and a molecular clock calibrated using island geological ages to infer colonisation history. Genetic and morphological evidence indicated the presence of 12 putative species, 11 of which were endemic to the islands. Only Baetis atlanticus, on Madeira, also occurs on the European mainland. Two lineages (B. pseudorhodani s.l. and B. canariensis s.l.) appear to have arisen in the past 15 million years (mya) and diversified in parallel throughout the Canary Islands. Within the canariensis lineage, sister species occur on the island of Gran Canaria and in North Africa. Pronounced island endemism contradicts previous taxonomic work, which reported a depauperate fauna that included several mainland species. Recent diversification among islands and a close link to North Africa suggest a complex evolutionary history. Owing to their small population size and ongoing habitat alteration, several of these island endemics are among the most endangered aquatic insects in Europe.

Global warming favours light-coloured insects in Europe.

Associations between biological traits of animals and climate are well documented by physiological and local-scale studies. However, whether an ecophysiological phenomenon can affect large-scale biogeographical patterns of insects is largely unknown. Insects absorb energy from the sun to become mobile, and their colouration varies depending on the prevailing climate where they live. Here we show, using data of 473 European butterfly and dragonfly species, that dark-coloured insect species are favoured in cooler climates and light-coloured species in warmer climates. By comparing distribution maps of dragonflies from 1988 and 2006, we provide support for a mechanistic link between climate, functional traits and species that affects geographical distributions even at continental scales. Our results constitute a foundation for better forecasting the effect of climate change on many insect groups.

Molecular Characterization and In Silico Analysis of the Pheromone-Binding Protein of the European Grapevine Moth Lobesia botrana (Denis & Schiffermüller) (Lepidoptera, Tortricidae).

The European grapevine moth Lobesia botrana (Denis & Schiffermüller) is an economically important insect in Europe. The species invaded vineyards in Chile, Argentina, and California during 2008-2010 causing severe problems. A major component of the sex pheromone, (E,Z)-7,9-dodecadienyl acetate (E7,Z9-12:Ac), is used in a mating disruption technique when grapevine moth populations are low or to monitor pest numbers. It is thought that these sexual pheromones are blends of volatiles that typically are specific to a species and are transported in the insect antenna by pheromone-binding proteins (PBPs) across the sensillar lymph to the olfactory receptors. Currently, an increasing number of Lepidopteran PBPs are being identified and cloned. However, there are no studies of the olfactory system and of proteins involved in the olfactory perception of L. botrana at the molecular level. In the present study, we report, for the first time, the sequence of a PBP from L. botrana (LbotPBP), which was determined using reverse transcription technology. Homology modeling was used to generate the three-dimensional protein structure. The model suggests that PBP consists of six α-helices as follows: Lys2-Met23 (α1), Thr28-Phe36 (α2), Arg46-Leu59 (α3), His70-Asn80 (α4), Glu84-Asn100 (α5), and Cys108-Lys125 (α6), held together by three disulfide bridges, Cys19-Cys54, Cys50-Cys108, and Cys97-Cys117. Docking simulations based on this model suggested that Trp114 is a key residue in the recognition of acetate pheromones, such as E7,Z9-12:Ac. In silico results in this study are consistent with previous findings in which E7,Z9-12:Ac acts as the most active compound in behavioral and electroantennographic assays.

The invasive Leptoglossus seed bug, a threat for commercial seed crops, but for conifer diversity?

Among the recent introductions of alien insects in Europe, the polyphagous western conifer seed bug, Leptoglossus occidentalis Heidemann (Het- eroptera; Coreidae) can seriously be regarded as a major threat for all the European conifer forests. In the current study combining laboratory and field experi- mentations, we characterized first bug damage by developing specific damage categories on seeds of different conifer species by the use of X-ray. Sec- ondly, we investigated the impact of the invasive bug on key conifer species used for afforestation in Western and Central Europe. For this purpose, we performed germination tests on predated seeds which revealed that even light damage (consumption of\1/3 of the whole seed content) strongly reduced the germination capability of the seed. We also compared the impact of feeding on the proportion of filled seeds. Second year cones of Pinus sylvestris and Pinus nigra have been enclosed and offered to different life stages (nymphs and adults) and the results showed a signif- icant reduction of filled seeds whatever the life stage. In field, we annually surveyed the bug seed damage for six different conifer species planted in southwestern French seed orchards. Taking into account the eco- nomic value of improved seeds in seed orchards, economic impact of bug damage was important although never exceeded 25 %. Two natural or semi- natural alpine pine stands were also surveyed and appeared to be highly affected by the bug (up to 70 % of damaged seeds). Therefore, bug damage could also be considered as a serious threat for seed production in natural stands.

Statement of EFSA on host plants, entry and spread pathways and risk reduction options for Xylella fastidiosa Wells et al.

Following a request from the European Commission, EFSA was asked to provide urgent scientific and technical assistance on the plant pathogenic bacterium Xylella fastidiosa. X. fastidiosa was detected in olive trees in Lecce province in Apulia, Italy, in October 2013. This is the first outbreak of X. fastidiosa under field conditions in the European Union. EFSA reviewed the host range and vectors, the pathways for entry and spread and the risk reduction options. Known hosts include many cultivated and spontaneous plants common in Europe, however a range of European wild plant species would meet this bacterium for the first time, increasing uncertainty on the host range. All xylem-fluid feeding insects in Europe should be regarded as potential vectors of X. fastidiosa and identification of the vector in the Apulian outbreak is pending. The main entry pathway for X. fastidiosa is the movement of plants for planting. Infective vectors of X. fastidiosa transported on plant consignments are also of concern. The only route for natural spread of X. fastidiosa is by insect vectors that generally fly short distances up to 100 metres, but can be transported by wind over long distance. The movement of infected plants for planting is the most efficient way for long-distance dispersal of X. fastidiosa. There is no record of successful eradication of X. fastidiosa once established outdoors due to the broad host range of the pathogen and of its vectors. Strategies for prevention of introduction from areas where the pathogen is present and for containment of outbreak should focus on the two main pathways and be based on integrated system approach combining, when applicable, the most effective options.

Irradiation as a quarantine treatment for the solenopsis mealybug, Phenacoccus solenopsis

Phenacoccus solenopsis is an aggressively invasive species that targets agricultural and ornamental plants, thereby threatening the world cotton industry and other crops. P. solenopsis has been listed as a quarantine insect in Europe and China. The utilization of phytosanitary irradiation as a potential treatment for disinfesting agricultural commodities in trade has expanded rapidly in recent years. A reasonable dose of radiation to eliminate P. solenopsis needs to be determined, taking into account the side effects of radiation on agricultural products and the species-specific tolerance of the insect to radiation. We applied radiation ranging from 50 to 200Gy to P. solenopsis to determine the optimal dose. Both the radiation dose and the developmental stage of the insect were independent variables. Higher doses of radiation or lesser mature insect stages provided more effective treatment. In nymphs, a radiation dose of 100Gy caused extinction of the irradiated population by disrupting ovary development, while 150Gy caused 100% mortality. In adults, all tested doses of irradiation did not affect longevity, but we were able to prevent reproduction with high (150 and 200Gy) doses. In P. solenopsis, a 100Gy dose of radiation could eliminate the irradiated population in two generations. The mortality curve showed a steep slope beyond 150Gy; thus, if killing all of the insects in a shorter amount of time is necessary, 200Gy may be a reasonable dose for the quarantine treatment of the solenopsis mealybug.

Recovery of African horse sickness virus from synthetic RNA

African horse sickness virus (AHSV) is an insect-vectored emerging pathogen of equine species. AHSV (nine serotypes) is a member of the genus Orbivirus, with a morphology and coding strategy similar to that of the type member, bluetongue virus. However, these viruses are distinct at the genetic level, in the proteins they encode and in their pathobiology. AHSV infection of horses is highly virulent with a mortality rate of up to 90 %. AHSV is transmitted by Culicoides, a common European insect, and has the potential to emerge in Europe from endemic countries of Africa. As a result, a safe and effective vaccine is sought urgently. As part of a programme to generate a designed highly attenuated vaccine, we report here the recovery of AHSV from a complete set of RNA transcripts synthesized in vitro from cDNA clones. We have demonstrated the generation of mutant and reassortant AHSV genomes, their recovery, stable passage, and characterization. Our findings provide a new approach to investigate AHSV replication, to design AHSV vaccines and to aid diagnosis.

Evolution ofCryptocephalusleaf beetles related toC. sericeus(Coleoptera: Chrysomelidae) and the role of hybridization in generating species mtDNA paraphyly

Gómez‐Zurita, J., Sassi, D., Cardoso, A. & Balke, M. (2011). Evolution ofCryptocephalusleaf beetles related toC. sericeus(Coleoptera: Chrysomelidae) and the role of hybridization in generating species mtDNA paraphyly. —Zoologica Scripta,41, 47–67.Autochthonous European insect diversity is the result of the very complex geological, geographic and climatic history of the Mediterranean area. The leaf beetle genusCryptocephalushas over 250 species in this area. Among them stands out a group nearly endemic from this region consisting of conspicuous metallic green or blue beetles which can be found visiting yellow Asteraceae flowers in most mid‐ to high‐altitude European grasslands: the Linnaean speciesC. hypochaeridis,C. sericeus, and all their relatives. In all, these are 32 species forming several taxonomically complex groups across Europe. We sampled all morphological diversity in this lineage and characterized it for two mitochondrial DNA genes. The mtDNA phylogeny of this assemblage was inferred, as well as the timing of its diversification using standard mtDNA substitution rates and a hypothetical Messinian vicariant split. The origin of the group can be traced back to western continental Eurasia in the Lower Miocene. Its subsequent taxonomic splits can be linked to specific periods in the formation of Europe, with a marked trend to east–west phylogenetic divides throughout time and space, and a nearly constant rate of diversification. Only during the Pleistocene, a significant increase in diversification rate can be associated with species formation in theC. hypochaeridisandC. sericeusspecies complexes. Within these latter groups, most taxa show some degree of mtDNA paraphyly as a result of their recent separation and remarkably by episodes of gene flow in areas of secondary contact among recently diverged species, possibly driven by climatic change.

Chemical Ecology and Management of Lobesia botrana (Lepidoptera: Tortricidae)

The moth Lobesia botrana (Denis & Schiffermüller) (Lepidoptera: Tortricidae) feeds on grapes (Vitis vinifera L.), reducing yield and increasing susceptibility to fungal infections. L. botrana is among the most economically important insects in Europe and has recently been found in vineyards in Chile, Argentina, and California. Here, we review L. botrana biology and behavior in relation to its larval host (the grapevine) and its natural enemies. We also discuss current and future control strategies in light of our knowledge of chemical ecology, with an emphasis on the use of the sex pheromone-based strategies as an environmentally safe management approach. Pheromone-mediated mating disruption is the most promising technique available on grapes and is currently implemented on approximately 140,000 ha in Europe. Experience from several growing areas confirms the importance of collaboration between research, extension, growers, and pheromone-supply companies for the successful implementation of the mating disruption technique. In the vineyards where mating disruption has been successfully applied as an areawide strategy, the reduction in insecticide use has improved the quality of life for growers, consumers, as well as the public living near wine-growing areas and has thereby reduced the conflict between agricultural and urban communities.

Incidence of the pinewood nematode Bursaphelenchus xylophlius Steiner & Buhrer, 1934 (Nickle, 1970) in Spain

The pine wood nematode (PWN) Bursaphelenchus xylophilus Steiner & Buhrer, 1934 (Nickle, 1970), is a pathogenic species in the genus Bursaphelenchus Fuchs, 1937. The introduction and establishment of PWN in the Setubal Peninsula, Portugal (Mota et al., 1999), and the evidence that Monochamus galloprovincialis is its insect vector in this country (Sousa et al., 2001), has stimulated studies on its further distribution and its potential vector insects in Europe, including Spain. There is a clear risk of spread of pine wood nematode into Spanish territory from Portugal, both as a result of natural spread and due to the important timber trade between the two countries. The occurrence of Bursaphelenchus spp. in Spain has been previously studied (Abelleira et al., 2002, 2003; Escuer et al., 2003, 2004; Robertson et al., 2008). These studies have shown, in addition, the distribution of insects belonging to the Cerambycidae, Curculionidae and Scolytidae families (Cobos, 1986; Gil Sanchez & Pajares Alonso, 1986; Vives, 2000; Verdugo Paez, 2004), which are known to be vectors of Bursaphelenchus spp. other than B. xylophilus and are often found to be associated with stressed, dead or decaying conifers. Although it is generally accepted that B. xylophilus is the only truly pathogenic species (reviewed by Jones et al., 2008), other species may be involved in pine forest decline in some European regions or, alternatively, may simply be opportunistic infectors of stressed trees (Mamiya, 1999; Caroppo et al., 2000; Skarmoutsos et al., 2000). Pinus pinaster is the most representative

Maria Sibylla Merian (1647-1717): an early investigator of parasitoids and phenotypic plasticity

AbstractIn a time when science and technology were almost exclusively dominated by men, the European naturalist and biological illustrator, Maria Sibylla Merian (1647-1717), studied arthropods, particularly American and European insects. rough careful observations, she documented and lavishly depicted phenomena that today we call parasitoidism and phenotypic plasticity, making her one of the first to inquire into these topics. Merian's legacy emphasizes the inextricable link between the environment and organisms.

Enantiomeric conservation of the male‐produced sex pheromone facilitates monitoring of threatened European hermit beetles (Osmoderma spp.)

AbstractHermit beetles of the genus Osmoderma (Coleoptera: Scarabaeidae: Cetoniinae) are known for their fruity odour, which is released in large amounts by males. Two species of the genus occur in Europe, the eastern Osmoderma barnabita (Motschulsky) and the western Osmoderma eremita (Scopoli). Previous studies on Swedish populations of O. eremita showed that the compound responsible for the characteristic scent, γ‐decalactone, functions as a sex pheromone for the attraction of conspecific females. Male O. eremita only release the (R)‐enantiomer of the lactone, and both sexes are anosmic to the opposite enantiomer. As the distribution areas of the two hermit beetle species partly overlap, it may be expected that they use different enantiomeric compositions of γ‐decalactone as pheromones to promote species discrimination. This paper reports on the identification of the sex pheromone of O. barnabita. Surprisingly, males from a Polish population produce only the (R)‐enantiomer of γ‐decalactone, and conspecific females show equal attraction to the (R)‐enantiomer and a racemic mixture of the compound, indicating that O. barnabita is anosmic to the (S)‐enantiomer, similarly to what was observed for O. eremita. A mtDNA sequence analysis of the cytochrome oxidase subunit I gene of Polish and Swedish beetles confirmed their taxonomical status as O. barnabita and O. eremita, respectively, with an average sequence divergence of 10.5% between beetles from the two studied areas. Although genetic data suggest that these species diverged several million years ago, they still rely on the same enantiomer of γ‐decalactone for mate finding. Thus, the male‐produced pheromone in Osmoderma spp. may be regarded as a territorial signal being exploited by females, rather than a cue for determining species identity. Our data show that the same compound can be used to facilitate monitoring of both beetle species, which are considered indicator species of the species‐rich fauna of saproxylic insects in Europe.

Biology and host specificity ofAulacobaris fallax(Coleoptera: Curculionidae), a potential biological control agent for dyer’s woad,Isatis tinctoria(Brassicaceae) in North America

Dyer’s woad, Isatis tinctoria, a plant of Eurasian origin is a problematic weed in western North America against which a classical biological weed control programme was initiated in 2004. Three European insect species were selected as candidate agents to control this invasive species, including the root-mining weevil Aulacobaris fallax. To determine its suitability as an agent, the biology and host specificity of A. fallax were studied in outdoor plots and in the field between 2004 and 2006 in its native European range. Aulacobaris fallax is a univoltine species that lays its eggs from March to August into leaf stalks and roots of dyer’s woad. Larvae mine and pupate in the roots and adults emerge from August to October. Up to 62% of the dyer’s woad plants at the field sites investigated were attacked by this weevil. In no-choice host-specificity tests, A. fallax attacked 16 out of 39 species and varieties within the Family Brassicaceae. Twelve of these are native to North America. In subsequent multiple-choice tests, seven species, all native to North America, suffered a similar level of attack as dyer’s woad, while none of the European species were attacked. Our results demonstrate the importance of including test plant species that have not co-evolved with the respective candidate agent. In sum, we conclude that the risk of non-target effects is too high for A. fallax to be considered as a biological control agent for dyer’s woad in the United States.

Habitat preferences of oak-feeding xylophagous beetles in a temperate woodland: implications for forest history and management

Oaks host the richest fauna of saproxylic insect in Europe. We studied habitat preferences of two beetle families, Buprestidae and Cerambycidae, by rearing the beetles from standardised oak timber baits. Species density was higher in the understorey than in the canopy; and in sun-exposed baits if within the understorey. Insolation was the most important factor affecting the composition of reared assemblages (explaining ca. 30% of variation in the data), followed by vertical stratum (ca. 10%). Local dead wood volume had no effect. The high preference for sun-exposed wood located near the ground suggests that: (i) open-canopy woodlands had to be rather common in temperate Europe; (ii) oak-utilising xylophages would benefit from restoration of management practices such as coppicing or woodland pasture; (iii) the policy of increasing dead wood volume in commercial forests is principally correct, but its success will depend on dead wood location within the forests.

Invasive insects in Europe - the role of climate change and global trade

Invasive insects in Europe - the role of climate change and global trade

How can alien species inventories and interception data help us prevent insect invasions?

Information relevant to invasion processes and invasive alien insect species management in Central Europe was extracted from two databases: a compilation of two inventories of alien insects in Austria and Switzerland, and a list of interceptions of non-indigenous plant pests in Europe gathered by the European and Mediterranean Plant Protection Organisation (EPPO) for the period 1995-2004. For one-third of the insects established in Switzerland and Austria, the region of origin is unclear. Others come mainly from North America, Asia and the Mediterranean region. Among the intercepted insects, 40% were associated with commodities from Asia, 32% from Europe and only 2% from North America. Sternorrhyncha, Coleoptera and Psocoptera were particularly well represented in the alien fauna compared to the native fauna. In the interception database, Sternorrhyncha were also well represented but Diptera accounted for the highest number of records. Sap feeders and detritivores were the dominant feeding niches in the alien insect fauna. In contrast, external defoliators, stem borers, gall makers, root feeders, predators and parasitoids were underrepresented. Nearly 40% of the alien insects in Switzerland and Austria live only indoors. Another 15% live outdoors but exclusively or predominantly on exotic plants. Less than 20% are found mainly in 'natural' environments. The majority of introductions of alien insects in Europe are associated with the international trade in ornamental plants. An economic impact was found for 40% of the alien insects in Switzerland and Austria, whereas none is known to have an ecological impact. The implications of these observations for further studies and the management of alien species in Europe are discussed.

Population structure and its implications for conservation of the great silver beetle Hydrophilus piceus in Britain

Summary1. The great silver water beetle Hydrophilus piceus is one of the largest aquatic insects in Europe. In Britain it is rare and endangered, and confined to a small number of low‐lying marshes. Very little is known about the beetle populations in any of these areas, or the connectivity between them.2. To investigate the population structure of H. piceus in Britain, four polymorphic microsatellite loci were identified and characterized. The genome of this beetle seems to have few microsatellites but contains a high proportion of a larger repeated sequence.3. All six of the main British populations (Somerset, Lewes, Pevensey, Romney, North Kent and Norfolk) showed substantial genetic diversity at the microsatellite loci. However, estimates of effective population size at one site (Pevensey) were remarkably low, at <10 adults for the period 2004–05.4. Most of the genetic diversity was partitioned within rather than among the populations, although there was, nevertheless, significant genetic sub‐structuring. Almost all population pairwise Fst estimates were significantly different from zero, and there was a clear isolation‐by‐distance effect. Assignment tests and cluster analyses demonstrated interpopulation relationships largely consistent with their geographical separations.5. Hydrophilus disperses by flight, and records from moth traps indicated that there was no month in which the beetles never flew, but that flight activity was highest in the spring.6. The genetic data highlight the need to maintain or regenerate habitat connectivity within flying distance for H. piceus, and to sustain large areas of suitable breeding marshes.

A lack of native congeners may limit colonization of introduced conifers by indigenous insects in Europe

We compared the recruitment of phytophagous arthropod pests onto exotic conifers introduced in Europe without any congeners with that of exotic conifers that have native congeners. In 130 years of extensive plantation forestry in Europe, Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) recruited only 87 arthropod species, i.e., only 33.9% of the number of associated arthropod species in its native range (257 spp.). Exotic species of Cupressaceae without indigenous congeners also recruited only a portion (3.4% to 57.9%) of the arthropod fauna observed in their native range. In both cases, the majority of the recruited species were polyphagous, i.e., that they can feed on plants of different families of conifers and (or) angiosperms. In contrast, exotic conifers with native congeners recruited most of the insects colonizing the native congeneric conifers. Differences in arthropod recruitment were observed according to both guild and feeding habit, with the externally feeding herbivores being dominant. Typically, the damage caused by native insects that had been recruited by exotic conifers without congeners was limited, whereas the damage caused by native insects that had been recruited by exotic conifers with congeners often led to severe outbreaks at the time the shift between hosts occurred. However, when a highly specialized exotic insect was introduced along with the host, the invasive insect tended to occupy the entire niche, causing more damage than in the original range, in the absence of natural enemies and indigenous competitors.