The primary aim of the present study is to analyse the introduction pathways, spread, and risk of establishment in wild of (potentially) invasive ants in the Netherlands.

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In total, 40 alien ant species were found in the Netherlands with established populations, of which 10 species can (also) occur outside buildings. New discoveries are made regulary, so these numbers can change quickly. These alien ants mainly hitchhike as benthic animals with nursery material and tourists. Via this pathway, they are introduced and distributed in the Netherlands and the rest of Europe. The number of first records of alien ants has increased over the years in the Netherlands and neighboring countries. For many species, (up-to-date) information about their invasiveness and settlement status is currently lacking in national databases, such as the Dutch Species Register (Nederlands Soortenregister). Furthermore, apart from Lasius neglectus and some Solenopsis species (fire ants), no risk assessments were available for alien ant species observed in the Netherlands. Based on sufficient availability of scientific literature, 21 species were assessed using the Harmonia+ protocol (Table 1). The results show that Crematogaster scutellaris, Lasius grandis, Lasius neglectus, Linepithema humile, Pheidole pallidula, Plagiolepis pygmaea, Plagiolepis schmitzii and Tapinoma nigerrimum-complex can have negative impacts on biodiversity and plants by competing with native (ant) species, promoting aphid populations and by reaching high densities that may affect food chains (and thereby biodiversity). When an ant colony has settled indoors, they mainly cause a nuisance because they are everywhere. They can crawl into electrical appliances and store food and debris in there, which can lead to short circuits. Furthermore, alien ants can bite and contaminate food, which can cause a lot of annoyance. Crematogaster schmidti, Crematogaster scutellaris, Lasius neglectus, Linepithema humile, Monomorium pharaonis, Paratrechina longicornis, Pheidole megacephala, P. pallidula, Plagiolepis pygmaea, Plagiolepis schmitzii, Tapinoma melanocephalum, Solenopsis geminata, Solenopsis invicta, T. nigerrimum-complex and Wasmannia auropunctata pose risks indoors. However, quantitative information on these effects is scarce. With the Harmonia+ protocol, risk scores per species have been calculated using an extensive questionnaire. The scores are subsequently classified as "low" (risk score lower than 0.33), "moderate" (risk score between 0.33 and 0.66) and "high" (risk score higher than 0.66). To calculate a risk score for a specific risk category, the maximum value is used to prevent the averaging of securities, as well as the average value. After all, the use of the maximum value prevents the averaging of risk scores within and between different effect categories. From the precautionary principle, such a calculation method is relevant because the high risks of effects within one or a few subcategories always remains visible in aggregated risk scores. In total, eleven species have a high risk score, and these are Lasius neglectus, Linepithema humile, Monomorium pharaonis, Monomorium trageri, Pheidole megacephala, Pheidole pallidula, Plagiolepis pygmaea, Solenopsis geminata, Solenopsis invicta, Tapinoma nigerrimum-complex and Wasmannia auropunctata. Of these, Lasius neglectus and Tapinoma nigerrimum-complex have the maximum risk score of 1. Monomorium pharaonis, Pheidole megacephala, Solenopsis geminata, Solenopsis invicta and Wasmannia auropunctata cannot establish outside buildings. Three species occurring outside buildings in the Netherlands have a moderate risk score, which are Crematogaster scutellaris, Lasius grandis and Plagiolepis  schmitzii. In addition, four species, Paratrechina longicornis, Solenopsis richteri, Tapinoma melanocephalum and Technomyrmex vitiensis, which occur only indoors, have a moderate risk score. The species Crematogaster schmidti and Tapinoma pygmaeum have a low risk score. The species Monomorium trageri and Plagiolepis invadens do not yet occur in the Netherlands, but are expected to survive in the Dutch climate. Monomorium trageri has a high risk score and Plagiolepis invadens a moderate risk score.Due to the limited availability of information about ants and their possible impact, the certainty of most risk scores is low to moderate. Therefore, it is recommended to periodically update the risk assessments based on new scientific data.
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