Agelenids of the World

Systematics and Taxonomy of Agelenidae, a Worldwide distributed Spider Family

The Hobo spider Tegenaria agrestis (Walckenaer, 1802) - occurrence and distribution in Denmark (Araneae, Agelenidae)

Publication Type:Journal Article
Year of Publication:2009
Authors:R. A. Jensen, Scharff N.
Journal:Entomologiske Meddelelser
Date Published:2009
ISBN Number:0013-8851
Keywords:Anthropogenically-modified habitats / distribution] [Introduction /, Dispersal, Ecology, establishment / collection localities list] [Man made habitat /, Eurasia, Europe, Habitat, Invasion status assessment / ] [Denmark / Zealand / ]., Land, Means of dispersal, Palaearctic region, Tegenaria agrestis (Araneae)., Tegenaria agrestis [Population dynamics / Breeding population, zones, Zoogeography

The media coverage was intense when the Hobo spider, Tegenaria agrestis (Walckenaer, 1802) (Fig. 1), was first recorded for Denmark in 2005 in connection with a joint Swedish-Danish entomological survey of the new artificial island Peberholm, situated in Oresund between Sweden and Denmark. Few people knew that the spider had already been found in eastern Jutland in 2002, since this finding had not been published. The Hobo spider was broadcasted on TV and in newspapers for a few days, and it even inspired two Danish medical doctors to publish a paper on the first possible Hobo spider bite in Denmark, even though neither patient nor doctors had seen or found any spider. The Hobo spider is widespread in Europe but had not been officially re-corded from Scandinavia prior to 2005. In the southern part of Europe it is found in natural habitats, often in dry open areas, but it can also be found in forest clearings and along forest edges. In northern Europe it is mainly associated with wastelands, around construction sites, railway areas and harbours. Except for the Danish "case" above, there are no records of medical problems with Tegenaria agrestis in Europe. It was introduced to North America in the beginning of the last century, and here it is mainly found in houses, where it is considered aggressive and dangerous. There are many medical reports on spider bites and necrotic wounds caused by Tegenaria agrestis. Due to the potential problems with this species, we decided to perform a survey in order to: 1) check the occurrence and distribution of Tegenaria agrestis on Zealand, 2) search for egg sacs to determine whether Tegenaria agrestis is established and breed in Denmark, 3) investigate potential routes of dispersal into Denmark, and 4) decide whether Tegenaria agrestis can be considered an invasive species. We checked 28 suitable localities on Zealand (Table 1, Fig. 13) in the period September 26 to November 3, 2008, and found adult Tegenaria agrestis on 22 of these. Most specimens were found under wooden logs, boards, etc. (Figs 4[long dash]9), at wastelands associated with railway areas (Fig. 2), construction sites or harbors (Fig. 3). Records from other parts of Europe were collected from a large number of helpful colleagues and from the literature, and geo referenced records were used to construct a map showing the records of Tegenaria agrestis within 10 years intervals. (Fig. 12). From this it can be seen that the oldest records are from southern Europe, and that northern Europe mainly has more recent records. The map (Fig. 12) shows that Tegenaria agrestis has been spreading northwards. For instance in the United Kingdom, where the species was first recorded from southern UK in 1949, then subsequently re-corded from Wales and England in the following years, from southern Scot-land in 1971 and northern Scotland in 2006. The records also show that it was first recorded from Ireland in 2000. Egg sacs (Figs 10[long dash]11) were found at most localities in Denmark, proving that Tegenaria agrestis is breeding in Denmark. Due to the late time of the year, only a single male was found. Tegenaria agrestis has not yet been recorded from Norway and Finland, but has been found at three localities in southern Sweden since 2005 (Malmo, Ystad and Kristanstad). The literature supports the general distribution tendency shown on Fig. 12, with the first records from southern Europe predating the 1950s and subsequent first records from northern European countries after 1950. All Danish records of Tegenaria agrestis are mapped on Fig. 13. Given the distribution of Tegenaria agrestis on Zealand, we expect the species to be wide-spread elsewhere in Denmark. All findings of Tegenaria agrestis in Denmark are associated with harbors, construction sites and railway areas, and it is therefore likely that the species has been introduced to Denmark with goods and building materials. It seems to be dependent on open warm habitats, like those associated with waste grounds, and it is therefore highly unlikely that the species will be able to spread to natural habitats in Denmark, except perhaps sand dunes and heathlands. In Denmark, Tegenaria agrestis has been found together with Tegenaria atrica, Amaurobius fenestralis and Textrix denticulatus [long dash] species that occur naturally on the localities where 7: agrestis have been found. Therefore, there is not yet any reason to assume that T. agrestis is invasive, i.e, an alien species whose introduction and/or spread threatens local biodiversity. There is no reason to fear Tegenaria agrestis more than any other Tegenaria species, since there is no proven connection between bites of this species and the medical problems reported in the literature. None of these medical cases seems to be based on a correctly identified T. agrestis. The authors of this paper can confirm that common house spiders, Tegenaria atrica and T. domestics, can indeed bite, when provoked, and their fangs can penetrate the human skin, but aside from the pain the fangs generate when they penetrate the skin (like a pin prick), the bites have no additional effect, and do not generate any tissue damage.

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