Invasive species constitute a growing problem in European and North American forests and can be a serious threat to forest sustainability (Ciesla, 2003; Roques, 2007). Despite efforts to combat introductions, the rate of establishment for exotic arthropods remains constant (Stendlid et al., 2007; Aukema et al., 2010). Regardless of ongoing introductions, little is known about the impact these pests may have on the forest ecosystem, and risk to these systems can be difficult to assess (Kenis et al., 2009). While many introduced forest insects pose little notable risk to forest stability, others, such as pine wilt nematode and processionary moth, are eliminating and damaging tree species in European forests.

SIRICIDAE AS FORESTRY PESTS

Members of Siricidae, commonly called horntails or woodwasps, are considered important, major forest pests (Viitasaari & Heliövaara, 2004). Due to their symbiotic association and ability to vector wood-decaying fungi, some siricid wasps are able to kill trees. While ovipositing, females introduce the eggs along with the fungus and their own mucus, which possesses phytotoxic effects (Coutts, 1970), into the sapwood. Many horntail species are considered as secondary pests in their native range because they tend to attack weak or dying trees or trees that have recently died. However, when they are introduced into non-native countries, the economic, environmental and forestry impacts of these horntails can be greater. Biology of Siricinae is well documented in Europe (Viitasaari & Heliovaara, 2004).

An invasive woodwasp, Urocerus albicornis, recently was added to the list of potential secondary forest pests in Spain as the result of several isolated interceptions in sawmills (Lopez et al., 2012). This species is native to North America, being present in Newfoundland to British Columbia, south to North Carolina, Michigan, Missouri, New Mexico, and California (Schiff et al., 2006). Due to timber importation to Europe, it has been also detected in the United Kingdom, the Netherlands, Iceland and Poland (Ólafsson, 1991; Liston, 1995; Witmond, 2001; Rasplus et al., 2010). However, its establishment in these countries is not fully confirmed due to insufficient data.

In North America, U. albicornis is reported to attack stressed or dead trees and does not cause excessive damage (Dodds et al., 2010). Conifers like Abies Mill., Larix Philip Miller, Picea (Dietr. A.) Link, Pinus L., Tsuga Carrière, Douglas-fir Pseudotsuga menziesii (Mirb.) Franco and western red cedar (Thuja plicata Donn ex D. Don) are listed as natural hosts in North America (Cameron, 1965; Schiff et al., 2006). Oviposition appears to take place in freshly cut or killed trees (Middlekauff, 1960) and the life cycle lasts 2 years (Belyea, 1952; Morris, 1967). Damage by larvae was described by Thomas (1881). In addition, Stillwell (1966) isolated the fungus Amylostereum (=Stereum) chailletii (Pers.) Boid. from the intersegmental sacs of U. albicornis reared in logs of Abies balsamea (L.) Mill. in New Brunswick and Nova Scotia, Canada. Several insect species are reported as parasitoids of U. albicornis in North America: Megarhyssa nortoni quebecensis (Provancher, 1873) (Hymenoptera: Ichneumonidae), Rhyssa lineolata (Kirby, 1837) and R. crevieri (Provancher, 1880) (Hymenoptera: Ichneumonidae), and Ibalia leucospoides (Hochenwarth, 1862) (Hymenoptera: Ibaliidae) (Middlekauff 1960; Cameron 1965; Schiff et al., 2012).

SIRICIDAE IN FORESTS OF THE IBERIAN PENINSULA

Little is known about the potential risk of the presence of U. albicornis in the Iberian Peninsula. Further studies are needed to know if it has been definitively established in the Basque Country, and on its ecology, focusing on the biology (e.g., flight period, hosts, development) and the presence of potential parasitoid species. U. albicornis is regarded as a secondary pest in its native range (Schiff et al., 2012) and there are no reports of damage caused by U. albicornis in Europe. However, as mentioned above, the association with A. chailletii has been reported in its natural distribution range (Wilson et al., 2009). It should be taken into account that a possible lack of defensive mechanisms of native host trees outside the pests natural distribution range may make them susceptible to U. albicornis infestation, and therefore this pest in a new host range may represent a serious threat as was the case with S. noctilo in Australia, South America and South Africa. Thus, the detection of wood-decaying fungi associated with U. albicornis should be stated as a critical point in order to assess the real risk of its presence. Moreover, additional work is needed to study in greater depth the biology of this insect in its new distribution, notably what kinds of trees are attacked (e.g., healthy or decaying). This information will allow determination of the role of U. albicornis as a potential forestry pest.

A first step in investigating Siricidae is to gain an understanding of species composition and flight periods in pine forests that are likely to be colonized by U. albicornis. Forest insects’ emergence and flight periods can be influenced by weather and factors such as temperature can cause a change in phenology between populations of the same species (Jepsen et al., 2011). Warmer temperatures can have an indirect effect by stimulating growth in pines and a direct effect by increasing insect metabolism (Thomson et al., 2010). The biology of Siricidae has been poorly studied in the Iberian Peninsula and no species have been documented within the Basque Country. Fauna Europaea (2013) listed eight Urocerus and six Sirex species in Europe. To date, for the Iberian Peninsula Urocerus has been represented by four species, i.e., Urocerus gigas, Urocerus fantoma (Fabricius, 1781), U. albicornis and Urocerus augur (Ceballos Jiménez, 1963; Lopez et al., 2012). Sirex is represented by one species, Sirex noctilio.

The objective of this study is to determine the species composition and flight period of native Siricidae within Basque Country pine forests and to confirm the status of U. albicornis. Knowing flight periods for native Siricidae can eventually lead to an understanding of dispersal, inter-species interactions and oviposition behavior.

SITES

Two sites were chosen in pine forests within each province. Sirex lures were erected at sites in six distinct Basque Country forest regions (Menagarai and Gordoa forests in Alava Province, Orio and Zegama forests in Gipuzkoa Province and Carranza and Muxika forests in Bizkaia Province). Traps were monitored throughout 2011 and 2012. At the same time in January of both years, ten dead logs with oviposition holes from the same localities were collected and placed in emergence cages for wasp emergence.

Logging occurred within each site in the summer or fall of 2010 and much of the slash was left in the sites through the 2011 and 2012 siricid collecting seasons. These stands in the sites were mostly pine with a component of hardwoods (Crataegus spp., Ilex spp., Acer spp., Rubus spp.) mainly in the understory. These stands had a mixture of tree age and diameter. Pines within the stands were Monterrey pine (Pinus radiata D. Don.). These sites ranged from moderately to heavily managed stands and two points were randomly selected in each site.

TRAPPING IN THE FIELD

Siricids were collected at each site with three interception traps baited with Contech Inc. Sirex lure (α-β pinene) plus 95% high-release ethanol (Fig. 1). Flight traps were used because it is suggested that they outperform Lindgren funnel traps in capturing large bodied insects, such as siricids (Johnson et al., 2009). The traps were 70 cm long and 25 cm wide and hung 1 m above the ground. Trapped specimens were captured in propanodyol in a collection container at the bottom of the trap. Traps were placed approximately 40 m apart at each site. Trapping was initiated from late April to early December in 2011 and 2012. Trap catches were collected every two weeks until early December in both years. Lures were changed according to manufacturer recommendations, approximately once every 4 weeks.

All trap catches were transferred into plastic vials in the field, brought back to the lab and stored at 1 °C. Specimens from each trap were categorized as siricids, parasitoids, and large and small arthropods. Each group was stored in separate vials in 95% ethanol. Siricids were further identified to species using the manual of Schiff et al. (2006, 2012), Benson (1943) and Gussakovskij (1935). Each specimen was placed in a separate vial and labeled with an identification number, site, trap, and collection date and entered into a database.

Emergence traps: From the stands where the traps were set up, I examined large numbers of dead and dying trees for symptoms of occupancy by Siricidae. In January 2012, 10 fallen logs were selected from each sampling site (60 logs total), with emergence holes that could have been produced by siricids according with Ayers et al. (2009) (Fig. 2). The logs were transported to a covered storage building without heating and introduced individually into hard paper barrels opened with an entomological cloth (Fig. 3). The barrels were checked twice a week from late March to the end of June, and all the insects were collected alive and stored in vials. The rate of parasitism of siricid wasps was calculated using the following formula:

COLLECTION STUDIES

Collections of Hymenoptera of the Entomological Societies of Galicia, Cantabria and Asturias (Northern Spain provinces) also were studied to determine the presence of U. albicornis.

SIRICID TRAPPING

In 2011, 27 U. albicornis, 7 Sirex noctilio, 4 Urocerus gigas, and 1 Xeris spectrum were caught in traps at 6 sites in the Basque Country (Table 1). In 2012, 17 U. albicornis, 7 S. noctilio, 6 U. gigas and, 2 Xiphydria camelus were caught in traps at the same sites.

EMERGENCE TRAPS

In 2011, 263 (186 males and 77 females) U. albicornis, 3 (2 males and 1 female) S. noctilio, 3 (2 males and 1 female) U. augur and 17 (10 males and 7 females) U. gigas emerged from the logs stored in the emergence traps (Table 2). Two siricid parasitoids were collected, 27 specimens of Ibalia leucospoides, and 22 specimens of Rhyssa persuasoria (Linnaeus, 1758).

COLLECTION STUDIES

Two females identified as Urocerus albicornis are in the collection of the Entomological Society of Galicia Province. The specimens were captured in Lugo in 2012 in a cultivated forest of Pinus radiata.

Four siricids were collected from these regions, Sirex noctilio, Urocerus augur, Xeris spectrum, and Urocerus albicornis, and one xiphydriid, Xiphydria camelus (Linnaeus, 1758) The four siricids trapped colonize pines and thus responded to Sirex lures during both years. Xiphydria camelus is a hardwood specialist and thus would not be attracted to the Sirex lures. However, each site had a minority hardwood component and the addition of an ethanol lure could have attracted this species or they were passively trapped. All these species are Palaearctic except U. albicornis which is Nearctic. All the specimens of S. noctilio and U. gigas trapped with lures in both years were female, although natural populations of U. albicornis seems to have a male-biased sex ratio since 32 males and 12 females were trapped. This is usual in siricid wasps according to the studies of Morgan & Stewart (1966) and Morgan (1968). Differences in numbers of U. albicornis trapped among regions were not the same in both years, but most specimens were collected during the summer. Urocerus gigas was trapped mainly in July and S. noctilio was trapped in September (Fig. 4).

U. albicornis (Fig. 5) is an exotic horntail native to North America and was discovered in Spain in 2012 (Lopez et al., 2012). Samples of males and females were obtained unintentionally during surveys for Ips sexdentatus using multiple funnel traps baited with an I. sexdentatus-specific attractant in different sawmills and timber processing industries of Basque Country (Northern Spain) during 2003 and 2011. However, its establishment in Spain was not fully confirmed due to insufficient data. During our sampling, 263 specimens were collected while emerging from the logs taken in the three provinces of the Basque Country. We observed that U. albicornis was the most abundant horntail emerging from the logs. During the experiment, 186 males and 77 females were captured in the nets of the emergence traps. We also observed that one log can be inhabited by both species of Urocerus at the Gordoa (Alava Province) site. Most of the specimen emerged from the logs in May and June. The presence of U. albicornis in Galicia Province (1000 km east from the Basque Country) indicates that this exotic species is probably widely distributed throughout Northern Spain.

Urocerus albicornis is attacked by the same parasites of S. noctilio and U. gigas. Adults of Rhyssa persuasoria emerged from middle April to the end of May while Ibalia leucospoides emerged from June to the end of July. These data agree with Spradbery (1970) and Spradbery & Kirk (1978) who studied the life cycle of the parasites. Percentages of parasitism varied from 0% (Zegama and Orio), 28% (Muxika), 35% (Menagarai), 39% (Gordoa) and 41% (Carranza).

Populations of species of Siricidae remain at low density in their native ranges, and the insect is considered as a secondary pest. This is in contrast to the invaded areas, where the insect populations are large and they have resulted in serious and damaging outbreaks (Boissin et al., 2012). According to my preliminary data, the density of this invasive species could follow this rule and must be monitored in the future for potential outbrakes that could produce negative economic impacts in our coniferous forests. Much effort is needed to avoid the introduction of this invasive species in the Iberian Peninsula, especially at seaports during the transportation of logs from the Americas and other continents.