Results and discussionTop

Distribution update of D. kuriphilus

The distribution of D. kuriphilus has been modified in some areas of Spain (Fig. 1), expanding throughout Galicia to cover almost the entire territory of this region (1). The adjoining infested areas to Galicia (1), the El Bierzo (León) and Alta Sanabria (Zamora), have also been extended. The infested area of Serranía de Ronda and Valle del Genal (12), south of the IP, has been expanded. The Catalonian region (5 and 6) has not been modified with the data we have, and the entire chestnut territory is invaded by D. kuriphilus. We do not have general data on the rest of the infested areas: Asturias (2), Cantabria-West Basque Country-North Burgos (3) and Navarra-East Basque Country (4). However, in the last surveys in the Navarra territory (4), near the French border, we observed an apparent decrease in the populations of D. kuriphilus. This observed D. kuriphilus population decrease may be related to the natural dispersion of T. sinensis into Navarra from France (Nieves-Aldrey et al., 2019), across the ecological corridor of the west of the Pyrenees, through the mass of chestnuts trees and the D. kuriphilus in them. The focal point of Madrid (11) has not changed due to its isolation from chestnut forests.

There were four newly detected focal points of D. kuriphilus. All were prospected by different official institutions, a priori discarding the presence of D. kuriphilus. in previous years, either for not having been detected or for having successfully destroyed the detected infested chestnut material without possibility of propagation. One of these new focal points was detected in the south region in the Tejeda-Almijara (13), between the focal points of Lanjarón (14) and Serranía de Ronda and Valle del Genal (12). This focal point is relatively isolated and separated from the other two nearest focal points, and its infestation by D. kuriphilus. is likely due to the movement of infested chestnut material from other areas to this one.

The other three new focal points are related to the Sistema Central mountain range: Sierra de Francia (7), Valle del Jerte (8) and southwest of Madrid (10). The appearance of these new areas might be due to 1) the natural dispersal of D. kuriphilus from neighbouring focal points or 2) the artificial dispersal caused by the movement of infested material to distant areas with no local presence of this invasive cynipid. A priori, the Sistema Central should has a low probability of the arrival of D. kuriphilus by natural dispersal due to this area is far away from the other infested areas of the Iberian Peninsula. These focal points are separated up to ~50 km, meaning it is unlikely that they were infested by each other or by the focal point of the city of Madrid through the natural dispersion of D. kuriphilus. In addition, the nearest focal point to the Portuguese frontier is ~50 km away. Thus, these recently produced D. kuriphilus focal points are probably due to the accidental mobilization of infested chestnut material, which caused the new introductions. As Gil-Tapetado et al. (2018) indicated in their survey, without quarantine measures, the chestnut forests at the centre of the IP were only vulnerable to D. kuriphilus dispersal mediated by human transport and could be colonized only in short-medium terms in this manner. In this brief communication, we inform about this statement, with the beginning of the D. kuriphilus problems forming from chestnut production in the Sistema Central.

In the focal area of Valle del Jerte, our prospections indicate that the presence of D. kuriphilus was earlier than 2019. In this area, the gall abundance did not coincide with a first-year invasion scenario, and it is probable that the infestation occurred in 2017 or 2018 and went undetected.

The Government of Castilla y León informed us that the different infested chestnut trees found in the Sierra de Francia (7) were eliminated by cutting off the tree canopy and burning it. However, these measures were taken in May and June, when the D. kuriphilus adult can emerge, and they may have oviposited and infested nearby chestnut tree buds. Therefore, the settlement of D. kuriphilus in the Sierra de Francia area remains in doubt in 2019. The infested chestnuts of this area came from Asturias and Galicia, confirming its introduction by transport through human forestry activities. A similar event occurred in the Sierra de San Vicente (9), near the Sierra de Gredos in Toledo in 2016. In this area, infested chestnut trees were reported and eliminated by burning galls. A priori, this control measure seems to be effective because no new presence of D. kuriphilus has been reported in this area since 2016 (Gil-Tapetado et al., 2018).

In 2019, at the time this distribution update was written, the Sierra de Aracena and Picos de Aroche Natural Park (Huelva, Andalusia) was the last most important Spanish chestnut forest without D. kuriphilus, after our own prospecting. This area is a priority area of control, and chestnut trees and Castanea forestry material should be quarantined to prevent new introductions of this alien species and an almost complete invasion of Spanish chestnut forests.

 

 

Figure 1. Distribution of D. kuriphilus in Spain until 2019. The red dots show an updated distribution of D. kuriphilus with respect to the focal points described in Gil-Tapetado et al. (2018). The yellow dots indicate the new D. kuriphilus focal points described in this brief communication. Different line types indicate the infested areas by D. kuriphilus in Spain. The colour green indicates the presence of C. sativa in the Iberian Peninsula and south of France. The names of each updated focal point related to each geographical area are in the Eurosiberian region: 1) Galicia-El Bierzo-Alta Sanabria; 2) Asturias; 3) Cantabria-West Basque Country-North Burgos; and 4) Navarra-East Basque Country. Catalonian region: 5) Prades; 6) Montseny. Central region: 7) Sierra de Francia; 8) Valle del Jerte; 9) Sierra de San Vicente; 10) Southwest of Madrid; 11) Madrid. South región: 12) Serranía de Ronda-Valle del Genal; 13) Sierra de Tejera-Almijara; 14) Lanjarón.

 

 

Parasitoid collection

Reared parasitoids from samples of galls ofD. kuriphilus collected in the site of Cadalso de los Vidrios (Community of Madrid) confirm the fast recruitment of native chalcid parasitoid fauna by D. kuriphilus, accounting for a minimum of nine different species. This recruitment of parasitoids by this alien cynipid would have occurred after only one year of presence in the chestnut forests of the Sistema Central. All emerged chalcid parasitoids were native and generalists and belonged to different native cynipid communities (Gómez et al., 2006) (Table 1). The species Cecidostiba fungosa (Geoffroy, 1785), Sycophila biguttata (Swederus, 1795) and Sycophila flavicollis (Walker, 1834) were rare in our D. kuriphilus gall samples throughout the IP from 2016 to 2018, with the focal point mainly located in Galicia (1) and Serranía de Ronda-Valle del Genal (12), and in the parasitoid list of Catalonia region (5-6) in the work by Jara-Chiquito et al. (2020) (Fig. 1). Although the data are very fragmented and scarce, it is possible there are differences in the communities of parasitoids in different areas of the IP as well as in the recruitment of native fauna (Gil-Tapetado et al., unpublished).

Considering the dissection data of D. kuriphilus galls, the early finding of Torymus (Syntomaspis) larvae in Cadalso de los Vidrios, preliminarily identified as T. sinensis, was intriguing and must be emphasized. This fact was later confirmed by the rearing of emerging T. sinensis adults from this sample (Table 1). The unexpected and surprising presence of this species in this locality of the Community of Madrid would be explained with two hypotheses: 1) the natural dispersal of T. sinensis from the nearest area where this biological control agent was experimentally released, the RJB Alfonso XIII in the city of Madrid; or 2) a possible unauthorized release of T. sinensis were made in this site, given that there are no records of such releases being authorized or made by the Community of Madrid. We think the first hypothesis is highly unlikely. In fact, although Madrid is 63 km distant from Cadalso de los Vidrios and, therefore, has a similar range about of 60 km/year of dispersal capacity for the species reported by Moriya et al. (2003), the date of establishment in Madrid is quite recent and with a low percentage (spring of 2018, 3.69%, Nieves-Aldrey et al., 2019). This would imply a very fast dispersal event, only one year, to colonize the distant apart chestnut trees of Cadalso de los Vidrios, due to our rearing of T. sinensis in this last site are dated in 2019 and the species needs an entire year for its establishment.

In conclusion, D. kuriphilus has continued to expand through the IP, both by natural dispersal and dispersal facilitated by human activities. This alien species has recruited as a minimum of nine parasitoid species at one site of the Sistema Central in the same year as D. kuriphilus was reported in this area and with the simultaneous presence of its biological control, T. sinensis, an alien species that is for the first time recorded from a natural area of the Community of Madrid.