Plagiolepis invadens, Bernhard, 2020

Bernhard, Seifert, 2020, Revision of the Plagiolepis schmitzii group with description of Pl. invadens sp. nov. - a new invasive supercolonial species (Hymenoptera: Formicidae), Deutsche Entomologische Zeitschrift 67 (2), pp. 183-196 : 183

Plagiolepis invadens sp. nov.

Etymology.

Meaning “invasive” (from Latin invado) The type colony in SW Germany is an anthropogenous introduction from an unknown origin, shows circannual outdoor nesting, but invaded houses in large numbers during the extremely dry summers of 2018 and 2019.

Type material.

Holotype plus one paratype worker on the same pin labelled "GER: 49.65703°N, 8.41775°E Hofheim, 92 m, supercolony in garden, known since about 4 years, leg. Heller 2019.08" and "Holotype (top) and paratype of Plagiolepis invadens Seifert"; three paratype workers from the same collecting data; five paratype workers labelled "GER: 49.65703°N, 8.41775°E Hofheim, 92 m, supercolony in garden, known since about 5 years, leg. Heller 2020.06"; all material is stored in SMN Görlitz.

Diagnosis and taxonomy

(Table 1 , key, Figures 6 - 8 ). With narrowly-spaced basal pits of pubescence hairs (BPdG 15.6 µm), low pubescence distance (sqPDG 2.69) and the 4th funiculus segment not being much longer than the 3rd (F4/F3 1.346), Pl. invadens sp. nov. is clearly separable from the species of the Pl. pygmaea and Pl. pallescens group and is characterised as a member of the Pl. schmitzii group. It is outstanding within the latter group by the very short 3rd funiculus segment: primary ratios of F3/CS are 7.37 ± 0.43 [6.85, 8.04]% in ten workers of Pl. invadens sp. nov., but 9.78 ± 0.61 [8.59, 11.59]% in 124 specimens of the other three Pl. schmitzii group species. The most similar species is Pl. atlantis and it may be asked if there is a risk of synonymy, considering that the description of Pl. invadens sp. nov. is based on workers from only a single supercolony. Running a PCA, considering absolute head size and the 16 RAV-corrected shape, pubescence and surface characters, resulted in a strong separation of all individuals by the 1st principal component:

Pl. invadens sp. nov. -1.946 ± 0.343 [-2.477, -1.368] n = 10

Pl. atlantis 0.348 ± 0.592 [-1.020, 1.338] n = 56.

This clear result (ANOVA, F1,64 = 140.4, p < 0.001), achieved without character selection, is a reasonable indication of heterospecificity. A vector considering the 1st and 3rd principal component with 1.893*PC1 + 0.563*PC3 provides an even stronger separation (ANOVA, F1,64 = 189.6, p << 0.001):

Pl. invadens sp. nov. -4.010 ± 0.649 [-4.962, -2.932] n = 10

Pl. atlantis 0.716 ± 1.046 [-1.597, 2.681] n = 56.

Further descriptive statements.

Head moderately elongated (CL/CW 1.105). Scape shorter than in related species (SL/CS 0.923). Eye medium-sized (EL/CS 0.258). Mesosoma width smaller than in related species (MW/CS 0.615). Cuticular surface of head, mesosoma, coxae and femora brilliantly shining and with a dilute appressed to decumbent pubescence. Scape and tibiae with a more dense decumbent pubescence. Head, scape, femora and mesosoma varying from dark brown with yellowish tinge to almost black. Antennal funiculus, coxae, tibiae and sometimes pronotum pale yellowish-brown.

Biology.

Pl. invadens sp. nov. is known so far from only a single supercolony in SW Germany in a settlement with about 30% greenery and 70% building or sealed area. Residents became aware of the ants in the gardens in about 2016. Ants were not perceived as plagues inside the houses in the years 2016 and 2017, but masses of workers invaded houses during the extremely dry summers of 2018 and 2019 in such numbers that the residents tried to get rid of the ants by using vacuum cleaners. Gerhard Heller observed in September 2019 and June 2020 the presence of a true supercolony with millions of workers and runways stretching along the roadside of at least two properties. Preferred nest sites were the most humid spots with much greenery where the ants constructed small hills made of soil ejections. The residents also reported that "black ants being clearly bigger" than the Plagiolepis - presumably Lasius niger (Linnaeus, 1758) - vanished after the development of the Pl. invadens sp. nov. supercolony. The species is obviously able to long-time survival under outdoor conditions within the current climatic scenario and will have to be considered as established neozoon in Germany if efforts to eradicate the population fail. Reproductive biology, demography and food ecology of Pl. invadens sp. nov. are not studied so far, but are expected to show the traits described in the concluding chapter of this paper.

Comments.

There is certainly some risk describing a new species based upon a single colony. Yet, this risk is calculable and apparently low. Firstly, the separation in the PCA is very strong and the next similar species Pl. atlantis was available for this PCA in a large sample. Secondly, the reported diagnostic characters, which are homogenously distributed over the colony in space and time, are unlikely to represent a spontaneous mutant. This would require a single founding queen which was homozygous for at least one allele, both determining length of scape and funiculus segments and slenderness of the mesosoma and it would require propagation of this mutant over millions of individuals in the supercolony. Thirdly, considering the Palaearctic region, 10 taxa of the Pl. schmitzii group (reported here) and 14 taxa of the Pl. pallescens and Pl. pygmaea group have been checked and excluded as senior synonyms (Kirschner et al. in prep.). There is only one taxon which seems to pose some risk: Plagiolepis barbara var. pyrenaica Emery, 1921, collected in the Eastern Pyrenees. Assessing a photo of a type specimen ( AntWeb 2020), it seems to have a gastral pubescence density approaching the situation in the Pl. schmitzii group, but further conclusions are impossible. A direct investigation of type specimen(s), which should exist in MCSN Genoa, is currently not possible due to long-term effects caused by the COVID19 pandemic. Fourthly, scrutinising the photos of type specimens of 44 Plagiolepis taxa from remote zoogeographic regions ( AntWeb 2020), there is no apparent candidate taxon for senior synonymy. All these taxa differ clearly. Arranged in alphabetic order of species-level taxon names, their unique specimen identifiers were CASENT0909850, CASENT0132814, CASENT0917582, CASENT0909845, CASENT0217200, CASENT0281160, CASENT0909846, CASENT0909847, CASENT0917579, CASENT0915713, CASENT0903144, CASENT0917578, CASENT0906247, CASENT0912417, CASENT0281161, CASENT0909848, CASENT0917484, CASENT0101305, CASENT0903146, CASENT0916654, CASENT0912418, CASENT0235986, CASENT0913616, CASENT0905142, CASENT0905143, CASENT0909852, CASENT0909861, CASENT0906471, CASENT0912419, CASENT0217737, CASENT0101224, CASENT0909853, CASENT0909854, CASENT0912411, GBIF-D/FoCol 2225, CASENT0787965, CASENT0903145, CASENT0906251, CASENT0905141, CASENT0217738, CASENT0909863, CASENT0912412, CASENT0909864 and CASENT0917485.

Figures

Figure 6. Head of the holotype worker of Plagiolepis invadens sp. nov.

Figure 8. Dorsal aspect of the holotype worker of Plagiolepis invadens sp. nov.