Distribution of the invasive Anisandrus maiche (Coleoptera: Scolytinae) in Switzerland, first record in Europe of its ambrosia fungus Ambrosiella cleistominuta, and its new association with Xylosandrus crassiusculus

In 2022, two independent insect surveys in canton Ticino (southern Switzerland) revealed the widespread occurrence of the invasive ambrosia beetle Anisandrus maiche from southern to central-upper Ticino. This species is native to east Asia and has previously been found as a non-native invasive species in the United States, Canada, western Russia, Ukraine and, in 2021, in northern Italy. Here, we present the results of several trapping studies using different trap types (bottle traps, funnel traps and Polytrap intercept traps) and attractants and a map of the distribution of the species. In total, 685 specimens of A. maiche, all female, were trapped, and the identity of selected individuals was confirmed by morphological and molecular identification based on three mitochondrial and nuclear markers (COI, 28S and CAD). Traps checked from early April to early September 2022 in intervals of two to four weeks showed that flights of A. maiche occurred mainly from June to mid-August. Isolation of fungal associates of A. maiche from beetles trapped alive revealed the presence of four fungal species, including the ambrosia fungus Ambrosiella cleistominuta, the known mutualists of A. maiche. The identity of A. cleistominuta was confirmed by comparing DNA sequences of its nuclear, internal transcribed spacer (ITS) gene with reference sequences in NCBI and BOLDSYSTEMS. This represents the first record of A. cleistominuta in Europe. Ambrosiella cleistominuta was also found in association with another non-native invasive ambrosia beetle, Xylosandrus crassiusculus, at a botanic garden in central Ticino. As ambrosia beetles usually show a high degree of fidelity with only one mutualistic fungus (in the case of X. crassiusculus normally Ambrosiella roeperi), this association is highly unusual and probably the result of lateral transfer among these non-native invasive species. Of the other fungal associates isolated from A. maiche in Ticino, Fusarium lateritium is of note as there is a possibility that A. maiche could act as a vector of this plant pathogen. We highlight several research needs that should be addressed to gain insight into the potential impact of these non-native species and to overcome problems with heteroplasmy in COI sequences in studies of invasion and population genetics of ambrosia beetles.