First detection of invasive Douglas fir needle midges from the genus Contarinia Rondani (Diptera: Cecidomyiidae) in Switzerland

Since 2015, North American Douglas fir needle midges of the genus Contarinia Rondani (Diptera: Cecidomyiidae) are known to spread across Europe. In 2022, these gall-inducing insects were detected for the first time in Switzerland, where their morphological identification was confirmed by DNA barcoding. Conducting a visual monitoring of Douglas fir stands in northern Switzerland, the current distribution range was delimitated and revealed the presence of Douglas fir needle midges in forest areas close to the borders of France and Germany, where their establishment has been reported earlier. Genetic analyses of five representative populations further proved the presence of two different mitochondrial haplotypes, of which one was only found in the westernmost population, indicating faint population structuring. When assessing co-occurrence of Douglas fir needle midges with other pests and pathogens originating from North America, Douglas fir stands were often found to be infested also by the insect Adelges cooleyi (Gillette) (Hemiptera: Adelgidae) and the fungus Nothophaeocryptopus gaeumannii (T. Rohde) Videira, C. Nakashima, U. Braun, Crous (Mycosphaerellales: Mycosphaerellaceae). The observed abundance of the newly established midges within the invaded area and their co-occurrence with other introduced pests and pathogens are an indication of the increasing biotic stress level on Douglas fir, which should be taken into account when considering Douglas fir as a potential future tree species for European forests.