DNA-barcoding identification of a hitherto unreported defoliating geometrid species on acacia in the Great Green Wall in Senegal: taxonomic challenges, genetic diversity, and additional threat to the plantations

When encountering an insect species that targets high-value plants, such as acacia in large-scale tree programs like the Great Green Wall (GGW), it is essential to assess the phytosanitary risks associated with this species. When it has not previously been described locally, accurate taxonomic identification is a crucial pre-requisite. The DNA-barcoding approach is relevant for delineating and assigning unidentified specimens to species or molecular operational taxonomic units already listed in reference DNA databases. From 2020 to 2023, in the Ferlo region of Senegal, where GGW's reforestation activities were implemented, a defoliator caterpillar species not previously reported in the region was observed causing damage to three indigenous acacia species (Vachellia tortilis raddiana, V. nilotica, Senegalia senegal). The barcoding approach coupled with external and/or internal morphological observations of the various stages collected enabled us to assign the specimens to Isturgia pulinda deerraria (Geometridae) but confronted us with the ongoing nomenclatural problem associated with this taxon. Interestingly, based on 122 mitochondrial barcode sequences (78 from our study, 44 retrieved from genetic databases), high intraspecific genetic diversity was revealed both in Senegal and worldwide, with two weakly divergent geographical lineages, an African-Eastern lineage and a Mediterranean lineage. The observation of the species from 2020 to 2023, both at the end of the rainy season and during the dry season on nurseries or plantations of acacia (Fabaceae) in the 5 localities studied and incidentally on Moringa oleifera (Moringaceae) suggests that, even at low numbers, it could pose an additional biotic threat to woody vegetation-based programs.