Leaf Mine Types and Associated Parasitism in the Invasive Tree Insect Pest <i>Macrosaccus robiniella</i> (Lepidoptera: Gracillariidae)

Since its first documentation on the European continent in 1983, the invasive micromoth Macrosaccus robiniella (Clemens, 1859) (Lepidoptera: Gracillariidae) was known only for its lower-side blotch mines occupying one of the leaflet halves (common mine type, hereafter mine type 1). In 2023, in abundant populations of M. robiniella in forest areas of Slovenia and Croatia (with more than 50% of leaves damaged), we observed that, in addition to the common mine, this invasive species is capable of forming three other mine types. One (type 2) resembles the common mine type by occupying one half of a leaflet, but the mine is situated on the upper side of the leaf. The two other mine types differ from the common mine in both appearance and position on the leaflet. These include a blotch above the midrib on the upper side of the leaflet (type 3) and a narrow blotch at the edge, causing strong downward folding (type 4). The mines of types 3 and 4 resemble the damage caused by two North American black locust herbivores that are also invasive to Europe: the gracillariid Parectopa robiniella Clemens, 1863 (Lepidoptera: Gracillariidae) and the gall midge Obolodiplosis robiniae (Haldeman, 1847) (Diptera: Cecidomyiidae), respectively. In the studied localities, the relative abundance of mine types 3 and 4 was about four times higher than that of mine type 1. The parasitism rate in these mines was 6.5 times lower compared to that in mine type 1. No statistical difference was found in the parasitism rate between mine types 3 and 4. We discuss how the leaf-mining behaviour of M. robiniella may provide an enemy-free space, conferring a survival advantage and supporting a high population density in forested areas.