Study system and field sampling
The Heteroceran Lepidoptera (moths) whose caterpillars feed onQuercus spp. leaves were used as study model. There is a high diversity of lepidopterans linked to the Quercus genus as host plant. The most abundant oak herbivore families are noctuids, tortricids, erebids, geometrids and nolids, which start feeding on the new shoots in early April (Elkinton et al . 2016). The community of caterpillars also changes during the season, with tortricids the first to feed on the new shoots and geometrid species the last (Soria 1988). Herbivory damage on oak differs among years too, being able to have outbreaks under specific conditions of temperature and climate (Schroeder & Degen 2008). These insects are widely spread over Europe and have been recorded feeding on different oak species (Soria 1988; Führer 1998). The number of DNA barcodes available for the southern European peninsulas (Balkans, Italy and Iberia) at public repositories was very low compared with the rest of continental Europe (see below). Thus, we carried out a field campaign in Spain to reduce such an imbalance. We sampled 10 oak forests along a latitudinal gradient (Table S1; Fig. 1) from April to June 2017, the period of maximum activity of oak defoliating caterpillars in the Iberian Peninsula (Soria 1988). Oak branches were shaken and the falling caterpillars collected on a white cloth of a fixed surface placed beneath (see Ruiz-Carbayo, Bonal, Espelta, Hernández & Pino 2017 for a detailed description of the sampling methodology). The caterpillars collected at each oak were placed within a plastic box and taken to the laboratory, where they were housed individually in Petri dishes and fed with fresh oak leaves. Caterpillars were identified to species level based on morphological characters following guides and dichotomous keys (Gómez de Aizpurúa 2002; Gaytán, Canelo, González-Bornay, Pérez-Izquierdo & Bonal 2018). In those few cases in which the caterpillar could not be identified it was raised to the adult stage and then identified using guides (Fibiger 1997; Goater, Ronkay & Fibiger 2003; Sihvonen & Skou 2015). In total 21 species of 9 families were collected (Table S2). All specimens (caterpillars and adults) were stored in 1.5 ml Eppendorf tubes filled with 96% alcohol for further molecular analyses.