In the last decades, among the invasive insects that have entered Europe, there is included also a series of mining microlepidopterans of the leaves of trees. As some of these species have become important pests, the attention of researchers, both from a theoretical point of view but also a practical one, has turned on the knowledge of the autecological aspects but also synecological ones of these species. In this article, we deal with the formation of the parasitoid complexes that, in certain conditions, can limit consistently the populations of invasive species. All these four species belong to the Family Gracillariidae. The first of these species that was the most extensively studied is - Cameraria orhidella Descka & Dimič, 1986 – which was reported in the South West of Romania in 1996, and in the cis - Carpathian area (Moldova) in 2002; in 2003, it spread within the entire Romanian space (including Dobrogea). During this period, big damages were produced to the ornamental chestnut-trees (Aesculus hippocastanum). The formation of the complex of natural enemies (parasitoid Hymenopterans) has also constituted the researchers’ goal in the territories newly conquered by C. ohridella in Europe. Thus, between 1997 and 2003, there were reported in these territories about 50 species among which the eulophid chalcidoids dominated evidently. Thus, it could be found that this parasitoid complex was formed of generalist, autochthonous species, the most frequent complex in Europe, being the following: Pnigalio pectinicornis (Z), P. agraules (Walk.), Sympiesis sericeicornis (Nees), Pediobius saulius (Walk.), Chrysocharis nephereus (Walk.), Chrysocharis pentheus (Walk.), Closterocerus trifasciatus (Westw.), Minotetrastichus frontalis (Nees), Pteromalus semotus (Walk.). In Romania, the researches were made: in U.S.A.M.V. Cluj-Napoca park with a great floristic diversity in the period 2000-2002; in Bacău town with a smaller floristic diversity and poorer ecological conditions (atmospheric pollution and chemical control) and in Hemeiuş dendrological park (Bacău) with high floristic diversity. The global parasitoid complex in Romania was made up of at least 22 species of which 3 ichneumonidae and 19 Chalcidoidea (17 species belonging to Family Eulophidae). These species are generalist (polyphagous), attacking the mining insects. In ClujNapoca, there were identified 16 species with a total relative abundance A(nr.) of 407 specimens, in Bacau 5 species with A(nr.). of 75 specimens and in Hemeiuş 13 species with A(nr.). of 470 specimens. On the whole, those 22 species identified in Romania were represented by A(nr.). = 952, the dominant species being the same as in the rest of Europe. Thus, the eudominant species were: Minotetrastichus frontalis, Pnigalio pectinicornis and Pnigalio agraules, the dominant species was Pediobius saulius and the subdominant were: Closterocerus trifasciatus and Pteromalus semotus. Other species with a lower D % (dominance), also present in the parasitoid complexes, present in other European countries, were found in Romania too: Pnigalio soemius Walk., Sympiesis sericeicornis (Nees) Scambus annulatus Kiss. and so on. Another invasive species, Phyllonorycter issikii Kumata-1963, was described from Japan, and in 1977, it was reported in Russia in the Far East; until 2002, it had reached Central Europe. The species was reported on Betula platyphylla and several East Palaearctic species belonging to the genus Tilia, becoming an important pest of some species of Tilia (cordata, platyphyllos). In Romania, in 2002, there was reported at Hemeiuş and Dărmăneşti (Bacău ), and Gheorghiţoaia şi Frumuşica forests, and Podu Iloaiei (Iaşi) and, in 2005, in Copou Park Iaşi and forest nature reserve Harboanca (Vaslui) on Tilia cordata, Tilia platyphylos and Tilia tomentosa. As it concerns the parasitoids complex, up to the present, there are known almost 20 generalist species, and, as in other cases, the eulophids are dominant. In Romania, the complex of parasitoids is in formation, consisting for the time being of 5 species: Gnaptodon pumilio (Ar % = 9), Orgilus obscuratus (Ar % = 9), Pnigalio soemius (Ar % = 18), Cirrospilus lyncus (Ar %= 9) and Minotetrastichus frontalis (Ar % = 55). Of these species, Minotetrastichus frontalis and Pnigalio soemius are also part of the dominant species found in the parasitoid complex in other palaearctic biogeographical zones. The third species is Phyllonorycter robiniella Clemes, 1859, originating from North America, reported for the first time in Basel (Switzerland) in 1983; later, it became a pest of the locust tree (Robinia pseudoacacia). In Romania, it was signalled near Drobeta Turnu Severin in 1988, and in Moldova (Romania), only in 2002. As it concerns the complex of parasitoids, in 2004, this consisted of 19 species in Hungary, 20 years after being reported in Europe. In the complex there were dominant Achrysocharoides cilla Walk. and less Holcothorax testaceipes and Minotetrastichus frontalis (Nees), and the parasitic percentages ranged between 38 % and 85 %. In Romania, the complex of parasitoids in formation is composed of five species: Sympiesis sericcicornis Nees, Pediobius saulinus Walk., Neochrysocharis formosa Westw., Achrysocharoides cilla Walk. and Minotetrastichus frontalis (Nees). All these five species are found in the respective complex in Central Europe, the most important also being Achrysocharoides cilla Walk. and the percentage of the parasitation of the host, being 8-24 % for 2008-2011 period. The fourth species also dealt with, associated with the locust tree is Parectopa robiniella Clemens-1889, originating from North America and signalled in Europe, in Milan (Italy) in 1970. The species spread in Europe so that, in 1988, it was also reported in Romania, near Drobeta Turnu Severin, and in 1999, in Central and South Moldova. The parasitoids associated complex, formed of autochthonous species, studied by Hungarian researchers in 2001-2003, is composed of 12 species: also present in the complex of the species Phyllonorycter robiniella Clemes; Achrysocharoides cilla Walk. is the dominant species in this complex, too. As it regards the percentage of parasitation of the host, it was 4.03 % - 5.9 %, much lower than in Phyllonorycter robiniella Clemes. In Romania, the parasitoid complex is identical to that of the species Phyllonorycter robiniella Clemes, with the exception of the species Closterocerus clarus Szel that was found only in Parectopa robiniella Clemens. The species Achrysocharoides cilla Walk is also dominant, Neochrysocharis formosa Westw is on the second place, and the percentage of parasitation was small, 0.3 % - 5 %. In conclusion, the parasitoid complexes are made up of species of Hymenopterans, especially autochthonous, generalist (polyphagous) Chalcidoidae-Eulophidae. The complex of the species Cameraria orhidella Descka & Dimič is best known from all points of view, and the other 3 species are in the course of formation and stabilization and in some local situations it can contribute consistently to the reduction of the host populations.