Native and introduced parasitoids in the biocontrol of Dryocosmus kuriphilus in Veneto (Italy)* F. Colombari and A. Battisti Department of Agronomy, Food, Natural Resources, Animals and Environment (DAFNAE), University of Padova, Agripolis, 35020, Legnaro, (Italy); e-mail: fernanda.colombari@unipd.it The invasive gallmaker Dryocosmus kuriphilus is a threat to chestnut stands and it is neces- sary to control it in order to maintain a satisfactory nut yield. A preliminary evaluation of the native natural enemies emerging from the galls, mainly parasitoids associated with native cynipid gallmakers, revealed that they were not able to contain the pest. Conse- quently, a classical biological control programme using Torymus sinensis started in 2010, with almost 500 releases being made in 6 years. The parasitoid achieved a satisfactory level of control of the pest over a period of time shorter than expected based on literature related to other countries. The success in the study area might be explained by specific traits intrin- sic to T. sinensis, such as its high spread capability via stratified dispersal (i.e. a combina- tion of short- and long-distance flights), together with a large availability of host galls and limited competition with other parasitoids. Introduction Factors influencing invasion of alien species Three fundamental stages must be considered in the invasion of alien species: (1) transport of organisms to a new location due to intentional or unintentional human activities, (2) establishment and population increase in the invaded locality, and (3) spread from the initial successful established popula- tions (Nentwig & Josefsson, 2010). Two factors are impor- tant for explaining the success of invaders: the invasiveness of the non-indigenous species and the invasibility of the ecosystem into which the non-indigenous species is intro- duced. The former factor includes traits that are intrinsic to the species and can guarantee the successful establishment and spread of the invader (e.g. pre-adaptation to novel envi- ronments, reproductive strategies, resource acquisition), the latter factor includes extrinsic factors in the environment that can decrease the resistance of the ecosystem to invasions (e.g. evolutionary history, disturbance, abiotic stress and community structure) (Lieutier, 2006; Hufbauer & Torchin, 2008; K€ uhn & Klotz, 2008). A recent framework for understanding invasions Recently, the concept of ‘niche opportunity’ has been com- monly recognized as a framework for understanding biologi- cal invasions in terms of establishment and population growth. This concept groups together different factors related to the opportunities that the particular invaded community provides for the invader and considers their possible interac- tions and variations over both space and time. Factors such as resources, natural enemies and the physical environment are involved, determining the ability of the species to invade and contributing to its rate of population growth. In particular, the availability and density of unused or underused resources in the environment give the invader a ‘resource opportunity’, whereas a low level or low efficiency of natural enemies to which invaders might be susceptible offer ‘a natural enemy escape opportunity’ (Shea & Chesson, 2002). This latter idea, also known as the enemy release hypothesis (ERH), plays a fundamental role in explaining the success of invasive alien species that become widespread and abundant not because they possess a suite of particular traits, but rather because they arrive in a new range where their population growth is not compromised by biotic constraints such as predators or para- sitoids. At the centre of this theory is the assumption that natu- ral enemies are important in regulating populations (Mack et al., 2000; K€ uhn & Klotz, 2008; Roy et al., 2011). From the ERH to classical biological control For invaders, specialist natural enemies may be relatively rare or even absent in the new range and, although there should be a shift of generalist natural enemies to which the novel species could be particularly vulnerable, their number and impact are generally low. A key prediction of the ERH, and one supported by growing evidence, is that intro- duced populations harbour fewer and less effective natural enemies than populations within their original range (Shea & Chesson, 2002; K€ uhn & Klotz, 2008). This prediction, implying that loss of natural enemies is sometimes an *Paper based on work presented at the Joint EPPO / COST-SMARTER Workshop on the Evaluation and Regulation of the use of Biological Control Agents in the EPPO Region, Budapest, 2015-11-23/24 ª 2016 The Authors. Journal compilation ª 2016 OEPP/EPPO, EPPO Bulletin 46, 275–285 275 Bulletin OEPP/EPPO Bulletin (2016) 46 (2), 275–285 ISSN 0250-8052. DOI: 10.1111/epp.12297