| Summary: | <i>Halyomorpha halys</i> (Stål) is an invasive Asian pest that causes severe crop losses on various crops. Nowadays, management strategies against this pest mainly rely on pesticide use, but biological control with egg parasitoids is considered the most promising long-term and sustainable solution. <i>Trissolcus japonicus</i> (Ashmead) and <i>Trissolcus mitsukurii</i> (Ashmead) are Asian egg parasitoids already present in Europe and are the most effective biological control agents of <i>H. halys</i>. Therefore, these two species are considered for biological control programs in Europe and other parts of the world. <i>Acroclisoides sinicus</i> (Huang and Liao) is a pteromalid parasitoid wasp that frequently emerged from <i>H. halys</i> egg masses collected in northern Italy. This species has been hypothesized to be a hyperparasitoid of <i>Trissolcus</i> spp. parasitoids. This study was carried out under laboratory conditions where <i>A. sinicus</i> was tested in no-choice and two-choice experiments to assess the host preference between <i>T. japonicus</i> and <i>T. mitsukurii</i>. Olfactory responses of <i>A. sinicus</i> from volatiles emitted from different potential hosts were also tested. In all trials, <i>A. sinicus</i> showed a clear preference for parasitizing <i>H. halys</i> eggs previously parasitized by <i>T. mitsukurii</i> compared to <i>T. japonicus.</i> In no-choice experiments, the impact of the hyperparasitoid on <i>T. japonicus</i> was low, showing an exploitation rate of 4.0%, while up to a 96.2% exploitation rate was observed on <i>T. mitsukurii</i>. <i>Acroclisoides sinicus</i> was also attracted by volatiles emitted by egg masses parasitized by <i>T. mitsukurii</i>, while no response was observed to egg masses parasitized by <i>T. japonicus</i> or not parasitized. Therefore, according to the results obtained here, <i>A. sinicus</i> could limit the population development of <i>T. mitsukurii,</i> while lesser effects are expected on <i>T. japonicus</i>.
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