The EFSA Panel on Plant Health was requested by the European Commission to assess the risk to plant health in the European Union if the Australian bud-galling wasp Trichilogaster acaciaelongifoliae was released for the control of the invasive alien plant Acacia longifolia in Portugal. T. acaciaelongifoliae feeds on A. longifolia and A. floribunda. In South Africa, following its intentional introduction in 1982 and 1983, the wasp is now present throughout the range of A. longifoliain that country, with most plants showing galls and seed set reductions of, initially, up to 95 %. Climatic conditions in the EU are largely suitable for establishment wherever A. longifolia and A. floribunda are present. T. acaciaelongifoliae is moderately likely to establish and spread in the EU, by natural means, but particularly if it is intentionally moved to control populations of A. longifolia other than those present in Portugal. The effects on native biodiversity and ecosystems resulting from invasive populations of A. longifoliaare likely to be reduced by the wasp. A. longifolia is grown as an ornamental plant in some EU countries. A. floribunda is not an invasive plant in the EU and is cultivated as an ornamental plant on a small scale in France, Greece and Italy. Any effects on cultivated ornamental A. longifolia and A. floribunda are rated as moderate, although likely to be transient, as the industry could switch to the cultivation of other Acacia spp. For plant species other than A. longifolia and A. floribunda, consequences are expected to be minor, with low uncertainty except for A. retinodes and Cytisus striatus, where further investigation is required. No risk-reducing options in the plant health context are considered necessary, except for monitoring, sentinel planting, and care with regard to quarantine facilities and release protocols to prevent accidental release in situations and locations other than those intended.