In 2022, EFSA was mandated by the European Commission's Directorate-General for Health and Food Safety (M-2022-00070) to provide technical assistance regarding the list of Union quarantine pests qualifying as priority pests, as specified in Article 6(2) of Regulation (EU) 2016/2031 on protective measures against plant pests. As part of Task C, EFSA conducted comprehensive expert knowledge elicitations on candidate priority pests, focusing on the lag period, rate of expansion and the impacts on production (yield and quality losses) and the environment. This report provides the rationale for the dataset on Choristoneura fumiferana and C. parallela, delivered to the European Commission's Joint Research Centre, to feed into the Impact Indicator for Priority Pest (I2P2) model and complete the pest prioritisation ranking exercise.