The recent sika deer irruption has had a significant impact on forest ecosystems. Many studies have reported that this increase has led to the dominance of plants unpalatable to the deer; however, few studies have investigated the effects of this dominance on the specialist sawfly species that feed on the unpalatable plants. We observed the mass emergence of Aglaostigma amoorensis (Hymenoptera: Tenthredinidae), rare specialist sawfly of Veratrum plants, unpalatable to sika deer, under severe feeding pressure. An exponential increase in the number of A. amoorensis adults was observed according as the increase in the number of Veratrum album shoots. The increase in the A. amoorensis density had also a negative effect on reproductive success of V. album. Our findings suggest that sika deer irruption changed the impact of specialist sawfly on the host unpalatable plant.