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Background Like most pines, radiata pine (Pinus radiata D.Don) is conventionally considered resistant to defoliation by European gypsy moth, i.e. it is only consumed by late larval stages, or when susceptible species are not available or are scarce. The ability of this moth to complete development on this host in field conditions has never been reported. Findings A gypsy moth outbreak in a pure radiata pine plantation was observed in north-western Spain, where this insect is endemic. During the 2 years of outbreak culmination (2012 and 2013), approximately 46 ha of radiata pine were severely defoliated (>75 % of leaf area removed) and no defoliation was evident in nearby stands of susceptible species. Large numbers of egg masses were present on stems of pine trees. Larvae were observed feeding exclusively on radiata pine needles beginning with the first instar and continuing to pupation. Nearly 100 % tree mortality occurred in stands severely defoliated. Conclusions The occurrence of a European gypsy moth outbreak in a pure radiata pine plantation contradicts previous observations that larvae of this species cannot complete development in stands comprised entirely of pines. These results suggest that European strains of the gypsy moth hold great potential for damage to commercial radiata pine plantations where this species is utilised in plantation forestry, such as in New Zealand or Australia