A new species of Closterocerus (Hymenoptera: Eulophidae: Entedoninae) is described from South Korea. The species is reported for the first time as an egg parasitoid of Pachynematus itoi Okutani (Hymenoptera: Tenthredinidae), of which recent sudden outbreak caused severe damage in larch forests. In total 61 species in Chalcidoidea are recorded as primary parasitoids of various tenthredinid sawflies in the Palaearctic area. However, no chalcidoid parasitoids have previously been recorded from P. itoi. The genus Closterocerus is cosmopolitan in distribution, and with an extremely wide host range from Symphyta, leaf-mining and gall-inducing insects to scale insects and plant lice. In Korea, one species of Closterocerus, three Chrysonotomyia, two Desmatocharis, one Neochrysocharis and one Teleopterus were known; most of them, except Chrysonotomyia sudoensis Paik, now are placed under Closterocerus. The new species is compared with 13 other species of Closterocerus that are known from the Far East.

Pachynematus itoi Okutani emerges three times a year, and overwinters as a pre-pupa in cocoon. First emergence of P. itoi was from 7th May to third June. Second and third emergence was from 20th June to 13th July and fourth to 25th August, respectively. Mean oviposited egg number was 59, and egg period was about 9 days. Hatching rate was ＞90%. Oviposited egg number was the highest in the first needle cluster of short shoot followed by second and third. P. itoi did not oviposit on single needle (long shoot) of Larix leptolepis (Sieb. et Zucc.). Larva had five instars, and larval period was about 20 days. Larva prefer needle cluster of short shoot to single needle of long shoot. Mature larva descended from host tree and pre-pupa in cocoon overwinter at topsoil.

We surveyed natural enemies of Pachynematus itoi Okutani, newly recorded sawfly pest of Larix leptolepis, and investigated their occurrence and life cycle at Cheolwon and Taebaek, Gangwon Province in 2006～2007. Endasys liaoningensis was identified as main natural enemy of P. itoi Okutani. Parasitism of E. liaoningensiswas was 42%. Emergence period of E. liaoningensis was from May to September, and the peak of emergence was from 21th May to 20th June. It was suggested that the occurrence number and period of E. liaoningensis were quite closely relative to those of host, P. itoi.

낙엽송잎벌은 1년에 3회 발생하는 전형적인 돌발 해충으로, 암갈색 고치 속에서 전용상태로 월동하며, 성충 우화시기는 강원지방에서 1화기가 5월 7일～6월3일, 2화기가 6월 20일～7월 13일, 3화기는 8월 4일～8월 25일이었다. 산란수는 59±8개, 난 기간은 8～10일, 부화율은 90%이상이었다. 낙엽송가지 정단부쪽 제1, 제2, 제3 단지엽에 산란수가 많고 단지엽 뒷면 조직에 알을 절반 정도 삽입하여 산란하였다. 유충의 령기는 5령이고 유충기간은 20일 내외이다. 1～4령 유충은 군서생활을 하고 5령 유충부터 분산하여 섭식한다. 장지엽(신초)보다는 단지엽을 선호하여 섭식하며, 노령유충은 지면으로 낙하하여 낙엽층 속이나, 표토 속에서 고치를 짓고 월동하였다. 낙엽송잎벌 천적종류는 기생성천적으로 맵시벌과 낙엽송잎벌살이뾰족맵시벌(Endasys liaoningensis), 알 기생봉 1종, 기생파리 3종(Tachinidae sp. Lydella sp. Scaphimyia sp)등 5종이 확인되었고, 포식성천적으로는 개미, 거미, 풀잠자리 유충, 조류, 병원미생물로 Beauveria basiana 균 감염이 확인되었다. 낙엽송잎벌살이뾰족맵시벌(Endasys liaoningensis) 기생율은 강원도 철원에서 42.0%로 상당히 높았다. 출현 시기는 5월～9월이며 1화기 노숙유충이 고치 짓는 시기인 5월 하순～6월 하순에 집중적으로 출현하였다.