Abstract: Monochamus alternatus Hope is the main vector of pine wood nematode Bursaphelenchus xylophilus. Sclerodermus guani Xiao et Wu is an important insect natural enemy of M. alternatus larva. In biological control of M. alternatus using S. guani in field, we found that S. guani not only parasitized but also delayed development of M. alternatus larvae, which decreased the overall emergence rate of M. alternatus. To verify this discovery, we conducted field tests, in which parasitoids were released at different quantities and timing on pine wilt trees with the over-winter larva of M. alternatus. Ratios of body length/prothorax width (ratio-Ⅰ) and body weight/prothorax width (ratio-Ⅱ) of M. alternatus larvae were measured for comparing normal larva and abnormal larva (including the development-retarded larvae). Results showed that percentage of abnormal M. alternatus larvae were 29.2%, 48.9% and 59.4% when the number of wasps/larvae were 0.5 wasps, 1 wasp and 2 wasps, respectively. When S. guani were released in March, April and May, percentage of abnormal M. alternatus larvae were 24.5%, 35.5% and 61.6%, respectively. There were also some changes on morphologenesis of abnormal larvae. The ratio-Ⅰ and ratio-Ⅱof normal larvae of M. alternatus in May were 6.14 and 0.11, respectively, but the ratio-Ⅰ and ratio-Ⅱof development-retarded larvae in August were 4.83 and 0.06, respectively, which means length and weight of abnormal larvae of M. alternatus were reduced by 21.36% and 44.28%, respectively, compared with normal larvae.