Brachymeria lasus Walker is a solitary endo-parasitoid that attacks the pupae of a wide range of lepidop-teran hosts,including an important invasive species,the fall webworm (Hyphantria cunea Drury).We studied the rela-tionship between temperature and development of B.lasus from egg to adult hatching.The results show a decrease in parasitoid development time from 34.4 days at 18 ℃ to 10.6 days at 32 ℃.The minimum threshold temperature of B.lasus was 13.2 ℃ ± 1.7 ℃,and the effective accumulated temperature was 210.3 ±28.7 degree days.These results pro-vide a basis for optimizing the production of this parasitoid.In addition,the effects of host size on offspring performance of B.lasus were investigated under laboratory conditions.Offspring longevity,size,and percentage of females were positively correlated with host size.Female offspring are larger and live longer than males.Furthermore,this research showed that parasitoid adults successfully emerged from approximately 27.9％ of pupae.However,eclosion or hatch-ing of H.cunea decreased dramatically,which may be due to damage caused by female B.lasus when testing hosts with their ovipositors or by feeding on them.The results suggest that B.lasus has the potential to become an efficient natural enemy for controlling H.cunea.