Abstract:This paper concerns the bioassay of virus activity and stability of the nuclear polyhedrosis of Eriogyna pyretorum.Early fourth-instar larvae were fed on camphorwood leaves immersed with polyhedral inclusion bodies (PIB)suspension of virus. The LC50 and 95% fiduciallimits were 1.49×105 and 3.88×104—5.72×105 PIBs/ml respectively. Mortality of larvae increased with the increasing of concentration of polyhedral inclusion bodies suspension and the decreasing instar in a limited range. Temperature had no influence upon total mortality but it did affect the death rate.Polyhedral bodies had a relatively stability. The dead larvae boeies and polyhedral bodies suspension still had a high infection ability after they were stored in the refregerator at 0—4℃ for 10 months.The effect of ultraviolet (UV-) rays on the efficiency of nuclear polyhedrosis virus was significant. Its efficiency was reduced considerably after 1×108 PIBs/ml was exposed under the strong sunlight for 16 hours; its efficiency was lost mostly under the Uv-lamp (30w, 40cm)for 30—40 minutes; and was almost lost completely under Uv-lamp for 60 minutes.The effect of field experiments of the nuclear polyhedrosis virus of suspensions on the larvae of Eriogyna pyretorum was effective. After tenth days the mortality of 43.5%, 51.4% and 80.3% was obtained by using nuclear polyhedrosis virus concentrations of 1.7×105, 1.7x106 and 1.7×107/ml respectively.