Department of Agricultural and environmental Biology, Graduate School of Agricultural and Life Sciences, The University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113–8657, (Japan).
An increase in LD50 and LT50 values with increased larval age was recorded in bioassay studies of a nuclear polyhedrosis virus (NPV) infected Spodoptera litura Fabr. larvae. The LD50 values were 224, 806, 20 692 and 519 381 PIBs/larva for the second, third, fourth and fifth instars, respectively. Based on LD50, the second instar was about 3.5-, 92-, and 2314-fold more susceptible to NPV than the third, fourth, and fifth instars, respectively. The estimated LT50 increased with the increased larval age, whereas it decreased with the increased doses. Larvae > 11 day-old were not infected through oral inoculation. However, intrahaemocoelic injection of NPV into mature larvae resulted into death due to viral infection, indicating a lack of susceptibility possibly due to gut mediated barrier that developed as the larvae approached pupation.
