Developmental resistance in orally inoculated mature larvae of Spodoptera litura Fabricius to its nuclear polyhedrosis virus (NPV).

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.