Effect of Viral Concentration and Body Weight on Mortality of Larvae of Trichoplusia ni (Lepidoptera: Noctuidae) Exposed to Wild-Type or Recombinant Strains of the Nuclear Polyhedrosis Virus of Autographa californica (Lepidoptera: Noctuidae)

A parental, wild-type strain of the nuclear polyhedrosis virus of Autographa californica (Speyer) (AcMNPV) and 2 recombinant strains, engineered to express a scorpion toxin (AcAaIT) or a juvenile hormone esterase (AcJHE-KK), were used to infect larvae of cabbage looper, Trichoplusia ni (Hübner). Initial mortality of 1st, 4th, and 5th instars, regardless of the polyhedral inclusion body (PIB) concentration, was not detected earlier than the 2nd d after exposure. At equivalent concentrations of virus, the recombinant AcAaIT was the fastest acting strain followed by the recombinant AcJHE-KK and then the wild-type AcC6. Each doubling of the viral concentration of AcC6 decreased the LT 50 by ≍5%. Therefore, an 8- to 16-fold increase in the concentration of virus of the wild-type AcC6 was needed to attain LT 50 s equivalent to that of the recombinants AcAaIT and AcJHE-KK. Six weight classes (ranging from 0.3 to 95 mg per larva) of T. ni larvae also were exposed to AcC6 or the recombinant strain AcAaIT. The recombinant AcAaIT always killed cabbage loopers more quickly than the wild AcC6 strain regardless of larval size. Differences in LT 50 between strains AcC6 and AcAaIT were significant for all weight classes except for larvae weighing 45–60 mg per larva. The greatest difference in the LT 50 of the strains was for 1st instars weighing 0.3–0.5 mg per larva and the least difference was for 3rd instars weighing 45–50 mg per larva.