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 Auto grapha californica (Speyer) (AcMNPV) and 2 recombinant strains, engine ered to express a scorpion toxin (AcAaIT) or a juvenile hormone estera se (AcJHE-KK), were used to infect larvae of cabbage looper, Trichoplu sia ni (Hubner). Initial mortality of 1st, 4th, and 5th instars, regar dless of the polyhedral inclusion body (PIE) concentration, was not de tected earlier than the 2nd d after exposure. At equivalent concentrat ions of virus, the recombinant AcAaIT was the fastest acting strain fo llowed by the recombinant AcJHE-KK and then the wild-type AcC6. Each d oubling of the viral concentration of AcC6 decreased the LT50 by appro ximate to 5%. Therefore, an 8- to 18-fold increase in the concentratio n of virus of the wild-type AcC6 was needed to attain LT(50)s equivale nt 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 expos ed to AcC6 or the recombinant strain AcAaIT. The recombinant AcAaIT al ways killed cabbage loopers more quickly than the wild AcC6 strain reg ardless of larval size. Differences in LT50 between strains AcC6 and A cAaIT were significant for all weight classes except for larvae weighi ng 45-60 mg per larva. The greatest difference in the LT50 of the stra ins was for 1st instars weighing 0.3-0.5 mg per larva and the least di fference was for 3rd instars weighing 45-50 mg per larva.