Virulence of Autographa californica Baculovirus Preparations Fed with Different Food Sources to Cabbage Loopers123

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Abstract
Polyhedral inclusion bodies (PIB) of an isolate of a nuclear polyhedrosis virus of the alfalfa looper, Autographa californica (Speyer), were propagated in larvae of the cabbage looper, Trichoplusia ni (Hübner), and prepared by one of three methods: (1) filtration and centrifugation through 40% sucrose in water; (2) filtration, centrifugation with urea, sodium dodecyl sulfate, and NaCl, and final centrifugation through a sucrose density gradient; and, (3) filtration, centrifugation through 40% sucrose in water, air-drying, and storage of the PIB as a dry mixture with sucrose. Three experiments were conducted with 4th instars of T. ni: the LD50 of preparation (2) fed with semisynthetic diet was determined in 1975; the LD50’s of preparations (1) and (2) were compared by feeding with semisynthetic diet and cabbage disks in 1976; and the LD50 of preparation (3) fed with semisynthetic diet was determined in 1976. The virus prepared with reagents and collected from a sucrose density gradient (2) was more virulent than a crude preparation of virus (1), with LD50’s of 10 and 68 PIB per larva on semisynthetic diet and cabbage disks, respectively, compared to LD50’s of 19 and 345 PIB per larva for the crude preparation on semisynthetic diet and cabbage disks, respectively. When results from the three experiments were compared, the slopes of the probit of mortality vs. log of dosage regressions ranged from 0.65 to 1.40 probits per log-dose, and the LD50’s ranged from 10 to 188 PIB per larva for viruses fed with semisynthetic diet and 68 to 345 PIB per larva for viruses fed with cabbage. The wide variation in results with the three virus preparations was related to preparation methods and bioassay food source.