Bioassay of a β-exotoxin of Bacillus thuringiensis against Heliothis zea larvae

A β-exotoxin of Bacillus thuringiensis was bioassayed on 1st-, 3rd-, and 4th-instar Heliothis zea larvae. Larvae were fed continuously on diet incorporated with concentrations of 1–700 μg AI/ml diet. Larval and/or pupal death was the measured response criterion. Dosage-mortality responses were determined at two evaluation times, 7 days post-initiation and after the entire larval-pupal development period, using probit analysis procedures. The LC₅₀ values for 1st-, 3rd-, and 4th-instar larvae at these two evaluation times were 4.9, 134.6, and 286.2 μg AI/ml diet, and 4.0, 17.6, and 66.4 μg AI/ml diet, respectively. Differences in responses between instars were more pronounced at 7 days than after the entire development period. The LT₅₀ values for 1st-, 3rd-, and 4th-instar larvae decreased from 7.1 to 3.1, 12.7 to 5.4, and 11.6 to 5.2 days post-initiation, respectively, as dosages were increased. The toxin did not act as a feeding deterrent, as all increases in dosage caused increases in mortality. Nineteen and 38% of those 3rd- and 4th-instar larvae, respectively, which survived β-exotoxin intoxication pupated later than untreated cohorts.