Interrelationship of Bacillus thuringiensis Berliner to the Diamondback Moth (Lepidoptera: Noctuidae) and its Primary Parasitoid, Diadegma insulare (Hymenoptera: Ichneumonidae)

The interrelationship of Bacillus thuringiensis Berliner var. kurstaki to the diamondback moth, Plutella xylostella L. (Lepidoptera: Plutellidae), and its primary parasitoid, Diadegma insulare Cress. (Hymenoptera: Ichneumonidae), was investigated using laboratory colonies of the insects. The differential response of third-instar diamondback moth, parasitized and unparasitized, to B. thuringiensis (Bt), and the ability of D. insulare to oviposit in Bt-stressed hosts were determined. No significant difference (P > 0.05) was found between the mean mortality of parasitized and unparasitized larvae at each of three concentrations (154, 334, and 2,237 IU/ml) of Bt endotoxin. The three concentrations were equivalent to the LC₃₀, LC₅₀, and LC₉₀ of Bt potency based on preliminary tests. Parallel line assay analysis, however, revealed that the linear dose-response regressions of parasitized and unparasitized larvae were highly significant (P = 0.0001). The LC₅₀s of parasitized versus unparasitized larvae were 373 and 175 IU/ml Bt endotoxin, respectively, indicating that parasitized larvae were less susceptible to Bt. Female D. insulare oviposited in Bt-stressed hosts. The percentage of D. insulare females emerging from Bt-treated larvae (41.4%) was not significantly different from that of untreated larvae (32.0%).