Two Protease Inhibitors Fail To Synergize Bacillus thuringiensis in Diamondback Moth (Lepidoptera: Plutellidae)

Two serine protease inhibitors were tested for direct effects and synergism with B. thuringiensis against larvae of diamondback moth, Plutella xylostella (L.). Concentrations of 1-1,000 mg/liter of trypsin inhibitor (Kunitz) and 1-500 mg/liter of trypsinchymotrypsin inhibitor (Bowman-Birk) caused little or no mortality to larvae in laboratory bioassays. No significant synergistic effects on mortality occurred between either protease inhibitor (100 mg/liter) and formulated B. thuringiensis in tests with larvae that were susceptible or resistant to B. thuringiensis. The lack of synergism suggests that enzymes mediating the toxicity of B. thuringiensis to diamondback moth larvae were not affected by trypsin inhibitor or trypsin–chymotrypsin inhibitor. The similar response of susceptible and resistant larvae suggests that altered processing of B. thuringiensis toxins by trypsin or trypsinlike proteases was not an important mechanism of resistance.