Synergism of Bacillus thuringiensis by Ethylenediamine Tetraacetate in Susceptible and Resistant Larvae of Diamondback Moth (Lepidoptera: Plutellidae)

We measured mortality of neonates and 3rd instars from susceptible and resistant colonies of diamondback moth, Plutella xylostella (L.), caused by Bacillus thuringiensis with and without ethylenediamine tetraacetate (EDTA) and trypsin. For susceptible larvae, the addition of EDTA + trypsin reduced the LC₅₀ of B. thuringiensis by 5-fold in neonates but had no effect on 3rd instars. For resistant larvae, the addition of EDTA + trypsin significantly reduced LC₅₀s of B. thuringiensis by 2- to 3-fold in neonates and 3rd instars. EDTA + trypsin without B. thuringiensis did not kill larvae. Further tests with susceptible and resistant neonates showed that EDTA significantly increased the toxicity of B. thuringiensis, whereas trypsin had no effect and no interaction occurred between EDTA and trypsin.