Chymotrypsin-like proteases contribute to Phthorimaea absoluta (Meyrick) susceptibility to Bacillus thuringiensis

Phthorimaea absoluta (Meyrick) is a globally significant pest of tomato crops, capable of causing up to 100 % yield losses under high infestation levels. The continuous use of chemical insecticides has led to the evolution of resistant P. absoluta populations, resulting in frequent control failures in the field. This scenario extends to the entomopathogenic bacterium Bacillus thuringiensis (Berliner), raising concerns about resistance development due to increased usage as an alternative to chemical control. This study tested the hypothesis that Brazilian populations of P. absoluta have developed resistance to Bt formulations and investigated the role of serine proteases in this process. We conducted concentration-response assays for eight P. absoluta populations. Resistance variation was observed, with a 2.93-fold increase for Dipel® and a 5.83-fold increase for XenTari®. The LC80 values for XenTari® exceeded the recommended dose, indicating potential field control failures. Serine protease activities, specifically chymotrypsin and elastase , were inversely proportional to the LC50 values for Dipel®, suggesting their involvement in toxin activation. However, no correlation was found between serine protease activity and LC50 values for XenTari®, implying that these enzymes do not contribute to resistance against this formulation. The differential toxin composition of the two products likely explains the higher efficacy of B. thuringiensis var. kurstaki (Dipel®) compared to B. thuringiensis var. aizawai (XenTari®) against P. absoluta . The presence of the Cry1Ac toxin in Dipel® appears to enhance its efficiency in controlling P. absoluta while exerting selective pressure on the insect's humoral defenses . • Toxicity of Bt-based insecticides varied against Phthorimaea absoluta populations. • B. t. kurstaki is more active against Phthorimaea absoluta than B. t. aizawai. • There seem not to have a sharing activity between both Bt strains against P. absoluta. • Cry1Ac is likely the most toxic Cry protein against P. absoluta , not present in B. t. aizawai. • Chymotrypsin-like protease was the major factor linked to P. absoluta resistance to B. t. kurstaki.