A rapid conductivity test offers a viable method to assess the quality of Bacillus thuringiensis products

Faster and less expensive quality tests are essential for measuring the quality of microbial pesticides formulations for users. In this work, the conductivity of the suspensions of six commercial Bacillus thuringiensis products was compared with the International Units of Potency (IU) in labels, the concentration of the pesticide protein, the presence of Cry-type, virulence in the larvae of Spodoptera frugiperda and Helicoverpa zea, spore and total viability, the presence of bacterial contaminants, pH, and humidity to establish relationships. The results showed that XenTari® had more viable spores (>4.5 × 1010 spores • mL−1), and Agree® had the most total viable cells. Dipel®, Agree®, and XenTari® had the most pesticide protein (>40,000 μg mL−1). The Cry-type 1 pesticide protein was present in Agree®, XenTari®, Turinsil®, and BtKrone®, and only Dipel® contained Cry-type 1 and type 2. Regarding virulence, a survival lower than 2.6% was obtained with Agree®, BtKrone®, DiPel®, and XenTari® products when S. frugiperda and H. zea larvae were used in the bioassay. Interestingly, while Bacimín® had the highest IU label claim, the pesticide protein and mortality of S. frugiperda larvae were not observed. However, viable cells and spores were detected at the lowest concentration (<2 × 107 spores • g−1). These results were useful for classifying products into null, medium, and high quality categories and establishing relationships with conductivity. An important relationship was found between conductivity and protein concentration (R2 > 0.85) or survival (R2 > 0.81). Our findings showed that conductivity predicts the quality and performance of commercial B. thuringiensis products, suggesting that this method is a faster and more reliable quality test that can be easily used by farmers.