Compatibility and Insecticidal Responses of Bacillus thuringiensis in Tank Mixtures with Spray Adjuvants against Spodoptera frugiperda

The fall armyworm, Spodoptera frugiperda, is a major agricultural pest whose management increasingly relies on Bacillus thuringiensis (Bt)-based bioinsecticides. However, tank-mix applications with adjuvants may alter Bt performance. This study evaluated the in vitro compatibility of Bt (Xentari) combined with five commercial adjuvants (Li-700, Fighter, Wetcit Gold, Protac, and Tween 80) and compared the insecticidal responses of these mixtures in laboratory bioassays. In vitro compatibility was assessed using nutrient agar to determine vegetative growth and sporulation, while pH variation was measured in laboratory spray solutions over time. Insecticidal response was evaluated through bioassays on artificial diet with second-instar S. frugiperda, using two complementary approaches: spore suspensions derived from in vitro assays and laboratory assays with spray solutions applied at 0, 2, 6, and 24 h after preparation. Tween 80 increased vegetative growth, whereas Li-700 reduced vegetative growth but enhanced sporulation. Wetcit Gold and Protac completely inhibited Bt development in vitro. Protac reduced solution pH below 2.5, indicating strongly acidic conditions. Under the laboratory conditions evaluated, microbiological compatibility observed in vitro did not consistently correspond directly to the insecticidal responses obtained in the artificial-diet bioassays. All Bt-containing spray solutions resulted in final mortality above 80%. Within the Wetcit Gold treatment, final mortality at 2 h was lower than at 6 and 24 h, while no significant differences among Bt-containing treatments were detected within individual resting times. These findings indicate that microbiological compatibility assays and laboratory bioassays provide complementary information for interpreting Bt–adjuvant interactions.