Insecticidal activity of Bacillus thuringiensis Cry1, Cry2 and Vip3 toxin combinations in Spodoptera exigua control: highlights on synergism and data scoring

Bioinsecticides based on Bacillus thuringiensis toxins can be a safe alternative to chemical pesticides currently used to control Spodoptera exigua (Hubner; Lepidoptera: Noctuidae)–a worldwide, polyphagous pest species. However, only a few B. thuringiensis proteins have been proven effective against S. exigua and currently used B. thuringiensis bioinsecticides are not efficient in control of this pest–therefore, new biocontrol approaches are needed. Some research suggest that synergistic interactions can occur between certain Bt toxins and increase their insecticidal activity. Multiple-toxin bioinsecticides are also recommended to prevent field-evolved resistance. In this study, the insecticidal efficacy of five selected B. thuringiensis proteins (Cry1Aa, Cry1Ca, Cry1Ia, Cry2Ab and Vip3Aa) has been assessed, together with eight mixtures containing two or three of these toxins. The composition containing Cry1Ca and Vip3Aa is the most active against S. exigua , followed by: Cry1Aa + Vip3Aa; Cry2Ab + Vip3Aa; Cry1Aa + Cry2Ab + Vip3Aa; Cry1Aa + Cry1Ca; Cry1Ca + Cry2Ab and Cry1Aa + Cry1Ca + Cry2Ab. The results gathered at 7 days after treatment (DAT) indicate synergistic interactions between most of these mixtures. Interestingly, the results obtained at 10 DAT suggest synergism only in three combinations (Cry1Ca + Vip3Aa, Cry1Aa + Vip3Aa, Cry1Aa + Cry1Ca). Moreover, the estimated synergism factors are 1.6-2.2-fold lower than at 7 DAT. The only exception is the Cry1Ca + Vip3Aa composition, for which high synergism factor (>4) was calculated at both scoring times. This work shows that the time of data record can substantially influence the interaction type estimation. The most potent compositions should be used to develop new bioinsecticides (transgenic crops or targeted recombinant strains) with high effectiveness against S. exigua .