Application timing affects the effectiveness of Bacillus thuringiensis kurstaki against Tortrix viridana larval populations

Abstract: The green oak leaf-roller moth, Tortrix viridana L. (Lepidoptera Tortricidae), is one of the main defoliators of pubescent oaks (Quercus pubescens Willd) in Mediterranean forests. The effect of Bacillus thuringiensis kurstaki (Btk) applications on different larval stages was tested in 2021 in Sardinia (Italy). The experimental design involved five replicates of the following four treatments: untreated control and application against first-second instars (i. e., early application), second-third instars (i.e., intermediate) and fourth-fifth instars (i. e., late). The efficacy of treatments was evaluated by assessing the larval density reduction at 7, 14, and 21 days after applications. In addition, defoliation due to T. viridana feeding activity was estimated for all treatments at the end of the larval development. The reduction of the larval density due to Btk changed depending on application timings. The efficacy of the Btk-based insecticide sprayed in the late application was significantly higher than other application timings, as the larval density reduction observed 7, 14, and 21 days after Btk application was approximately 75, 89, and 90 %, respectively. On the contrary, the larval density reduction was always below 50 % when Btk was sprayed in early and intermediate applications. In general, the degree of defoliation was lower in trees sprayed in intermediate and late than in early applications. Our results showed that the efficacy of Btk applications against T. viridana could be more likely related to the level of Q. pubescens sprouting than to a different susceptibility of larval instars to Btk, as leaves were unfolded and exposed to Btk when treatments were applied against 4th-5th instars (i. e., late application).