Field and laboratory trials to compare different Bacillus thuringiensis formulations against Lymantria dispar in Sardinian forests

Abstract: The effectiveness of two Bacillus thuringiensis subsp. kurstaki (Btk) formulations was tested against Lymantria dispar larval populations in field and laboratory conditions. Field trials were carried out in North and Central Sardinian cork oak forests in 2018 and 2019, respectively, by applying (1) Foray® 76B AVIO (strain ABTS-351, Sumitomo Chemical Agro Europe S.A.S., 20 BIU/l) and (2) Rapax® AS AIR (strain EG-2348, Biogard, division of CBC Europe, 24 BIU/l). The experimental design involved three replicates for each of the following treatments: (i) untreated control; (ii) Foray® 76B at the dose of 2.0 l/ha; (iii) Foray® 76B at the dose of 2.5 l/ha; (iv) Rapax® AS AIR at the dose of 2.0 l/ha. All formulations were sprayed undiluted using helicopters equipped with 4 electronic rotary atomizers adjusted to sprinkle 160 micron-sized drops. In order to evaluate the effectiveness of Btk in the field, larval density was assessed at each area by counting larvae on 40 shoots, randomly-chosen from 10 tagged trees, before spraying and 7, 14 and 21 days after the application. Moreover, samples of 100 II-III instar larvae randomly collected from each treatment were fed in the laboratory with foliage collected from the respective plot, and mortality data were recorded until pupation. All Btk-based products proved to be effective in the field against gypsy moth larvae, being able to reduce the mean insect population density at levels lower than the untreated control. These results were confirmed by survival observations conducted on laboratory-reared larvae, indicating a similar effectiveness of all three Btk treatments.