Biological control of the nun moth, <i>Lymantria monacha</i> : different formulations of <i>Bacillus thuringiensis</i> for large-scale applications

Abstract The nun moth, Lymantria monacha (L.), is one of the most destructive pests of coniferous forests, causing considerable damage during cyclical outbreaks across Eurasian forests. In view of the rising mean temperatures and the associated intensification in pest pressure, it is crucial to implement control strategies that minimize reliance on chemical insecticides. The primary objective of this study was to evaluate the insecticidal efficacy of two formulations containing crystals and spores of Bacillus thuringiensis subsp. kurstaki: Delfin water-dispersible granules (WG) and Foray 76B, which differ in the strain of B. thuringiensis used and in their formulation type. Delfin WG (strain S-11) is a powder for preparing a sprayable aqueous suspension, while Foray 76B (strain A BTS-351) is a liquid concentrate used without diluting with water. Both are intended to reduce the number of Lepidoptera pests. The experiment included three treatments in the 58–72 year-old Scots pine stands threatened by Lymantria monacha: (i) stands treated with a spray solution of Delfin WG, (ii) stands treated with undiluted Foray 76B, and (iii) untreated control stands. Aerial applications were performed using aircraft equipped with Micronair AU 5000 atomizers. Insecticidal efficacy was assessed in two stages: based on (i) 21-days-after treatment monitoring of larval fallout onto canvas sheets placed under the representative trees, followed by (ii) the numbers of alive and dead larvae found in the crowns of representative trees after their cutting. Larval fallout on canvas sheets allowed assessment of early larval mortality, while subsequent crown sampling captured the remaining population, providing a comprehensive evaluation of treatment efficacy. Observed larval mortality reached 97.6% in Delfin WG-treated stands and 95.8% in stands treated with Foray 76B. Statistical analysis confirmed that both treatments resulted in significantly higher mortality compared with the control (P &amp;lt; .05), while no significant difference was found between the two biopreparations. These findings demonstrate that aerial application of Delfin WG is highly effective against L. monacha and supports its use in integrated forest protection strategies in Poland.