Abstract: The present study investigates whether treatments with Bacillus thuringiensis kurstaki(Btk) at the beginning of the progradation phase affect the natural oscillation in Lymantria dispar(L.) populations, by decreasing their abundance to the levels of the latency phase, thus avoidingoutbreaks. During the spring of 2005, a bioinsecticide based on Btk was applied on 3500 hectaresof cork oak wood in north-eastern Sardinia where infestation foci of the gypsy moth had beenrecorded. Spray applications reduced the population abundance and interrupted the progradationphase. However, from the third generation after the treatments, the gypsy moth populationabundance increased, reaching a peak in 2009. Our findings indicate that treatments applied at thebeginning of the progradation phase postpone the outbreak by 2 years.