Prolonged pupal diapause drives population dynamics of the pine processionary moth (Thaumetopoea pityocampa) in an outbreak expansion area

Due to its negative impact on tree growth and human health, pine processionary moth is a major concern for forest managers, especially in recent outbreak expansion areas. As some individuals have prolonged diapause for more than a single year, population dynamics of this species is hard to understand. To decipher the mechanism of prolonged diapause and its role in population dynamics, we started a pest surveillance program in 1999 in a pine processionary population in the southern-central Alps of northern Italy, a recent range expansion zone for the species. The 16-year study used a pheromone trap network and four large rearing cages. We found some individuals could diapause for up to a maximum of seven years. With few exceptions, prolonged pupal diapause increased and retrieval rate of moth in cages decreased with increasing elevation. Moreover, we found prolonged diapausing individuals to emerge in advance of non-diapausing individuals. This trait allowed us to infer the proportion of prolonged diapausing individuals caught in the pheromone traps. Prolonged diapause was responsible for maintaining high population density for eight years in spite of annual applications of the biocontrol agent Bacillus thuringiensis kurstaki. This sustained density relied on individuals from cohorts before the application of insecticide started, and from cohorts not completely suppressed by the insecticide applications.