A practical method for predicting the short-time trend of bivoltine populations of Ips typographus (L.) (Col., Scolytidae)

Abstract: Ips typographus is the main spruce pest of European forests. In most areas of the Italian Alps there are two generations per year; overwintering adults fly in May looking for trees suitable for breeding, their offspring emerge in summer, 7–8 weeks after tree colonization, and the adults of the second generation emerge in spring of the following year after overwintering under the bark or in the litter. A long‐term population monitoring was carried out in north‐east Italy with the aim at developing a prediction model able to estimate the population density of the following year. Between 1996 and 2004, pheromone traps monitored populations of I. typographus annually. Monitoring lasted 4 months (May–August), with replacement of pheromone dispensers after 8 weeks. Insects trapped before dispenser change were called ‘spring captures’ (May–June), and included both overwintering and re‐emerging adults. Beetles caught after dispenser change were called ‘summer captures’ (July–August), and included the adults of the first generation. The results show a high positive correlation between the ratio of summer and spring captures of one year (Summer_x/Spring_x), and the ratio of total captures of the following year (Y_x+1) and those of the current year (Y_x) (Y_x+1/Y_x). Summer_x/Spring_x lower than 0.62 indicate decreasing populations in the following year (Y_x+1/Y_x <1), whereas Summer_x/Spring_x higher than 0.62 indicate increasing populations (Y_x+1/Y_x >1). The applicability of the model in the study of I. typographus risk of outbreak and in the forest management is discussed. The prediction of the short‐time trend of the population allows assessing its density in the following year, and therefore the risk of outbreak.