A practical method for predicting the short-time trend of bivoltine populations of Ips typographus (L.) (Col., Scolytidae) M. Faccoli 1 and F. Stergulc 2 1 Department of Environmental Agronomy and Crop Productions, University of Padua, Legnaro (PD), Italy; 2 Department of Plant Protection, University of Udine, Udine, Italy Ms. receieved: April 25, 2005; accepted: October 17, 2005 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 Ôspring capturesÕ (May–June), and included both overwintering and re-emerging adults. Beetles caught after dispenser change were called Ôsummer capturesÕ (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. Key words: Ips typographus, forecasting, Italian Alps, monitoring, pheromone traps 1 Introduction The spruce bark beetle Ips typographus (L.) (Col., Scolytidae) is one of the most destructive pests attacking European spruce forests (Christiansen and Bakke, 1988; Gre ´goire and Evans, 2004). The species causes great damage also to Italian stands growing in the Alps (Lozzia, 1993; Faccoli, 1999; Frigimelica et al., 2000; Stergulc et al., 2000), where it is usually bivoltine (Faccoli, 1999; Faccoli and Stergulc, 2004). Adults of the parental generation leave the overwin- tering sites in spring (May), when the mean air temperature is about 18°C(Annila, 1969), and fly looking for suitable host trees where to begin the first generation. Then, at the beginning of July, the offspring emerges and starts a second generation, which will end in September. After overwintering under the bark or in the litter, offspring of the second generation will emerge in spring of following year (Faccoli, 1999; Netherer, 2003). Actual strategies for I. typographus control may include the use of traps baited with aggregation pheromones (Bakke et al., 1977). Traps can be used both for control by mass-trapping (Raty et al., 1995) and for population monitoring (Lindelow and Schroeder, 2001). They are in use during spring and summer, when the insects are flying looking for host trees. In general, a monitoring network based on pheromone traps can give information concerning the flight period and the mean number of trapped insects, which can vary year-by-year and stand-by-stand in relation to the population density. The number of trapped insects and the amount of damage occurring in the monitored area are, however, useful for a cost/benefit analysis, which allow fixing a Ôrisk-thresholdÕ to decide about additional control measures (Faccoli and Stergulc, 2004). Catches higher than risk-thresholds mean high risk of damage, and therefore control strategies could be applied, e.g. making a more careful survey of spruce stands, applying sanitation felling, increasing the number of traps and setting up trap trees (Raty et al., 1995). Nevertheless, the monitoring data are available only at the end of summer, when the flight period is over and, in case of an outbreak, a large part of damage has already occurred. In many cases, unexpected outbreaks can occur suddenly causing serious damage. In a short time, the populations pass from low density to outbreak density, without giving the time to set up J. Appl. Entomol. 130(1), 61–66 (2006) doi: 10.1111/j.1439-0418.2005.01019.x Ó 2005 Blackwell Verlag, Berlin No claim to original US government works Journal compilation Ó 2005 Blackwell Verlag