The gypsy moth appears almost all over Europe and is one of the most dangerous insect species in the deciduous stands causing outbreaks at about 8-10-year intervals. It was also introduced into the United States in 1869. Since then the gypsy moth has spread there over a large area due to lack of its natural enemies and has caused huge losses in forestry.
In Poland this species appears over all the country except the mountain region, but, so far, it has not been as harmful as the nun moth. However, the economic importance of the gypsy moth at the end of XX century became more significant due to the increase in both the forest area infested with this species (up to about 9 thous. ha in 1992) (Figure 1) and the percentage of the area of deciduous forests in the total forest area (13% and 22.5% in 1945 and 1996, respectively) in the state. Several outbreaks of the gypsy moth have been recorded in Poland since 1953. The outbreaks in the north-east Poland, particularly in the area of "Biebrza Marshes" (the Biebrza National Park since 1993 with an area of 59233 ha) are among the best described. The last outbreak was in 1991-1995. The area, which seems to be a focus of the outbreaks in this region, was chosen for our research into the gypsy moth population.