Forecasting the impact of the Gypsy moth on lowland hardwood forests by analyzing the cyclical pattern of population and climate data series

This paper presents the analytical results of cyclical behavior of Gypsy moth population dynamics monitored from 1970 in the continental part of Croatia. The analysis of the gathered Gypsy moth population series, in addition to temperature and precipitation data measured at the Jastrebarsko meteorological station, was provided with the aid of Fast Fourier Transform. Strong Gypsy moth outbreaks occur regularly with a significant periodicity of 10.6 years. A peak in the Gypsy moth population density can be expected 5.2 years after the onset of the latency period, i.e., upon the ending of the previous gradation. In the climate data, significant periodicities of 2.1-2.9 were found in yearly temperature and precipitation, in monthly temperature (February, April, October, November, December) and precipitation series (May, September). Gypsy moth outbreaks of 10.6 years are synchronized with temperature and precipitation as harmonics, i.e., as an integer multiple of the short periods of climate data series. From the beginning of the latency phase to maximal increase of population density (5.2 years) two periodic increases of temperature usually occur. It was confirmed by use of logistic regression that the strong increase of population density is related to a significantly higher temperature in December. Strong collapse of outbreaks is related to a significantly higher precipitation in March. These events occur simultaneously during the Gypsy moth population cycle and moderate its population density. Knowledge of the systematic cyclical behavior of the Gypsy moth series and climatic data can be used in the short term to forecast outbreaks with the aim of reducing forest damage in future.