The influence of solar radiation on the development of forest insects

The mass outbreaks of pest insects occurring in many countries on different continents have not been sufficiently explained so far. The causes have mainly been seen in abiotic factors insect physiology and even in genetics. A hypothesis was put forward some years ago that there might have been, in addition to the said factors, a number of less apparent causes, such as solar radiation that might have had a significant effect on many biological processes. The occurrences of mass outbreaks of forest insects suggest that there is a correlation between the intensified activity of sunspots and the development of insect populations. Over the last 100 and 200 years, outbreaks of nun moth (Lymantria monacha L.), (Panolis flammea Schaff.), (Dendrolimus pini L.) and many other insect species have been reported in Polish forests. The highest nun moth population abundances occurred in the years: 1806-1809, 1916, 1927-1930, 1837-1841, 1892-1898, 1908-1912, 1933-1938, 1945-1952, 1955-1960, 1962-1968,1970-1975,1978-1985,1991-1994, 1999-2000, usually one or two years after the maximum of sunspot activity (about the Wolf number 150) which fell on the years: 1837, 1848, 1870, 1947, 1957-1959, 1979-1981, 1990. The intensified solar activity cycle varies between 7 and 17 years, the average cycle being 11 years. A similar 11-year periodicity in the Sun's activity was sought in the mass occurrence of pest insects (e.g. nun moth) which was confirmed in a number of cases. However due to various factors an inter-outpreak cycle was slightly different and was 6-10 years or more. It should be emphasized that natural environmental factors (i. a. abiotic and biotic) are a barrier limiting insect reproduction. These factors can, in a short time, lead to a very quick reduction in the population density of active pest insects.