Over-wintering of the Colorado potato beetle (Leptinotarsa decemlineata Say) in field conditions and factors affecting its population density in Estonia
Estonian University of Life Sciences, Institute of Agricultural and Environmental Sciences, Kreutzwaldi St. 64, 51014 Tartu, Estonia; e-mail: firstname.lastname@example.org
The adaptation of Colorado potato beetles (CPB) (Leptinotarsa decemlineata Say) to low temperatures has been the basis for the formation of a local permanent population in Estonia. The number of this pest fluctuates in different years on a large scale. The hibernation of the beetles in field conditions during the years 2000–2005 was observed and some factors influencing the beetles’ mortality was discussed in this study. Soil with a lighter texture – loamy sand proved better for the hibernation of CPB than clay loam soil with a heavier texture. 30 cm confirmed to be more suitable depth for hibernation than 50 cm, demonstrating lower mortality rate. Every autumn CPB populations were differently prepared for hibernation: part of the beetles burrowed themselves into the soil considered to be ready for over-wintering, and there were more survivals than among the beetles staying on the soil surface: that could not complete their maturation feeding. The temperature may become lethal for a majority of the hibernating population only during extreme winters when the temperature falls to -30oC for a longer period of time. During our observation period, it happened only once: in 2002/2003. In more mild winters there were no problems with over-wintering: about two third of the beetles survived in clay sand and about a half in loamy clay soil. It is difficult to predict the annual damage caused by CPB because the Estonian population consists of adapted over-wintered beetles and beetles migrated from southern regions. In some years we have had a great number of immigrant beetles and, in some years, no immigration has occurred.