Phenology and formation of local population of Colorado potato beetles in 2008–2013 (Leptinotarsa decemlineata Say)

In the long term five field crop rotation experiments of organic and conventional growing systems on potato plots the phenology and formation of local-population of Colorado potato beetles were monitored from 2008 to 2013. The beetles started to colonize the field from the establishment of the experiment. The initial population density of beetles was low in both, conventional and organic plots because various cereals and rape had been cultivated on the field and in the vicinity for many years previously. The number of beetles and damage level increased year after year because the potato field remained at the same block, the suitable overwintering places were nearby and the winter mortality was low. As a result of coincidence of several favourable factors like constantly high temperatures, optimal precipitation and availability of fresh food until the late autumn, the second generation of beetles developed in 2010 in Estonia for the first time. This year the number of pest increased sharply when the summer beetle’s offspring reach the adult stage and started to reproduce. The unprotected plots were entirely defoliated, in the conventional plots application of Fastac 50 killed the larvae and therefore the plants had less damage. As the spraying did not destroy the adults and the eggs, the new larvae hatched soon. Early spring and warm summer promoted fast emergence of beetles and enabled development of two generation also in 2013 for the second time. During the observation period the numerous vital local Colorado potato beetles population has been developed in this area. This was facilitated by the neighbouring position of organic and conventional plots as the organic plot acted as a reservoir of the pest and the conventional plot as a food reserve