Monitoring of developmental stages of the Western corn rootworm (WCR) (Diabrotica virgifera virgifera) is an important aspect of its control. In the experiment there were analysed randomly selected plants of maize on six localities in western Slovakia, in the years 2008-2010 a 2014-2015. The aim of the study was to identify exact time of the emergence of the WCR adults in the consequence of different years and different localities and to find the best date for their eradication. The most of the WCR individuals at the northern locality were observed on 29 June and the first adults were found on 07 June, during colder year 2006. The development started about nine days earlier in the next year in southern Slovakia, because of extremely hot weather. The population of pest increased slower in 2008 than in previous years, and the highest appearance was monitored up-to 02 July. The first adults were monitored on 27 June in the years 2009-2010 as well as 2014-2015. Generally, the highest numbers of the first instar larvae were observed on 22 June at the locality Čiližská Radvaň. The second instar larvae and the most of adults was found on 20 June in Sokolce. On 29 June it was monitored the largest amount of the third instar larvae at the locality Čiližská Radvaň. The differences in the time of the emergence of adults depended on the year. Based on the monitoring in Slovakia, it can be expected the third instar larvae from second decade of June and the first adults in the last decade of June and in the first decade of July.