Insects are strongly dependent on climatic factors, especially temperature. For this reason, changes in insect phenology are predicted to be one of the many effects of climate change. We analysed the effect of climate warming on Agrotis segetum (Denis & Schiffermüller) in Denmark. Agrotis segetum is an agricultural pest that causes root crop damage. Extensive registers of root crop damages by A.segetum in Denmark were kept between 1905 and 1980. These records show a correlation between A.segetum outbreaks and hot, dry summers. The last outbreak in Denmark, in 1976, prompted the implementation of pheromone trap monitoring. Data from these pheromone traps, for the periods between 1981-1991 and 1997-2009, show a decline in the number of A. segetum individuals during 2000-2009. We used this pheromone trapping data to test the hypothesis that temporal changes in phenology and abundance are correlated with temperature and precipitation patterns. As phenological fixed points, we use the time of capture (first, 10%, 50%, 90% and last) of 1st generation A.segetum together with the 10%-90% capture period and the total capture per location for a statistical comparison of the period 1981-1990 versus 2000-2009. We also analysed the effect of temperature on the timing of fixed points and the effect of precipitation on the mean total capture from the summer and the preceding winter. Our analyses showed an earlier occurrence of A.segetum during 2000-2009 than during 1981-1990: all phenological fixed points advanced by 6-9 days. Trap captures of the 1st generation roughly halved from the 1981-1990 decade to the 2000-2009 decade, and a much larger 2nd generation of moths developed in the latest decade. When a second generation occurs, populations in following years decrease because the larval offspring of the 2nd generation cannot successfully overwinter. Our analyses further indicated that increased precipitation during the summers and mild winters contributed to an overall decline in the populations of this pest. The presence of A.segetum remains sufficient to be a potential threat to yields. Forecasting an economically important attack of A.segetum is challenging because of the early emergence of A.segetum and more variable local weather conditions than 20 years ago; forecasting relies on the timely and correct delivery of monitoring results. Farmers are a vital source of information for monitoring, for estimating control requests and for timing the emergence of A.segtum. However, the growing frequency of no-risk forecasts has reduced the interest and lowered the motivation of farmers to participate in monitoring programs.