Conservation measures of endangered zygopteran species are frequently accompanied by minimal monitoring. Such a monitoring comprises two censuses per year, in which the number of imagines is determined. For the Southern Damselfly, Coenagrion mercuriale we have evaluated whether minimal monitoring results in a reliable assessment of population size. Therefore, we compared two different datasets, collected from the same populations on the Swiss Plateau in 2009. One dataset represented minimal monitoring with two censuses per year. The other dataset presented a more profound estimation of real population sizes. For this latter dataset, counts were conducted on every day with suitable weather conditions during the reproductive phase. Statistical evaluation showed a high agreement of the two methods. Therefore, we conclude that even minimal monitoring results in a reliable assessment of population size. This result may also hold true for other damselfly species.