Insect populations across the globe are declining and the biodiversity of insects is threatened. To deal with the threats, conservation biology aims to identify species that are most vulnerable to extinction, and also to understand mechanisms that make some species more prone than others to population decline, range contraction and extinction. For the extinction risk assessments it is crucial to study and monitor the possible changes in the numbers and ranges of species. Furthermore, it is important to examine the drivers of declines. This knowledge sheds light to the nature of extinction processes and enables conservation management planning, as the most threatened species often require specific management and conservation measures. Out of all insect groups declining worldwide, butterflies (Lepidoptera) are among the taxa most affected. In Finland butterflies are considered as one of the most endangered organism groups. Among the endangered butterfly species is the Apollo butterfly (Parnassius apollo), which has declined in numbers throughout the species range since the 1970s. Two decades ago the Apollo and its sole host plant orpine (Sedum telephium) was studied in one of its few strongholds in southwest Finland in the Archipelago Sea. The host plant was found to be the most important factor affecting the occupancy of the Apollo larvae. Today the species and the study area are included in CoastNet LIFE project that plans to implement restoration work of habitat for the Apollo butterfly. For successful restoration work, it is important to update the data on the Apollo and its host plant. In this thesis I study the possible occupancy changes of the Apollo larvae and possible changes in their host plant abundance by comparing historical data from survey years 1997, 1999-2003 to the data I collected in the same study area in 2019. I also explore if abundance of S. telephium affects the occupancy of P. apollo larvae. To examine the probable change in the larvae occurrence I compared naïve occupancy estimations and occupancy model estimates between data sets of historical years and the year 2019. In the historical data the number of S. telephium plants per site were scored in categories: 1 (1-10 plants), 2 (10-100 plants), 3 (100-1000 plants) or 4 (> 1000 plants). I used these same categories to study the possible change in the abundance of S. telephium in the comparison between earlier survey years and the year 2019. I found a very strong decline in the occupancy of the larvae and no apparent difference in the abundance of their host plant between the historical data and the data I collected 2019. However, there were difference between survey sites. The Apollo larvae were mainly detected on sites with average amount of the host plant (10-100 plants) instead of sites with high abundance of the host plant. This finding is unexpected as it suggests that the abundance of the host plant is not as important as was predicted based on previous studies. In addition, I did tentative grouping of survey sites with kmeans cluster analysis. I included the island groups (0-3) to top occupancy model and the analyze showed a clear difference between the groups. This difference between island groups indicates the importance of the spatial location to the probability of occupancy of the Apollo larvae. In light of these results, it is critical to continue monitoring the endangered Apollo larvae in order to understand if the decline is merely a temporary change or a signal of a possible trend. Additionally, for successful conservation management it is important to research the drivers affecting the population decline.