Land use intensification is a general threat to biodiversity, but many species depend on low-intensity agricultural ecosystems. One example is European mountain meadow ecosystems, traditionally managed by hay harvesting or livestock grazing. Abandoning management often causes population declines, local extinctions and biotic homogenisation in these meadows. We studied changes in the Myrmica host ant communities of the xerophilic form of the ant-parasitic Alcon Blue butterfly (Maculinea alcon) in four hay meadows in the Bükk mountains of Hungary between 2000–2007 and 2018. Abandonment started in this region in the 1970s, accelerated in the 1980s, and culminated in the 1990s. We found that the Myrmica ant community has changed substantially in less than two decades. Diversity of the ant community always decreased, and species composition became more homogeneous at two sites. Habitat suitability for Maculinea butterflies decreased at three sites and increased at only one site, where management was restarted 20 years after abandonment. The number of M. alcon caterpillars and pupae, the rate of infestation of ant nests and the mean number of caterpillars and pupae per ant nest decreased between the two periods, whereas host ant specificity did not differ from random in either period. We conclude that the unfavourable changes in the host ant community due to abandonment have negative consequences for the persistence of Maculinea populations. Our study highlights the need for detailed monitoring, and the maintenance of low-intensity management by mowing or grazing to avoid the decline of biodiversity dependent on low-intensity agriculture.