Landscape complexity can benefit natural enemy communities and the biological pest control services they provide in agricultural landscapes. Harvestmen are generalist predators consuming a large range of prey in terrestrial ecosystems including agroecosystems. However, their ecology and their role in controlling pest populations in such ecosystems remain poorly studied. In this study, we examined predator-prey interactions between the European harvestmen (Phalangium opilio L.) and several potential prey species found in a vineyard agroecosystem. We sampled 20 populations of harvestmen in vineyards selected along a gradient of proportion of semi-natural habitats and used gut-content molecular analyses to quantify interaction strength between harvestmen and the grape berry moth, the main insect pest of grape, and two alternative prey species, springtails and the grape phylloxera. We found a high proportion of harvestmen positive to each type of prey with, on average, half of the individuals collected that had consumed grape berry moths. Increasing the proportion of semi-natural habitats in the landscape enhanced the proportion of harvestmen preying upon grape berry moths. Despite a significant number of harvestmen preying on springtails and grape phylloxera, the strength of the feeding interaction between harvestmen and these alternative prey species never significantly explained predation rates of the grape berry moth. Our results indicate that conserving semi-natural habitats increases biological pest control services delivered by harvestmen in vineyard landscapes and highlight the potentially important role of harvestmen in those agricultural systems.