Supplying high-quality alternative prey in the litter increases control of an above-ground plant pest by a generalist predator

•In biocontrol, alternative food for predators is normally supplied on the plants.•We used the litter as source of alternative food for plant-inhabiting predators.•The food in the litter was of superior quality than was the pest.•Nevertheless, predators commuted between the litter and above-ground plant parts.•Predators controlled thrips better with food in the litter than without it.