Population growth and persistence when prey is diminishing in single-species and two-species systems of the predatory mites Euseius finlandicus, Typhlodromus pyri and Kampimodromus aberrans

Population growth and persistence of Euseius finlandicus (Oudemans), Typhlodromus pyri Scheuten and Kampimodromus aberrans (Oudemans) (Acari: Phytoseiidae) were studied in single-species and two-species systems on apple seedlings primarily infested by Panonychus ulmi Koch (Acari: Tetranychidae) in an environmentally controlled greenhouse. During the experiment, the seedlings developed natural infestations by Tetranychus urticae Koch (Acari: Tetranychidae), Frankliniella occidentalis Pergande (Thysanoptera: Thripidae), and powdery mildew. Several weeks after the start of the experiment a condition of diminishing prey availability was created by use of hexythiazox treatments. Without heterospecific competitors, T. pyri attained a higher population level than E. finlandicus or K. aberrans when similar amounts of food (spider mites) were available to each. Population growth of T. pyri was decisively favoured by the presence of T. urticae. In the single-species systems each predatory species persisted to the end of the experiment in spite of diminishing prey. In two-species systems with T. pyri/E. finlandicus and T. pyri/K. aberrans that were started with the same number of individuals of each species, only T. pyri was left at the end of the experiment. Typhlodromus pyri became more numerous than the other species when prey was abundant (which was in accordance with the results of the single-species groups) and finally displaced E. finlandicus and K. aberrans towards the end of the experiment. The following factors may have contributed to the dominance of T. pyri: (1) the ability of adult females to survive longer without food than those of E. finlandicus and K. aberrans, (2) the ability to complete juvenile development and to sustain reproduction with phytoseiid prey and (3) an advantage in foraging behaviour over K. aberrans and E. finlandicus at low spider mite levels. Euseius finlandicus predominated in the two-species system E. finlandicus/K. aberrans, but both species persisted to the end of the experiment.