Host-plant-mediated interaction between populations of a true omnivore and its herbivorous prey

Western flower thrips, Frankliniella occidentalis (Pergande) (Thysanoptera: Thripidae), are competitors with twospotted spider mites, Tetranychus urticae Koch (Acari: Tetranychidae), for plant resources and potential predators on spider mites when the opportunity arises. Which interaction predominates may depend on relative population densities and individual species’ responses to the plants on which they co‐occur. We examined interactions between populations of thrips and spider mites on several cultivars of two bedding plants: impatiens (Impatiens wallerana Hook.f) cultivars ‘Impulse Orange’ and ‘Cajun Carmine’, and ivy geranium Pelargonium peltatum (L.) L’Her ex Aiton] cultivars ‘Sybil Holmes’ and ‘Amethyst 96’. Four combinations of thrips and mite numbers were studied: thrips alone, mites alone, and two densities of thrips and mites together. We compared population numbers after 4 weeks. Overall, mite numbers increased more rapidly than thrips did, but both species increased more rapidly on impatiens than on ivy geraniums. Between impatiens cultivars, thrips and mites increased more slowly on ‘Cajun Carmine’ (i.e., it was more resistant) than on ‘Impulse Orange’. On ivy geraniums, spider mites increased more slowly on ‘Sybil Holmes’ than on ‘Amethyst 96’ but the reverse was the case for thrips. Regardless of plant species or cultivar, thrips had a strong negative effect on spider mites whenever they co‐occurred, suppressing mite population growth by around 50% compared to when mites were alone. However, the effect of spider mites on western flower thrips depended on the quality of the plant species. On impatiens, thrips co‐occurring with spider mites increased slightly more than thrips alone did, while on ivy geranium mites had a small negative effect on thrips. Contrary to expectations, thrips had a larger negative impact on spider mites on plants that were more susceptible to thrips than they did on plants more resistant to thrips. We suggest that host plants mediate the interaction between an omnivore and its herbivorous prey not only by altering individual diet choice but by changing the relative population dynamics of each species.