What omnivores eat: direct effects of induced plant resistance on herbivores and indirect consequences for diet selection by omnivores

1. Omnivory, where an animal crosses trophic boundaries, is thought to be c ommon in natural and managed communities. Foraging theory predicts that omn ivores will balance their diet as a result of nutritional needs, food quali ty and availability of alternate foods. We investigated diet selection of t he western flower thrips [Frankliniella occidentalis (Pergande)], a herbivo re under some circumstances, and a predator under other circumstances. 2. We demonstrate that induced plant resistance can indirectly affect diet selection by thrips. The density of herbivorous spider mites is reduced on induced plants compared to controls and is also positively correlated with the number of mite eggs that the thrips consume. 3. In choice tests, mite eggs from induced plants were less preferred by th rips than eggs from control plants. Mite egg size was also smaller on induc ed plants compared to controls. However, this alone did not explain the die t selection by thrips. In a field experiment, thrips avoided colonizing ind uced plants, but they were attracted to induced plants that we inoculated w ith mites. Thus, the presence of herbivorous prey on induced plants attenua ted the negative effects of induced resistance on thrips colonization. 4. Feeding decisions of thrips can be influenced by several factors. We sho wed previously (Agrawal, Kobayashi & Thaler 1999a) Influence of prey availa bility and induced host plant resistance on omnivory by western flower thri ps. (Ecology, 80, 518-523.) that reducing plant quality can cause omnivores to shift towards relatively more predation than herbivory. We show now tha t on induced plants, reduced prey density and quality may antagonize this s hift towards increased predation. 5. Induced plant resistance has negative effects on both the plant-based fo od resource (direct effect) and the animal-based food resource (indirect ef fect) of omnivorous thrips. Thus, variation in the quantity and quality of food items interact to determine the diet selection of omnivores.