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 common in natural and managed communities. Foraging theory predicts that omnivores will balance their diet as a result of nutritional needs, food quality and availability of alternate foods. We investigated diet selection of the western flower thrips [Frankliniella occidentalis (Pergande)], a herbivore 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 thrips than eggs from control plants. Mite egg size was also smaller on induced plants compared to controls. However, this alone did not explain the diet selection by thrips. In a field experiment, thrips avoided colonizing induced plants, but they were attracted to induced plants that we inoculated with mites. Thus, the presence of herbivorous prey on induced plants attenuated the negative effects of induced resistance on thrips colonization. 4. Feeding decisions of thrips can be influenced by several factors. We showed previously (Agrawal, Kobayashi & Thaler 1999a) Influence of prey availability and induced host plant resistance on omnivory by western flower thrips. (Ecology, 80, 518-523.) that reducing plant quality can cause omnivores to shift towards relatively more predation than herbivory. We show now that on induced plants, reduced prey density and quality may antagonize this shift towards increased predation. 5. Induced plant resistance has negative effects on both the plant-based food resource (direct effect) and the animal-based food resource (indirect effect) of omnivorous thrips. Thus, variation in the quantity and quality of food items interact to determine the diet selection of omnivores.