Summary 1.Omnivorous arthropods are known to change their diet when host plant quality is low. Consequently, it has been suggested that decreased plant quality has a twofold negative effect on herbivore populations: (1) a decrease in growth rate of herbivores; (2) omnivores include more herbivores in their diet. We hypothesized that decreased host plant quality may also cause omnivores to feed on predators, including their own enemies. 2. We tested this hypothesis, using the omnivorous western flower thrips. This species is known to feed on many plant species, but also on the eggs of another herbivore, the twospotted spider mite. Previous research has shown that a decrease in plant quality leads to increased feeding on spider mite eggs by western flower thrips. Western flower thrips also kill the eggs of various predatory mites, including those of the specialist predator of spider mites and those of a predatory mite that attacks western flower thrips itself. 3. In this paper we investigate whether thrips larvae kill predator eggs to feed on them and whether this predation depends on host plant quality. 4. Larval survival as well as developmental rate increased when plant tissue of low quality (sweet pepper) was supplemented with eggs of two predatory mite species or when it was supplemented with pollen, a high-quality food type. 5. Supplementing high quality leaf tissue (cucumber) with predator eggs did not lead to increased survival and developmental rate. Thrips larvae fed significantly less on predatory mite eggs when pollen was available. 6. Thus, thrips larvae indeed feed on predator eggs, including those of their predator, and they feed more on predator eggs when host plants are of low quality.

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