A tiny cuckoo: Risk-dependent interspecific brood parasitism in a predatory mite

1. Many animal species protect their eggs against predators, and other species may profit from this by adding their eggs to those of the protecting species.2. We studied two tiny species of predatory mites that share a food source, are engaged in intraguild predation, and which eggs are attacked by the same egg predator.3. One of these predatory mites (Neoseiulus californicus) adds its eggs to those of the other species (Gynaeseius liturivorus) which guards its eggs from egg predators, resulting in reduced predation of the eggs of the nonguarding species. The guarding species experiences costs in the form of intraguild predation of her offspring, and the addition of eggs to the guarding species therefore qualifies as brood parasitism.4. In the presence of egg predators, the brood parasite preferentially adds its eggs to those of the guarding host species, not to those of another, nonguarding mite species.5. This cuckoo behaviour comes with a cost for the parasite when egg predators are absent, and therefore only occurs when egg predators are present.6. Our findings emphasize the importance of the risk of egg predation as a factor driving facultative brood parasitism.