Do intraguild prey protect their eggs from intraguild predators that share their oviposition site?

Intraguild (IG)-prey prefer to oviposit at sites with a low IG-predation risk of their offspring. However, IG-predators sometimes show oviposition preferences similar to those of IG-prey. In such cases, IG-prey eggs might need protection against IG-predators to survive. We tested this possibility using a system that consisted of an IG-prey, the predatory mite Gynaeseius liturivorus, and an IG-predator, the predatory mite Neoseiulus californicus. Both mite species feed on larvae of the western flower thrips, Frankliniella occidentalis, as a shared food source. When offered plastic discs as substrates for oviposition, both mite species preferred to lay eggs on the discs, regardless of the presence of heterospecifics. Subsequently, we examined G. liturivorus egg survival in the presence of only conspecific mothers or N. californicus, and the presence and absence of both mite species. The survival of G. liturivorus eggs was significantly reduced when kept with only N. californicus, but this reduction was not found in the presence of both G. liturivorus and N. californicus. These results indicate that G. liturivorus mothers improved the survival of the eggs in the presence of N. californicus. Behavioural observation revealed that adult female G. liturivorus mostly remained on the plastic discs with their own eggs during experiments. Furthermore, the presence of G. liturivorus mothers reduced the residence time of N. californicus on plastic discs with G. liturivorus eggs, whereas the residence time of G. liturivorus mothers was not affected by the presence of N. californicus. We conclude that mothers of G. liturivorus are able to increase the survival of their eggs by deterring IG-predators.