Some solitary wasps and bees exhibit peculiar structures, i.e., acarinaria, which are invaginated chambers harbouring certain mite species for transfer to brood cells of hosts. Acarinaria have long been considered morphological adaptations that evolved to securely transfer beneficial mites into nests but there is little compelling evidence to support this hypothesis. The dispersal deutonymphs of the mite Ensliniella parasitica are housed in acarinaria of the host A. delphinalis during phoresy. The mite life cycle has been investigated in detail; the deutonymphal mite using acarinaria invades into a host cell during wasp oviposition, the tritonymph feeds on heamolymph from lepidopteran prey, then adults from the juvenile host, the female begins laying eggs on the host after the host pupates, and larvae and protonymphs acquire nutrition from the pupa, developing into deutonymphs by host eclosion. Although totally parasitic to the juvenile host throughout its life cycle, the mite did not negatively affect on the host. Nests were sometimes infested with other natural enemies, which included a parasitoid wasp (Melittobia acasta), a kleptoparasitic fly, and unknown pathogens. When the parasitoid and adult mites co-existed in a host cell, we found that either all mites or all parasitoids died. A single adult parasitoid (female) and adult mites were placed in transparent acrylic tubes containing a single prepupal host. In some cases, the parasitoids harbouring mites eventually died. However, in other cases, the parasitoid counterattacked mites by possibly biting them on their dorsum. This experiment demonstrated that the probability that the parasitoid was killed depended on the number of mites. Although mutualisms to protect a partner are common, this wasp-mite interaction is exceptional, because the mite is non-predacious and much smaller in body size than the enemy but protects the host wasp.