Metaphycus spp. are endo-parasitoids of soft scales. Some of these eggs will melanize and will not hatch because the soft scale hosts have a defense mechanism. The elucidation of the defense mechanisms of the soft scales insects against this parasitoid’s eggs is currently subject under investigation (humoral and/or cellular responses). Further research suggests that the host immunity or alternatively parasitoid virulence depends on factors such as oviposition sequence (ovipositing on multiple hosts subsequently), temperature, and plant growth. A comparative study between two Metaphycus species parasitizing brown soft scale (Coccus hesperidium L.) has been conducted. These species although they have similar biology it appears that they are not equally successful in parasitizing brown soft scale, in respect to the host defenses. This cross species comparison study might allow shading more light into the physiological interactions between these parasitoids and their hosts. Furthermore, both Metaphycus species are facultative gregarious parasitoids of soft scales. They tend to measure the size of their hosts and allocate a clutch of eggs. Host defenses are more successful if the wasps are interrupted while ovipositing and lay an incomplete clutch. In that case the wasp’s eggs will be destroyed more frequently. This may have tremendous implications on how these parasitoids make decisions about their clutch size and lifetime reproductive success.