conducting feeding test providing variety of host spp followed by study on feeding potential, survival of larvae and fecundity on particular host will be help to identify the host witch enhance the effectiveness of the predator
You can increase the preference and then the performance/effectiveness if you know the preferential feeding behaviour of a given predator. Which species you meant by black aphids, there are many black aphid species; I guess you meant Aphis fabae. If this is the case, this species is known as a highly ant-attended aphid, and these ants often defend strongly their aphids from predators like Coccinella spetempunctata (ants attack and often exclude successfully natural enemies of aphids). Hence, for increasing the preference/ effeciency :
1. manage ant population away from aphid colonies
2. apply a specific managment on the target landscape : flowering strip, intercropping system, banker plants, ..