Host-associated differentiation (HAD) is a form of ecologically mediated host-race formation between parasite populations. Since HAD can ultimately lead to speciation, it has been proposed as a way to account for the vast species diversity observed in parasitic arthropods. However, the importance of HAD to species diversity is unclear because the factors explaining the occurrence of HAD are only partially understood. Still, there are several examples of parasite-host case study systems for which there is a known cause of reproductive isolation between host-associated parasite populations. Thus, several biological and ecological factors (e.g., immigrant inviability or allochrony) have been proposed as explanatory factors for HAD occurrence. The body of research presented here represents the first quantitative assessment of the generalized relationship between HAD occurrence and the incidence of the proposed explanatory factors. This research was supported by field experiments that assessed the co-occurrence of HAD and particularly important explanatory factors. These experiments were conducted in a community of Aphidomorpha species living on pecan and water hickory trees. I found that HAD can be explained in general based on the incidence of specific explanatory factors (i.e. immigrant inviability, gall-making, short generation times, volatile preference, morphological differentiation, and host-shifting opportunities). These factors were used to create a hierarchy of conditional probabilities that can successfully separate the presence of HAD from its absence. The field experiments corroborated that the occurrence of HAD is correlated with immigrant inviability as well as allochrony.