The abundance of parasitoids on plants that harbor their monophagous herbivorous host often correlates with host numbers. However, when hosts are polyphagous, the species-specific characteristics of the plants can affect parasitoid abundance as well. We asked whether parasitoids that attack a polyphagous host aggregate on individual plants with high host densities, and whether plant-related factors (plant species and flowering state) also account for the parasitoids’ abundance on the plants. We sampled Encarsia (Förster) (Hymenoptera: Aphelinidae) parasitoids and their host Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) from three plant species, six times during the summer of 2013. We analyzed the effects of host abundance, plant species, and flowering state on parasitoid abundances. The abundances of three parasitoid species were significantly and positively affected by the abundance of B. tabacion plants, regardless of plant species. In contrast, the abundance of the fourth species was not affected by host numbers, but rather by plant species identity as well as by flowering state: parasitoid numbers were lower on flowering plants than on non-flowering plants. Although previous field studies have shown correlations between parasitoid and host abundance, our research demonstrates additional, plant-related variables that can influence this relationship when hosts are polyphagous. We also show that although having the same host, different parasitoid species respond differently to host- versus plant-related variables.