The U.S. Southern Plains is a critical region for cereal production, where aphids (Hemiptera: Aphididae) represent the predominant pest threat. For decades, producers have effectively relied on the conservation of natural enemies, instead of insecticide application, to suppress aphid populations. Two native aphid parasitoids, Lysiphlebus testaceipes (Hymenoptera: Aphidiinae) and Aphelinus nigritus Howard (Hymenoptera: Aphilinidae) provide a large proportion of aphid biological control. L. testaceipes has consistently been the dominant parasitoid, outcompeting A. nigritus for aphid hosts. However, climate change and the introduction of a novel invasive pest, Melanaphis sorghi sorghum aphid are altering the established parasitoid assemblages that provide essential biological control. Since M. sorghi’s initial detection, A. nigritus is the only parasitoid observed utilizing the novel species in most of the region and has increased in abundance in both summer and winter crops. An increase in A. nigritus abundance has resulted in increased interspecific competition for limited resources, which appears to be reducing L. testaceipes fitness, particularly in winter crops, threatening biological control that is relied upon for the management of aphid pests. Experimental data from laboratory and greenhouse studies was used to quantify the outcomes of competitive interactions between these parasitoids. This research will help analyze the long-term impact of altered parasitoid dynamics and provides recommendations for changes to integrated pest management programs for cereal aphids. Adapting existing integrated pest management recommendations to address current natural enemy dynamics will help ensure stable aphid pest suppression, cereal crop productivity, and stakeholder success in the Southern Plains.