For phytophagous insects, ecological speciation occurs when reproductive isolation evolves by divergent natural selection stemming from use of alternate resources such as different host plants. Typically models of ecological speciation consider the process at only one geographic location. However, diverging populations may be interacting in many different locations. Divergence could be accelerated at some locations and constrained at others. In pea aphids in the eastern US, host races on alfalfa and red clover are highly ecologically specialized. However, pea aphids in the west on clover were found to be less specialized using a reciprocal transplant design, and may be at an earlier stage of divergence. We then genotyped aphids from alfalfa and clover from across the US using four sequence-tagged codominant markers. Aphids from clover in the east and Midwest were genetically similar to one another but very different from aphids from alfalfa and aphids from clover in the west. The process of divergence and host plant specialization may occur at different rates at different locations or may be completely derailed in some places. Thus it is critical to study these processes throughout the ranges of these insects.