A primary focus of evolutionary biology is the origin of biodiversity through speciation. Though allopatric speciation has been largely accepted as the major mode of speciation, the role of sympatric speciation needs to be understood. The cerambycid beetle, Dectes texanus LeConte, an indigenous, univoltine species, is a pest of cultivated sunflower. During 1960’s this species emerged as a new pest on soybean. The close relationships between herbivores and their host plants for reproduction and development make their populations susceptible to disruptive selection following shifts to novel host plants. The recent shift of D. texanus to soybeans from sunflower facilitates to understand the initial dynamics of the host shift. Microsatellite markers (n=10) were used to quantify the pattern and levels of genetic variation. Adults and larvae were sampled both allopatrically and sympatrically. A total of two and three sympatric and allopatrically distributed fields were sampled, respectively. The results indicated that the combined genetic variation among the populations infesting soybean and sunflower was moderate (FST=0.12) with a mean gene flow (Nm) of 1.82. The results and their usefulness in understanding the adaptive genetic divergence will be discussed.