Among the best methods I can cite: RNeasy Mini Kit (Qiagen) or SV Total RNA isolation system (Promega).
However, more specifically, you can find very useful information through the link below linked to a research project (matching your topic of study mentioned above) at the University of California Brkeley:
USING FUNCTIONAL GENOMICS TO LINK BEHAVIOR, CHEMICAL ECOLOGY, AND CHEMORECEPTION IN A WIDESPREAD AND DAMAGING AGRICULTURAL PEST