The molecular interaction between plants and insects is dynamic and multifaceted. We are interested in understanding the molecular mechanism that insects utilize to overcome plant defense proteins, as well as discovering novel plant insecticidal proteins. Three projects were developed. First, we evaluated the effects of soybean cysteine protease inhibitor (soyacystatin N, scN) on the growth and development in southern corn rootworm. Both subtractive suppressed hybridization (SSH) and cDNA microarray analyses were used to uncover the changes of gene expression profiles in southern corn rootworm under the scN challenge. The counterdefense-related genes were identified, suggesting that southern corn rootworm deployed several regulatory mechanisms to overcome the dietary scN. Second, to identify and confirm insecticidal properties of vegetative storage protein 2 in Arabidopsis (AtVSP2), the gene was cloned and expressed in E.coli. This protein showed acid phosphatase activity. Feeding assay indicated that AtVSP increased the mortality and delayed the development of two coleopteran and one dipteran insects. Third, to identify the molecular mechanism of this novel insecticidal protein, P element mutagenesis was utilized to generate AtVSP resistant mutants (VRs). Two balanced VR mutants and their revertants were generated, and can be used to further characterize the genetic loci of P element inserted in the mutants.