RNA interference (RNAi) is an effective method to knock down gene expression in insects and other organisms. It has been adopted for basic research, to elucidate gene function, and applied research, to control insect pests. Here, I examined parameters needed for effective RNAi in the milkweed bug, Oncopeltus fasciatus, an emerging insect model species. For two developmental genes, Sex combs reduced, and even-skipped, very small amounts of dsRNA trigger a robust parental RNAi response. The higher the dose of dsRNA applied, the longer the duration of embryos laid with defects. Testing length-dependence, effectiveness decreased with dsRNAs in the 150 bp to 75 bp range. These developmental genes resulted in subtle, gene-specific defects which provided a more sensitive assay than lethality. Finally, effects of RNAi were transmitted across generations through trophic interactions, the first such discovery to our knowledge. This suggests potential unanticipated environmental risk to non-target insects from RNAi-based insecticides.