Monoterpenoids are natural substances which are found in many higher plant species. These plant secondary metabolites are generally considered as self-defense tactics against the plants' enemies. Our study was aimed at the anticipation of improved biological activities through the synthesis of derivatives of the natural monoterpenoids. We have also developed a systematic bioassay system to evaluate the spectrum of toxicities of the monoterpenoids. The results show that the monoterpenoids, either natural or synthetic, have a relatively wide spectrum of activity against agricultural and public health insect pests. Derivatization, especially, to the acyl derivatives of the monoterpenoids, has significantly improved the acute, fumigant, larvicidal and ovicidal activities against the above insects. The ether derivatives also showed enhanced insecticidal activity. When mosquito larvae were treated with the monoterpenoids at sublethal dosages, insect growth and development activity was observed. The enhanced biological activity of the synthetic derivatives of monoterpenoids indicates that optimal chemical structures for insecticides can be possibly elucidated through the study of structure-activity relationships.