Plants and insects have co-evolved for millions of years. Plants have developed the capability to produce secondary metabolites in order to protect themselves against different types of pathogens and herbivores. The pathogens include fungi and bacteria, and the herbivores include insects, birds, mammals, etc. Secondary metabolites, such as monoterpenoids, are potentially effective naturally occurring insecticides because of the co-evolution through which they were developed. In this thesis I investigated the toxicity of naturally occurring monoterpenoids on house flies (Musca domestica), honey bees (Apis mellifera), and varroa mites (Varroa jacobsoni). Along with the investigation of natural occurring monoterpenoids, ester and ether derivatives were synthesized to examine what effect modifying the parent monoterpenoids has on toxicity. Using monoterpenoids and their derivatives' LK50 values, I obtained several quantitative structure-activity relationships (QSARs). House fly LD50 values were used to construct QSAR relationships for thymol, carveol, carvacrol and their derivatives, and for cyclic aliphatic monoterpenoids. Mulliken population was used to construct individual QSAR relationships for thymol, carveol, caravacrol and their derivatives' LD50 values. Another QSAR model was developed for a more dynamic set of monoterpenoids. These cyclic aliphatic monoterpenoids showed a good relationship between house fly toxicity and an electrotopological state descriptor and a GETAWAY (Geometry, Topology and Atom-Weights AssemblY) descriptor. Monoterpenoids' and their derivatives' toxicities to the varroa mite and the honey bee were also tested to examine the insecticidal effectiveness of monoterpenoids to a real world problem. The data showed that some of these monoterpenoids have selective toxicity to the varroa mite but not the honey bee. Acetate derivatives of the parent alcohol or phenol increased the selectivity of these compounds. Most acetates had a greater toxicity to the varroa mite than to the honey bee. A QSAR relationship was found between honey bee toxicity and a GETAWAY descriptor.