Plant-derived compounds: acute toxicity, synergism, and effects on insect enzyme activity and flight motor responses.

Botanical extracts may contain compounds that have insecticidal properties that may be developed as inexpensive insecticides. In this thesis, I used a series of techniques to identify the acute toxicities and modes of action of plant-derived compounds against the Yellow Fever mosquito Aedes aegypti and the blowfly Phaenicia sericata. Initially I evaluated the acute toxicity of 16 phytochemicals on aquatic and terrestrial insects alone or with the synergist piperonyl butoxide (PBO) to quantify their lethal and sublethal effects. From this study 5 compounds, thymol, eugenol, pulegone, α-terpineol and citronellal, were selected for further study. I then evaluated the biochemical mechanisms underlying the activity of these phytochemicals and the basis of their increased toxicity in the presence of PBO. These phytochemicals affected the biotransformational capacity of these insects to detoxify the compounds, and their effects were enhanced by PBO. I then developed an electrophysiological system to evaluate the physiological effects of the plant-derived compounds and several commercially available insecticides on flight muscle impulses and wing beat signals of the blowfly, P. sericata. These compounds readily penetrate the insect cuticle and interfere with flight muscle and/or central nervous system function. All 5 compounds depressed flight-associated responses, and acted similarly to compounds that block sodium channels and facilitate Ɣ-amino butyric acid (GABA) action. I compared these responses to those induced by several synthetic insecticides whose mode of action is well known to allow us to make a more precise prediction of how the 5 compounds affect the target insects. I then evaluated the effect of the 5 phytochemicals, and octopamine on the octopaminergic system of insects by comparing the production of a second messenger molecule, cAMP, after treatment. Some monoterpenoids interfere with the octopaminergic system by targeting the octopamine receptors. The acute toxicity observed in Ae. aegypti and P. sericata may be the collective result of these compounds on complex biological systems in the insect and may depend on their structure, concentration, or exposure time. The overall results indicate that plant-derived compounds directly and indirectly affect aspects of insect physiology and could possibly be developed as new insecticides.