Garlic-derived diallyl polysulfides: Biochemical mode of action and application for green insecticides

Garlic (Allium sativum) has been used for thousands of years to treat various ailments, due to its bactericidal and fungicidal effects. These protective effects are associated with the presence of organosulfur compounds. Crushing of garlic leads to the production of thiosulfinates such as allicin. Allicin can decompose into diallyl polysulfides and other constituents. The polysulfides can react with glutathione, leading to the generation of reactive oxygen species (ROS) such as hydrogen peroxide. The ROS-generating properties of garlic constituent compounds suggest that it also might have insecticidal effects, and hence might be used to control serious pests such as the cabbage root fly, Deliaradicum, which continues to be a significant problem for crops with the genus Brassica. New methods for controlling such pests are especially important because current insecticides can cause toxic effects on non-target organisms, one example is chlorpyrifos, which was banned for residential use in 2000. In this thesis Drosophila melanogaster was used as a model system to test the insecticidal effects of garlic and garlic derivatives. Insecticidal experiments were performed at all life stages from egg to adults. The garlic products tested were garlic extract, Mexican garlic oil, and China garlic oil, with concentrations of 0 to 5%. Both the garlic extract and garlic oil had concentration- and age-dependent insecticidal effects. The garlic oils were more effective than the garlic extract, due to the presence of greater concentrations of diallyl polysulfides. Eggs and adults were most susceptible to the effects of garlic solutions and these stages are also expected to be easier to target in the field. The pupae were the least susceptible life stage. Biochemical studies showed that diallyl polysulfides lowered glutathione and cysteine levels whilst increasing reactive oxygen species, thus, causing an increase in oxidative stress. Using redox sensitive green fluorescent protein (roGFP) transgenic Drosophila demonstrated that garlic oil caused changes in redox couples, thus adversely affecting redox homeostasis. The results are important as they show that botanical insecticides have significant potential for use as alternatives to ecologically harmful insecticides that have traditionally been used in crop protection