Most garlic oil repellents, like Garlic Barrier or Guardian Spray, seem to work best against sucking pests such as aphids and whitefly. Although these oils look and smell anything but palatable, some have no odor after the initial application. It is important to stop aphids before they attack the plant because once they feed, the plant may contract a virus from the aphids. As either a repellent or an insecticide for sucking pests, horticultural oil represents a good choice. It does not persist in the environment long after application and so conserves the beneficial insects available in your garden to perform biological control.
Many repellents perform well against chewing insects.
While more than 20 insecticidal compounds have been isolated from raw neem oil, the predominant compound in neem products is azadirachtin. Azadirachtin works in two ways. It is a growth regulator acting to disrupt hormone levels in insects, and it is a very potent insect repellent. Although adult pests such as flea beetles will only be repelled by neem and not affected by its growth regulator action, neem may be mixed with other compounds, such as pyrethrum, to extend its punch and persistence as a plant protection agent. Remember that although neem has limited toxicity to mammals, it can pose a threat to certain beneficial insects, such as bees and parasitic wasps.