I am a well experienced gardener and farmer with a growing interest in alternative methods of using herbal plants instead of chemicals for pest management and have came across a notoriously bad rep plant called common ragwort tansy. I have done a lot of research into this plant and have not found any information on the plant used as a mild pesticide spray for common insects like cabbage moth caterpillars and aphids. My question arose from observing that no insects or animals go near it due to its unpleasant smell and poisonous alkaloids. I am wondering if a person can use such a plant for such purposes?
You can use it as a pesticide since it has alkaloids just like you can use plants with nicotine. But I would advise against it because you do not know if you or other animals accidentally ingest it. Livestock can get liver damage from ingestion a lots of it and there is no known cure for poisioning.