But you can observe that pyrethrins are used on large scale as food.

No, you can not. Pyrethrins are basically insoluble in water and people don't eat chrysanthemums from their tea.

I'll take that thousand year observational study

Be careful about that. For example,

The species Aristolochia clematitis was highly regarded as a medicinal plant since the ancient Egyptians, Greeks and Romans, and on to until the Early Modern era; it also plays a role in traditional Chinese medicine.

You actually have more than a thousand years of observational studies, and across different cultures, too. There is only one little problem -- "Aristolochia has been shown to be both a potent carcinogen and kidney toxin."