Theo is a gardener tending to his plants. He makes sure to apply an organic insecticide to all of his plants except his coffee plants. Why does Theo skip his coffee plants

Coffee contains a natural insecticide known as xanthines.

Explanation:

Coffee contains xanthines, a purine base chemical that functions as a drug precursor in some medications and as an ingredient in the manufacture of pesticide.

Xanthines can be also be found naturally in other substances such as theobromine and theophylline.

Hence, Theo must have skipped his coffee during the application of organic insecticide to his plants due to the fact that coffee contains a substance that can naturally act as an insecticide.