The question posted in the Conservation file reminded me of a little experiment that I did this summer. Since a lot of Monarch caterpillars munching on milkweed in a pen or old aquarium produce a lot of frass (droppings), I wondered if this Monarch manure was of any value as a plant fertilizer. After all, look at worm castings and how useful they are.
I grow plants of the Gesneriad family, which includes the African-violet. I had a number of Sinningia seedlings of equal size and thought that they woud be good for this test. All seedlings were potted in a regular soil-less mix (Pro-mix BX) but half the seedlings has Monarch manure added to the mix. All other conditions and care were the same.
After a coupe weeks it became clear that instead of helping the plants by providing extra nutrient, the exact opposite was happening. The plants with Monarch manure became stunted and pale. I guess the chemicals that make the caterpillars distasteful also hinder the growth of other plants. When I repotted the suffering plants into the mix without manure, they recovered and began to grow normally.
I thought I would mention this in case anyone else gets the idea to sprinkle the frass on their houseplants.