Are Eastern Black Swallowtail caterpillars picky eaters? It depends.

During the pandemic summer of 2020, my mom and I found a couple of caterpillars in our garden. They ate a lot of the dill plants we had, but the season for the dill was ending. So we brought them into the house and fed them parsley instead. We had an abundance of parsley in the garden and I read that the EBS caterpillars will also eat parsley. Unfortunately, that was not the case, I found that the caterpillars will travel towards the dried dill plants, rather than to eat the fresh parsley available. In fact, one caterpillar had somehow “mummified”, himself on the stalk of the parsley and died because he refused to eat. We went to the grocery store for some dill in order to save the other caterpillar. When the dill was offered, the caterpillar ate it ravenously. But that was not the end of our story.

About a week later, we found no less than 20 newly laid caterpillar eggs on the single dill plant remaining in the garden. I knew that if we didn’t “convert” them to parsley plant feeders, they were doomed to starvation.

The Experiment: Will EBS caterpillars found on dill eat parsley?

We knew that the caterpillars will not eat another plant type once they were larva. So I began to work to move the eggs to the parsley cuttings from our garden, this way they will know only parsley when they emerge. Getting the eggs off the dill leaves was difficult as they were stuck on tightly. I worked to just lay the eggs on the leaves without removing the dill leaf that the egg was laid on. This was very time consuming because each egg was stuck to a dill leaf the size of a needle. The dill leaves dried out quickly and left eggs that sat on fresh parsley leaves. About three days after moving the eggs to the parsley leaves, the eggs developed a small brown line. Later the eggs turned black.

I was very excited to see what was going to happen since I knew that the newly born caterpillars will need to eat the parsley to survive. The first of the caterpillars emerged from their shell, and they ate their shell immediately after coming out from it.

After eating their shells, the little caterpillars travel away from the shell and proceeds to explore the leaf. After a long wait, the larva ate some of the parsley leaf. This repeatedly happened with every one of the larvae that came out of their shells.

ANALYSIS: Sadly, some of the caterpillars that were emerged onto the dill plants did not survive when they were placed onto parsley plants. The caterpillar that hatched on a dill plant, when placed onto a parsley plant will travel around the container searching for dill to eat. They even ate the dill stem that was dried up and was not fresh. The same is true for the reverse situation where I had caterpillars hatched on parsley plants. Those “parsley” caterpillars refused the dill and moved to search for parsley.

CONCLUSION: The caterpillar’s first taste of a plant is the food of choice.

The Eastern Black Swallowtail caterpillars will eat parsley even if their eggs were laid onto dill plants. It seems the caterpillars will eat plant leaves only if that is the first taste of the plant. I think the taste of the plant leaf is imprinted into their system and they will refuse plants that they are unfamiliar with. I guess that the Eastern Black Swallowtail butterfly laid its eggs only on plants in which it has deemed safe for the larvae to eat. Although there may be other plants that caterpillars can eat, and because it does not identify those plants as a food source, they will reject the edible plants.

THE JOURNEY CONTINUES… Read on the next blog as I follow the development of the Eastern Black Swallowtail caterpillars!

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