As soon as I see the first Black Swallowtail butterfly flitting around the yard I begin to check the fennel plant for eggs.
Without my reading glasses on, I would never spy the rounded yellow eggs on the thin fronds of fennel. Dill, Parsley, Rue, and Fennel are a few of my garden plants that are host plants for Black Swallowtails.
When the eggs hatch, and I find small caterpillars on the plants, I bring them onto the back porch for safekeeping. You might wonder why I don’t leave them on the plant and let nature take its course. The answer is the predator bugs that share the same leafy fennel plant. Ladybug larva coexist and quickly eat anything else they find on the plant. Praying mantis babies also roam the leafy green fronds. Small spiders are a threat to newly hatched caterpillars. I know from experience, if I don’t remove the caterpillars when I first see them, the next day the numbers will be greatly diminished, even to the point of none to be found.
Before I gather the caterpillars, I always have vases of water ready to hold the host plant of fennel. Most importantly, I cover the top of the vase with some sort of barrier to keep the small cats from drowning, Unfortunately, I know from experience, they often wander into the water if the opening is left uncovered. I use garden cloth and a single rubber band. A small hole snipped in the center allows me to insert the fennel. At the start of their feeding, one frond of fennel will be enough. They will soon need several stems a day as they progress through their instars: a period of time between the caterpillar molting.
To water the fennel without removing the covering, I use a spouted water bottle that easily adds a few inches of water. The fennel is a very thirsty plant. It will stay fresh for days if the water level in the vase is kept near the top.
I never pick the small caterpillars up with my fingers when transporting them to the vased fennel. They are extremely small and easily crushed. Instead, I pull the strand of fennel they are munching on away from the plant and place it amongst the fronds of vased fennel on the porch. I check back several times a day to make sure the caterpillar has transferred to the fresher fronds.
The caterpillars will go through several instar phases on this first piece of fennel. When the fennel begins to get dry or is eaten away by the caterpillars, I fill a second, sometimes also a third vase, and place it close beside the first so that the fennel mixes into the first bunch. As they eat, and the first fennel dries up, the caterpillars move onto the fresher plants.
A great resource for Black Swallowtail Butterflies can be found on the site: Butterfly Fun Facts.