As I was planting some veggies yesterday, I saw a female Black Swallowtail butterfly in her characteristic search and hover mode as she investigated various plants in the garden. I knew from this fluttery flight behavior that she was searching for the right type of plant on which to lay an egg (aka host plant). As with many species of butterflies and moths, Black Swallowtail females tend to be discerning when it comes to which plants they choose for their eggs. Host plants of the caterpillar include members of the parsley family (Apiaceae) including Carrot, Parsley, Dill, Fennel and Queen Anne’s lace and some members of the Rutaceae such as common garden Rue (Ruta graveolens).
I have four of these in the garden right now: Parsley, Bronze Fennel, Sweet Fennel, and Rue. From my experience, the Bronze Fennel seems to be the preferred host, especially early in the season. The tops of the Carrots I grew last year were also very popular with the caterpillars. As any herb gardener knows all too well, Parsley is also immensely popular as a host plant, with the larvae often totally denuding your herb supply if you only have a few plants. That is one reason I plant the fennels as they tend to get tall (3 or 4 feet) which is usually enough to provide an adequate food supply. Rue becomes especially important as a host plant in my garden in the late summer and early fall as Black Swallowtails complete their final generation before winter. Last year I had one large rue plant with over 20 caterpillars on it and rue tends to be less completely devoured compared to some of the other hosts. Rue stems also tend to sprout quickly after being eaten. But I have also read that Rue can cause skin irritation in sensitive people, so be cautious if you plant it.
Naturally, I was excited to see what I thought were the first butterfly eggs of the season in the garden. I grabbed my camera and took a few shots after finding eggs on several fennel and parsley plants. The eggs are spherical and cream-colored (or slightly yellowish). The other swallowtail species eggs I have seen are also spherical although different species tend to have different colors. The eggs of other butterfly and moth groups can be quite ornate with many shapes, colors, and ornamentations (perhaps a blog topic later this season). Each butterfly egg is surrounded by a hard outer shell, called the chorion, to protect the developing larva. The shell is lined with a layer of wax, which helps keep the egg from drying out. There is a small opening near one end called a micropyle, which allows sperm to enter the egg for fertilization. The egg shell also is dotted with microscopic pores called aeropyles which allow gas exchange. The butterfly glues the egg to the plant leaf using an adhesive-like substance produced in the colleterial glands. Black Swallowtails lay the eggs singly (generally on the top of leaves) although she may lay several eggs on the same plant.
As I wandered the garden looking for more eggs, I realized the one I photographed is not the first of the season…I found a tiny caterpillar on one of the Rue plants. Since it takes 3-5 days for these eggs to hatch, I had apparently missed a few from the past weekend.
This is what is known as the first instar larva, the stage after emergence from the egg. It looks quite different from how this species is pictured in most caterpillar field guides. As is common with many species, Black Swallowtail caterpillars undergo a noticeable change in appearance as they molt five times on their way to becoming a chrysalis. This early stage is considered a bird poop mimic, with a dark background color containing a whitish splotch, just like a bird dropping. Many other species have this basic color scheme, especially as early stage larvae.
I’ll hang onto this little guy and try to photograph it as it develops over the next couple of weeks. I think I’ll have plenty of opportunity to get the various life stages as I found 11 eggs (and two more first instar larvae) on one Bronze Fennel plant this morning. While searching the Internet for a few details on these eggs, I found what could be my moneymaker in retirement – seem like a few companies sell butterfly eggs for people to raise and I saw one site that had Black Swallowtail eggs for $2 each! If I had the time and inclination, it could be a busy (and profitable) summer.