In seasonal regions such as Maryland, nature goes through cycles, with some seasons reserved for growing and reproducing, and others for resting and waiting for conditions to get better. One of the groups of organisms that in my opinion represent clearly those changes in seasons are butterflies, which go through extreme modifications in their bodies and ecologies to closely match the changing seasons. Today’s post is going to explore two beautiful butterfly species that we can find right here in Maryland: the common Black Swallowtail and the beautiful but imperiled Baltimore Checkerspot.
First Things First: The Life Cycle of Butterflies
Before going into details about these butterflies, I think it is important to explain how these organisms develop because their life cycles are usually tightly related to our seasons. Caterpillars have indeed pretty special and fascinating life cycles. In these insects, a female lays eggs on the preferred host plant of the species. This way, the first larva (a tiny caterpillar) that emerges from that egg will not need to move far to feed on its favorite and most nutritious plant.
Once larvae hatch they start feeding on plant material, becoming bigger as they eat. Because insects such as butterflies are covered with a special hard ‘skin’ called an exoskeleton (this is really an external skeleton!) that gives them support and structure, every time the caterpillar gets too big, the exoskeleton becomes too tight (imagine a kid outgrowing a T-shirt). At that point, the caterpillar breaks the old exoskeleton and grows a new larger one in which it can fit.
While going through these ‘changes of skeleton’ (called molts), the caterpillar is able to grow until it is large enough to make their last change: pupation. At this stage, the very large caterpillar is ready to become an adult. For this, the caterpillar will molt a last time and become a pupa, which is the form that builds the cocoon in which the last body changes happen before the adult butterfly emerges.
As one can see, because the life cycle of butterflies has so many stages, there are many chances for things to go wrong during their development, which can also explain some annual fluctuations in butterfly populations. For example, if a wave of particularly cold or hot weather happened during one of the stages at which the caterpillars are sensitive (e.g., pupa, first instars), we may not see many butterflies later in the season. The same is true if there are important disease outbreaks, if predation was particularly high earlier in the season, if the host plants were not as abundant as other years, or if insecticides were applied close to some of the preferred host plants.
Now that we have a better idea of how the life cycle goes, let’s take a look at what our two species do and how they differ in their food preferences, life cycles, and how that affects how we can promote their presence in our surroundings.
Black Swallowtails
Even though it may seem obvious to some, let me start by saying that this species gets its name from the shape of its hind wings, which look like the pointy tails of swallows. Black Swallowtails (Papilio polyxenes) are common butterflies in our region and are present across the whole eastern USA.
In Maryland, this species has between two and three generations per year, with the first generation(s) of a season reaching adulthood within the season, and the last one spending the winter in pupal phase and emerging as an adult the following spring. (Check this other blog to learn more about butterflies in the winter: Where are all the pollinators?)
Like all butterflies, Black Swallowtails are specialized on what they feed. What defines what makes a plant yummy or not to the caterpillars are the chemical compounds the plant carries. In fact, plants have evolved to produce different chemical compounds that protect them against the multitude of herbivores that exist. Black Swallowtails in particular have evolved to tolerate the chemical compounds present in plants of the parsley family (Apiaceae). It is for this reason that these caterpillars can be found in your garden feeding on carrot, parsley, or dill leaves. Adults (butterflies) are usually seen collecting nectar from flowers such as clover, milkweed, and thistles.
Black Swallowtails are currently considered to be a species that is not particularly at extinction risk. However, to maintain their populations it is recommended that open grasslands with plants that serve as caterpillar hosts are present. Interestingly, because they are so common in our area, they can be easily reared indoors, something that is really fun and can be a great summer project for kids (and adults!).
Baltimore Checkerspot
If Maryland has a state dessert, I feel it’s only fair that it also has a state insect! The Maryland state insect is the exquisite Baltimore Checkerspot (Euphydryas phaeton), which was chosen because its colors remind us of those of the Maryland flag. In our area, Baltimore Checkerspots are not as common as Black Swallowtails, and, unlike Swallowtails, have only one generation per year.
Their diets are also significantly more specialized than those of the Swallowtails: young caterpillars feed exclusively on white turtlehead, on which their eggs are laid. While later on in their development they are able to feed at least partially on alternative plants, white turtleheads are required for them to survive the early caterpillar stages.
Finally, unlike most butterflies in our region which spend the winter as pupae, this species spends its winter as a caterpillar and pupates only in the spring. The caterpillars of this species are super cute and they always remind me of the Dust Bunnies of the movie “My friend Totoro”. Adults of this species feed on milkweeds, dogbanes, and wild blackberries.
Unfortunately, even though these beauties are our state insect, they are currently imperiled in our area. The reasons for this have to do with changes in land use, which led to less white turtleheads being available to the caterpillars both because less natural habitats are present and because the deer populations are so large that they eat most of the host plants!
If you would like to try to contribute to these butterflies’ populations, you can plant white turtleheads in your yard, but in particular support conservation actions already happening in Maryland, such as the Baltimore Checkerspot Recovery Team.
By Anahí Espíndola, Assistant Professor, Department of Entomology, University of Maryland, College Park. See more posts by Anahí.