Swallowtail eggs are easy to overlook because they are tiny, usually under 2 millimeters across, and blended against the leaves where they are laid. But once you know what to look for, finding them becomes one of the more satisfying parts of raising swallowtails at home. Spot a clutch of eggs on your parsley or dill, and you know you have swallowtails visiting your garden.
This guide covers what the eggs look like, where females lay them, how long they take to hatch, and what to do if you want to raise them indoors through to adulthood.
Key Takeaways
- Swallowtail eggs are small, round, and typically pale yellow to yellow-green, turning darker as the embryo develops inside.
- Females lay eggs singly on the leaves of specific host plants rather than in clusters, so you usually find one egg per leaf.
- Hatching takes 4 to 10 days depending on temperature, with warmer conditions speeding up development.
- Eggs raised indoors need humidity, the correct host plant, and protection from ants and parasitic wasps.
What Swallowtail Eggs Look Like
Fresh swallowtail eggs are spherical and about 1 to 2 millimeters in diameter, roughly the size of a pinhead. They start out a pale yellow or creamy white color that gives them a slightly translucent quality. In good light you can sometimes see the interior of the egg through the shell, called the chorion.
As the embryo develops over the following days, the egg changes color. A healthy egg typically deepens to a darker yellow or develops a grayish tint as the caterpillar forms inside. Just before hatching, you may notice a dark spot near the top of the egg, which is the head of the caterpillar visible through the thinning shell.
An egg that has turned completely black or shriveled has usually either been parasitized by a tiny wasp or dried out and died. Parasitized eggs sometimes show a tiny exit hole where the adult parasitoid wasp emerged. Knowing the difference between a healthy darkening egg and a dead one comes with a bit of experience but is not hard to learn.
Different swallowtail species lay eggs that look subtly different. Black swallowtail eggs tend to be very smooth and pale cream at first. Giant swallowtail eggs are similarly round but may have a slightly more yellowish color. Spicebush swallowtail eggs are often found on the underside of leaves and have a characteristic greenish-yellow color. Check out this overview of swallowtail butterfly species for help identifying which species is using your yard.
Where to Find Them
Swallowtail females are very specific about where they lay. Each species has a set of host plants it uses, and females will fly past perfectly good-looking leaves to find the right species. They locate host plants using chemoreceptors on their feet, tasting the leaf surface as they walk across it before deciding to deposit an egg.
Black swallowtails lay on plants in the carrot family, which includes parsley, dill, fennel, and Queen Anne’s lace. If you grow any of these in your garden, check the upper surface of younger, tender leaves. Females tend to prefer leaves that are not the very newest growth but are not yet tough and old either.
Spicebush swallowtails lay on spicebush and sassafras, placing eggs on the underside of leaves. Giant swallowtails prefer citrus trees, prickly ash, and Hercules’ club. Tiger swallowtails use wild cherry, tulip poplar, and sweetbay magnolia, often laying on leaves in dappled shade along forest edges. A thorough guide to caterpillar host plants can help you figure out which plants to grow to attract specific species.
Looking for eggs is easiest in the morning when light comes in at a low angle and makes the reflective surface of eggs visible. A 10x hand lens or loupe makes a big difference. Scan leaf surfaces systematically, starting with younger growth where females most often choose to lay.
How Long Until They Hatch
Temperature is the primary driver of how long incubation takes. In warm summer conditions, around 80 to 85 degrees Fahrenheit, swallowtail eggs typically hatch in 4 to 6 days. In cooler spring or fall weather, the same eggs may take 8 to 10 days. Extremely cool conditions can extend development even further without harming the embryo as long as temperatures stay above freezing.
The newly hatched caterpillar, called a first instar larva, eats the eggshell immediately after emerging. This is normal behavior and provides the caterpillar with nutrients for its first hours of life. After consuming the shell, it moves to nearby leaf tissue and begins feeding in earnest.
If you are watching eggs closely, the timing cues are useful. By day three or four in warm weather, the egg should look noticeably darker than when it was first laid. If an egg looks exactly the same color it was on day one by day six or seven, it has probably failed to develop and will not hatch.
Caring for Swallowtail Eggs Indoors
Bringing eggs indoors protects them from parasitic wasps, predatory spiders, ants, and birds, all of which take a significant toll on outdoor eggs. In the wild, only a small fraction of laid eggs survive to adulthood. Indoor rearing dramatically improves those odds and gives you a front-row seat to the whole life cycle.
The easiest approach is to bring the leaf the egg is on indoors rather than trying to move the egg itself. Swallowtail eggs are sticky and adhere to the leaf surface, but they are fragile enough that prying them off risks damage. Place the leaf in a small container with the stem end in water to keep it fresh, or lay it flat in a plastic container lined with a damp paper towel to maintain humidity.
Keep the container in a warm room, around 75 to 80 degrees Fahrenheit, and out of direct sunlight, which can overheat the interior quickly. Check on the eggs once or twice a day without disturbing them. If condensation is beading on the inside of the container, you have enough humidity. If the leaf is drying out and curling, add a small amount of water to the towel.
Once the egg hatches, the tiny caterpillar needs fresh host plant immediately. Have a supply of the appropriate plant ready before hatching day. The first instar caterpillar is extremely small, a few millimeters long, and will not survive more than a day or two without food. From there, the rearing process shifts to feeding the caterpillar through its five growth stages until it eventually forms a chrysalis.
Frequently Asked Questions
How many eggs does a swallowtail butterfly lay?
A female swallowtail can lay between 100 and 200 eggs over her adult lifetime, though not all in one session. She distributes them across many plants and many days, laying singly rather than in batches. This dispersal strategy reduces the risk of all her eggs being destroyed by a single predator or parasite.
Why do swallowtails lay eggs one at a time?
Laying singly is a survival strategy. If a female laid 20 eggs on one leaf and a caterpillar predator found that leaf, it could wipe out the entire clutch. By spreading eggs across many plants, she hedges her bets. It also prevents competition for food between siblings once the caterpillars hatch and begin feeding.
Can I move a swallowtail egg to a different leaf?
You can, but it requires care. Slide a thin piece of plastic or a fingernail gently under the egg and lift slowly. The egg will usually come free with a tiny amount of leaf tissue attached to the bottom. Place it on a fresh leaf of the same plant species and it will adhere again without any glue or tools required.
What does a parasitized swallowtail egg look like?
Parasitized eggs often turn a dull brownish-black color and may shrivel slightly. If the parasitoid wasp has already emerged, you will see a tiny, irregular exit hole in the top or side of the egg. Healthy eggs that are darkening due to normal embryo development look more uniformly dark rather than shriveled or punctured.
Do swallowtail eggs need sunlight to hatch?
No, direct sunlight is not necessary and can actually be harmful by overheating the container. The embryo develops based on temperature, not light exposure. A consistently warm room temperature of around 75 to 80 degrees Fahrenheit is sufficient. Indirect bright light is fine and will not interfere with normal development.
