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Why Black Swallowtail Caterpillars Are a Gardener's Friend

Good v Bad Bugs

So, if you are new to gardening you may be having a difficult time separating the good and the bad bugs that are found in your garden. And it's no wonder considering the thousands of different insects that can be found in a garden. In order to determine if an insect is good for the garden are not there are a few things to keep in mind. However, you should understand that everything needs to eat in order to survive and this does include the good guys.

Bad insects can do one or more of the following:

  • cause damage to plants that can decrease harvest yield or kill the plant
  • cause harm to the gardener
  • kill the wanted bugs

Good insects can do one or more of the following:

  • may cause minimal damage to some plants
  • eat the unwanted bugs
  • pollinate

Examples of bad insects

As this article is mainly about the swallowtail caterpillar too much detail will not be spent here. Some of the most commonly found insects that are detrimental to a garden in North America are:

  • tomato hornworm
  • squash vine borer
  • aphids
  • scale
These are a very found black swallowtail caterpillars. They were so small my camera had a hard time focusing on them.

These are a very found black swallowtail caterpillars. They were so small my camera had a hard time focusing on them.

Are there neutral insects?

You may be wondering if there are insects that can run down the middle of the road and the answer would be yes. These insects may help or harm the garden.

  • Praying Mantis will often eat unwanted insects, like aphids, but they will eat pollinators as they grow larger
  • Ants can act as pollinators but some species have been known to farm aphids and they can be aggressive towards the gardener
  • Spiders are well known for their ability to rid a garden of insects, however they will eat some of the good insects as well

Examples of the good guys

While not a comprehensive list of all the good bugs that can be found in a garden this a good place for a beginner to start.

  • Ladybugs are known for their intense appetite for aphids
  • Bees, as well as wasps and fellow jackets, are fantastic pollinators
  • Caterpillars may have a large appetite for leaves the plant should be able to bounce back and when the caterpillar grows into a butterfly it will return to pollinate flowers
  • Worms, not hornworms but rather earthworms. These little guys are fantastic for maintaining high quality soil.

The black swallowtail caterpillar

Now that we have an understanding of the insect world in the garden let's look into my favorite garden critter: the black swallowtail caterpillar. Caterpillars are well known for their extreme transformation, the juvenile stage is a grub like creature with many legs and an intense appetite but the adult stage is a delicate winged insect that feeds off nectar.

The monarch has become well known for the fact that the caterpillars will only eat the milkweed plant. However they are not the only spices that are host specific. The swallow tail caterpillar will only eat certain plants, though they do have more options than the poor monarch.

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Black swallowtail butterflies will lay their eggs on only certain plants, and these plants are the ones the little caterpillars will feed on until they become butterflies. Black swallowtail caterpillars can be found on:

  • carrots
  • parsley
  • dill
  • Queen Anne's lace
  • fennel
  • rue

I'm sure you noticed that several of these plants are very common in gardens.

How much will they eat?

So, I've told you now that black swallowtail caterpillars are good insects to have in the garden and I've told you that they are going to eat your plants. I'm sure you are wondering how much they are going to eat. Well, these little guys do have a big appetite, however they won't eat you out of house and home unless you have a lot of caterpillars.

The first time I had a caterpillar I found him in my parsley, happily eating. He ate an entire parsley plant, it was a small plant but it was still the entire plant. Ever since I have planted extra parsley just for the caterpillars. This summer I planted a rather large amount of parsley. I've had two batches of caterpillars. The first was only two, the second was ten. You see, as the summer season goes on the butterflies live out their life cycle and will come and lay eggs. These eggs hatch and the cycle continues. It is possible that before the first frost I may have another few batches of caterpillars.

My parsley grows fast. Fast enough that there is enough to attract the butterflies to come lay more eggs. Some gardeners might be annoyed by how much the caterpillars eat, especially if they have a lot of caterpillars, but I like to think of it as a small price to pay for having the butterflies pollinate the flowers.

If you are worried about the number of caterpillars in an individual area you can move them. However, they will only eat the plant species previously mentioned and if they go too long between meals they will die. But if you notice that there are a lot of caterpillars eating the carrot greens and none on your parsley or dill you can move a couple to help share the load.

How long will they be around?

The eggs can hatch between 4-10 days once they have been deposited on a host plant. Then the caterpillar will eat from the host plant and grow for 3-4 weeks. The pupal stage, where the caterpillar has form a chrysalis and is undergoing a metamorphous, can last 10-20 days. It is important to note that the black swallowtail will overwinter in the pupal stage. Once they have emerged as a butterfly they typically live for 6-14 days.

Butterfly nettering

If you have kids, want to protect your caterpillars, or you are just fascinated by the process and want to actually see the adult butterflies you may want to put up some netting around the plants. If the plant is potted then you can get a butterfly tent and simply put the entire pot and plant inside. If you have a caterpillar collection on a raised bed or plant in the ground you can make your own tent with some hoops and netting, like I did.

I had the hoops from when I started my garden a little early and needed to protect the young plants from frost. I had a set of adjustable hoops for my raised bed and used garden plastic to make a small greenhouse. In the summer I use the same hoops and clips to hold up netting over the plants once I have caterpillars. It is really neat to be able to see the butterflies inside the netting when they emerge, especially since without the netting they would have flown away before I could see them.

I like that I am able to see the beautiful adult butterflies and that I am able to protect the little caterpillars from predation. After the butterflies have emerged I remove the netting as more butterflies will come and lay more eggs until the end of the growing season.

In the end

By allowing the little caterpillars to live and eat in your garden you'll be rewarded by the adult butterfly. The black swallowtail butterflies are rather large and beautiful to see. They will feed from various flowers in the garden, which will pollinate these plants and help them continue their lifecycle.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

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