Banking on caterpillars for next year

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What fun to watch a tiger swallowtail nectar on a flower! Beware: After the first look, it's a slippery slope to becoming very involved with these winged works of art. In this issue of What's Coming Up, we enjoy and learn from a reader's attachment to a hatch of caterpillars. Where is a butterfly before we see it on our butterfly bush? What happens to it at the end of a year? How can we attract more to our garden? 

(Third week of September)

Dear Janet & Steven, Thank you for telling me that fennel is a nice host plant for black swallowtail butterflies. I planted a small plant in July, and within a few weeks I had approximately 10 eggs on the plant. All the eggs hatched, and all the caterpillars made it to the point of leaving the host plant.

Here's my problem. The fennel seems to work too well. A couple of days ago, we stopped by a nursery to see if there were any plants that we could rescue. I talked to them about my success with black swallowtail caterpillars, and they gave me three leftover fennel plants that they were going to throw away. Before I could even get them into the ground, a black swallowtail flew up to the plant and started laying eggs on it. Do these eggs have a chance to hatch, or is it too late in the year?

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Above: Eastern black swallowtail caterpillar on fennel. Below: The adult butterfly just emerged from the chrysalis.BlkSwllwtlEmerg.jpg

If they do hatch, should I plant the fennel in a container and bring it in and out of my garage when the nights are cold, or should I just plant the fennel in the ground and let nature take its course?

I also have a couple of small black swallowtail caterpillars on my original fennel plant (they are about 10 days old). I haven't been able to find out if small caterpillars can survive really cold nights. Once again I'm not sure whether I should bring the little guys in at night or just leave them out on the fennel plants and let nature take its course.

Any help would be greatly appreciated. There could be quite a few caterpillar lives depending on me, and I'm not quite sure I'm ready for all this responsibility.  - G. -

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Not The End for insects

The end of the growing season is not The End for insects, although for many it requires a change in state.

  • Some butterfly species hunker down in plant debris to weather winter as a caterpillar (you've probably seen wooly bear moth caterpillars do this),
  • Some ride it out as eggs (you can reduce a gypsy moth or tent caterpillar problem by destroying the egg masses in fall).
  • A few butterflies and moths winter as adults (big, dark blue and brown mourning cloaks hole up in loose tree bark to fly the next spring as the north's earliest big, bold butterfly).
  • Some such as the monarch migrate south.
  • Eastern black swallowtails use still another strategy, waiting out the frozen months as pupae in their chrysalises.

It's possible for late season black swallowtail eggs to progress through caterpillar-ness to their overwintering state, provided they have enough warm days. As caterpillars they can survive cool nights -- but not freezing.

By day they need to eat and metabolize what they ingest to develop from egg to a size large enough to pupate. That requires temperatures above about 55 degrees F for a couple hundred hours -- about 18 daytimes in summer when it's warm, or perhaps four weeks in fall.

Your caterpillars might beat the 18-day minimum if you raise them in a warm house with grow lights turned on 24-7.


Above: It's remarkably easy to have the thrill of fostering a caterpillar and seeing it emerge as a butterfly or moth. The black swallowtail toward the top of this page made its debut as a butterfly in our house.

Most important is knowing what the caterpillar can eat. Dill (pictured above) is a host plant for eastern black swallowtail.

This species can also eat dill relatives including fennel (the plant with the caterpillar at the top of this page), parsley, carrot, rue and queen Anne's lace. We've raised many, keeping a caterpillar on host plant stems in a vase in our kitchen. When one of our pet caterpillars finished growing, it would migrate to the twigs we offered, pupate within its chrysalis, and rejoin us after about 12 days.

Always some eggs lost...

Adult butterflies don't know how many warm days are left. They just keep laying until the frost kills them. So there are always some eggs and caterpillars that don't make it.

...but cold hardy chrysalises endure

In the chrysalis these insects are quite cold hardy. However, the chrysalis is subject to physical damage such as crushing and also predation (birds, mice). Thus to support this species through winter, be less hasty about cutting everything down in fall and tolerate some winter mess. On those "messy" plant stems are next year's first butterflies.

Below: Each year's final generation of eastern black swallowtail caterpillars overwinters in chrysalises. Here's one we spotted in a shrub near some of their larval food plants (in our garden, we find the caterpillars mostly on annual dill and perennial rue and fennel -- Anethum graveolens, Ruta graveolens and Foeniculum vulgare). We also find chrysalises on ornamental grasses and sturdy perennial stalks near the larval food plants.


May be kept at the back of the 'fridge

You've already given your neighborhood black swallowtails a boost. Your guardianship of the first ten gave helped them beat the odds -- infant mortality's pretty high for insects! That boosted the local population going into winter. However, if you're keen to go above and beyond you can bring the host plants indoors to let this last brood keep eating, ride herd on them as they progress to pupae (they're wanderers), and finally cool those chrysalises down to 40F. Store them in the refrigerator and set them into the garden in April, or put them outdoors in a place mice or birds may not find them.

Below: Alex Grady is already a caterpillar pro, helping other kids discover the black swallowtail caterpillars on this fennel at our Detroit Zoo Adopt-a-Garden


Below: Every year we learn new caterpillars. This year, it's this neighbor moth caterpillar. Hard to imagine it of this soft, 2-inch critter, the neighbor moth caterpillar, yet it will overwinter in this form, then pupate in spring.

NeighbrCatr3883s.jpg NeighbrCatrp3880s.jpg

Resources to learn by

We use a number of books to find information like this. Gaps are common -- for instance, some species' listings may lack larval food plants or overwintering strategies. We often check several to piece together the whole story. The Audubon Society Guide to Eastern North American Butterflies has good life cycle info, usually including overwintering stage.

Another great resource is Brenda Dziedzic of Garden City, founder of the Southeast Michigan Butterfly Association. Her book, Learn About Butterflies in the Garden, is a marvelously illustrated one-stop resource. You can buy a copy online or from Brenda herself. She's acquired a mind-blowing amount of experience about butterflies including raising them, keeping the chrysalises and overwintering caterpillars overwinter, and she loves to share.

We had never seen a neighbor moth caterpillar until this year. Like many people, we want to have more butterflies. So we welcome them one species at a time, as we are doing with this new find, "the neighbor:"

  • First we identify the species. In this case, Peterson's Guide to Eastern North American Caterpillars helped us identify it as neighbor moth, Haploa contigua. Its adult form is small, boldly patterned, white and black.
  • Second, we learn its larval food (host plants; what its caterpillars eat). Neighbor eats many different plants, but probably prefers aster family.
  • Third, we protect its habitat, including banning all pesticide use in the area and preserving the winter habitat.

So far we have done this for more than a dozen butterfly and moth species in our own yard. We hope to keep adding more all our lives.