(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
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Coming Up 203
Above: Eastern black swallowtail
caterpillar on fennel. Below: The adult butterfly just emerged from
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. -
Continue this conversation on our
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
- 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
- Some such as the monarch migrate
- Eastern black swallowtails use still
another strategy, waiting out the frozen months as pupae in their
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
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
...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.
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.
firstname.lastname@example.org. 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,
- 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
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