Black Swallowtail Butterfly Larvae on Dill

If you have ever found caterpillars on your dill plants in your garden they are almost certainly the Black Swallowtail Butterfly caterpillars like the ones seen in these pictures.  My son found these on his dill. 

Black Swallowtail Butterflies lay their eggs exclusively on plants in the carrot family.  In the wild in this area that means Golden Alexander plants, and, south of Minneapolis, Queen Ann's Lace.  In your garden it means dill, parsley, and carrot.


The caterpillars eat the entire dill plant, right down to the ground.

 Butterfly and moth catterpillars all grow through what are called 5 "instars" or growth stages.  With each new instar the caterpillar sheds its skin and is able to grow larger.

In each instar the larvae usually look a bit different, or maybe a lot different.

This picture is of the 3rd instar for the Black Swallowtail caterpillar.  It is preparing to shed its skin here.  It quietly sits still on the plant stalk for several hours, not its normal behavior.

Notice all the spines and tubercules on the caterpillar's back.  These will get smaller with each instar until they are not present in the 5th or last instar.

Also notice the white 'saddle' in the middle of the back in this picture.  This will disappear in the next instar, as can be seen in the picture below.

Notice also the silk threads the caterpillar has woven all around the stem so the old skin will stay attached as it splits and the 'new' caterpillar moves forward.

 Here the caterpillar above has just shed its skin.  Basically it just crawled out of it during the night time.  The skin fell off the stem before I could reposition the camera for a better angle.

Here is the caterpillar in the 5th and final instar.  All the spines and tubercules are gone now, and the caterpillar uses its colors to blend in and not be seen by predators.

The caterpillar is about 3 inches long here, 3 times the length it was in the 3rd instar, which is the 2nd picture above.



It is pretty easy to see the difference between the six 'true' legs up by the head and the 10 'pro-legs' or 'false' legs that protrude from the caterpillar's abdoman.

These false legs are like little velcro pads that stick to plant surfaces very well, enabling the caterpillar to use the true legs to help grab and pull food towards its mouth.

 Here you can see the sharp edged 'jaws' pulling in the dill flower, along with a lot of pollen that has stuck to its face from all the other flowers it has eaten in the previous day.

I don't think these caterpillars help with fertilization of the flowers since they completely eat every flower, and stem, they can find.

Notice the aphid trying to escape at the bottom of the flower stem.  I didn't notice the aphids until after I was looking at the pictures on the computer. 

 It soon became obvious that every flower had its group of aphids that were soon to be deprived of their home and food as the caterpiller ate it all. They were very tiny, and trying to figure out where to go. 

The caterpillars seemed bothered by  the aphids when they crawled onto their bodies, especially when the caterpillars were little.  But I never saw one get eaten, even if it was on  the flower as the caterpillar ate the flower.

 All swallowtail butterflies have a brightly colored scent gland that is flashed out and emits a strong odor when danger threatens.  Black Swallowtail caterpillars are no exception.  But these caterpillars did not seem  threatened by me poking them, so this is the best photo I was able to get.  The orange gland is only about half way out here, and the air was filled with a strong dill/citrus odor.

At about 2 weeks of age the caterpillars start to wander, and after doing this for 12 hours or so, they settle on a plant stem to their liking, and attach themselves with a silk thread as can be seen below.  Now it is ready to transform into a chrysalis, which will appear in an upcoming post.