Sunday, September 17, 2017

Walnut Sphinx Moth

Walnut Sphinx Moth - REK
Over the last several weeks I have photographed many different moths coming to our Bull Creek deck light.  My favorite wasn't one of the showy giant silk moths but this more humble walnut sphinx moth, Amorpha juglandis.  The picture above is its usual resting position that I had been seeing several every night.

The female moth emerges from its cocoon and spreads its wings to dry.  It also spreads its pheromone over the air to attract a male.  Once fertilized she will fly off to lay 200-300 eggs on an appropriate food plant.  Her caterpillars will eat the leaves of a variety of trees including butternut, hazelnut, hickory, hop-hornbeam, and of course, walnuts. Like the giant silk moths, they have no intestine and live less than a week.

 Walnut sphinx caterpillar with its horn -  MJ Hatfield CC
Like other moths, the caterpillars serve as a food source for lots of spiders, birds and insects.  A dramatic example is the walnut sphinx caterpillar parasitized below.  Chris Barnhart found it on a dried up leaf and we identified it by the color and its distinctive "horn" at the end of its abdomen.

Parasitoid cocoons on a dead walnut sphinx moth caterpillar - REK
The most likely parasitoid is a Braconidae. The female lays eggs on the caterpillar and the young burrow into the caterpillar which continues on with life, chewing leaves happily while the young grow in its body.  Eventually they make cocoons and the caterpillar's health fails.  They are careful to avoid killing the caterpillar until they are full grown and ready to emerge.
"Most life histories involve parasitizing hosts as diverse as aphids, bark beetles, and foliage-feeding caterpillars. Many species are egg-larval parasitoids, laying eggs within host eggs and then not developing until the host is in the larval stage. Unlike ichneumon wasps, many pupate in silken cocoons outside the body of the host and others spin cocoons entirely apart from the host.(3) Also unlike ichneumonid wasps, very few braconids use host pupae to complete their life cycles, except for fly parasitoids in Alysiinae and Opiinae." from Bugguide
Empty cocoons - click to enlarge
Unfortunately we found this one when they had all left their cocoons.  I kept it in a sealed box in case there was one left but had no luck, just like the poor caterpillar.

The sphinx moth itself is a cool creature which is able to whistle at a high pitch, out of our hearing range but heard by birds. We wrote a blog about this some years back.  The sound comes out of the most distal spiracle and I wonder if that may be why it holds its rear end up.  On the other hand, it may just be showing off.

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