Monday, October 28, 2013

Even Hornworms Get the Blues

Toxin free hornworm - Picture by  Addicted2hymenoptera
"You are what you eat."  - Victor Lindlahr

Most people have seen tobacco or tomato hornworms, a fat bright green caterpillar with a horn decorating its rear.  Given the right diet they can be a bright blue.  This is about the how and why.

Why are hornworms green in the first place?  Their color comes from what they eat, but surprisingly it has nothing to do with chlorophyll they consume.   Chlorophyll is completely metabolized, leaving no trace of pigment.  When fall comes, the chlorophyll of plants breaks down, leaving another pigment called xanthophyll which gets less press.  It is a carotenoid that gives carrots and fall leaves their color.  It can even give you carotenemia, an orange color to your skin and especially the palms of your hands if you eat excessive amounts over a long period of time.

Unlike our red blood cells carrying hemoglobin, many invertebrates, including insects and crustaceans, transport oxygen in their blood with a chemical called hemocyanin.  It is copper based and thus blue.  If you add the yellow of xanthophyll, as you will recall from grade school art class, blue + yellow produces green!

Typical hornworm, Manduca sexta
Now why are there blue hornworms?  It has to do with a commercial market.  Some time back, biologists discovered that feeding a non tomato/tobacco diet could produce blue hornworms.  Leave it to business to find a market for hornworms raised on special diets.

There is an industry of exotic pets such as geckos and bearded dragons that need live food.  The hornworm feeds on tomato and tobacco plants which are members of the Solanaceae family which produce toxic alkaloid-type nicotine related chemicals that protect them from most leaf eating species.  The hornworms, Manduca sexta, gradually adapted over time to live with the plant, eating them voraciously - Manduca is Latin for "glutton."  They selectively sequester and secret the nicotine, protecting them from the toxin.

To avoid the toxins, and because it is easier to use, there is now commercially available horn worm food which is free of the toxins found in their normal Solanaceae diet.  Truly a product that you have been waiting for.  Now we need studies to see if birds will avoid the blue hornworms.

For an overview on Manduca sexta, see this site.
Tobacco or tomato hornworm?  See this August blog.

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