It's a good year for tomatoes, and a big year for hornworms. These caterpillars fatten on tomato leaves so they can become huge Carolina sphinx moths (Manduca sexta). As fantastic as they are, they are very hard to find. Even when I see lots of tomato leaves missing, I often can't find the culprit because they are so well camouflaged. In a fleeting moment during which I considered that I was smarter than a hornworm, I decided to use science to overcome their defenses. Although they are the same green color as tomato leaves, everybody knows that caterpillars don't absorb red or blue light for photosynthesis, so they should look distinct to an Infragram camera. Or so I thought.
Tobacco hornworm on my tomato plant.
I took three different kinds of infrablue photos of the hornworm above:
- Canon A810, Rosco #74 filter, white balanced on blue paper under blue sky in the shade.
- Canon G11, professional Superblue filter (Schott BG3), white balanced on blue cloth under blue sky in the shade.
- Canon G11, professional Superblue filter (Schott BG3), white balanced on 18% gray card in the sun.
The false color infrared image (NBG) was made in Photoshop the same way for each photo. The normalized difference vegetation index (NDVI) image was made in Ned's Fiji plugin the same way for each photo (no stretching).
Infragrams do a poor job distinguishing tobacco hornworms from tomato foliage. It seems that the caterpillars reflect the same proportion of blue and infrared light as do tomato leaves, which was puzzling. So I Googled it. Sure enough, the larvae take ß-carotene from the leaves and move it to their skin. Tobacco hornworms fed a diet without leaves turn blue. I'm not sure that is why the larvae look like leaves to an Infragram camera, but apparently the ß-carotene in their skin is still absorbing blue light and reflecting near-infrared. Very clever larvae.