Monday, June 29, 2020
Black Cohosh, caterpillars, and ants
I ran across a vigorous colony yesterday on a steep slope in a Hocking County woods, which instantly caused a lepidopteran red flag to fly. There is a very cool - amazing, really - relationship between this plant, a little butterfly known as the Appalachian azure, Celastrina neglectamajor, and ants. The only host plant for this azure is black cohosh, and with a bit of knowledge one can often find the caterpillars.
The search window is narrow, as the cats primarily eat only the flower buds and then the flowers. The two little racemes to the left in the photo above, and the upper half of the larger raceme, are still in the bud stage. They'll quickly develop into flowers, at which stage the caterpillars will be pretty well grown. So, the azure caterpillar hunter will do best to look for black cohosh still in bud, and then watch for ants swarming a localized area of the budded out flower raceme.
So, the million dollar question: Why the ants?
Their presence is part of a fantastic relationship between caterpillars (and other organisms such as aphids) and ants known as myrmecophily. This is especially common with gossamer-winged butterflies or Lycaenids, a very large family with some 6,000 species currently known. Basically, the ants involved in myrmecophic relationships "farm" their subjects, and in return are rewarded with highly desirable "honeydew".
The ants (I think a number of species may be involved and I don't know the identity of those in the photo) tend the azure caterpillar closely. They are essentially the cat's private security force, warding off attacks by parasitoid wasps, flies, and other would-be predators. In return, the caterpillar secretes nutritious "honeydew", a liquid excreted by the larvae. This stuff is clearly quite valuable to the ants, as they not only do not let the cat out of their sight, they often remain atop it. In my photo, seven ants are in the photo. No bad guys are breaking through these bodyguards.
So, if you run across black cohosh in flower, take a closer look for a squadron of ants swarming around the buds and flowers. Closer inspection may reveal one of these interesting caterpillars.
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