It's Complicated

With the waxing of the moon has come some glorious weather here in SW Florida and some spectacular migrating butterflies. This one is called cloudless sulphur, Phoebis sennae, traveling down from the north in search of warmth and food. Senna ligustrina, this butterfly’s host plant has emerged from the seed bank and thrived in the garden. This plant is still blooming and there are plenty of nectar plants around to nourish the adult butterfly. This summer the plant was covered with these caterpillars, eating both the leaves and flowers. As they ate the flowers their color turned from green to yellow and vice versa when they ate the leaves.

But I’ve noticed something else in my quest for connections. I am fascinated by extra floral nectaries, EFN, and this plant has them at the base of each leaf stem. To me it resembles a modified stipule. Nectaries are usually associated with the flower, secreting a sweet liquid to insure pollination. But these appendages also secrete sugars with other nutritional compounds and have nothing to do with pollination but they help to preserve the flowers. The tip of the gland is yellow or orange while the base that attaches to the stem is green. Ants are travelling and congregating around them. I have read that the ants will attack the larvae but I haven’t seen this onslaught yet. Interestingly, when the eggs are laid they look white and pitcher shaped. As they mature they turn yellow like these nectaries. I haven’t seen any eggs since August.

The ants were absent when the caterpillars were eating the flowers this summer but they are abundant now. Do the nectar producing glands respond to stress in the plant producing more nectar, attracting more ants? Now, I am seeing the flowers fertilized and seed pods forming. If ants are preventing the butterflies from flourishing on this bush are they laying them on a cassia instead, an alternative host plant without EFN? Or will they forego breeding here and return north in the spring? Obviously, I need to examine this relationship between host plant and butterfly more closely.