When my friend Kathy was visiting from Vermont, she was alarmed to see a gang of fat, red-faced caterpillars attacking a poor little tree. She was even more dismayed when she went to get a closer look and one of them dropped down onto her shoe.
It was big and it wouldn’t get off. These caterpillars have grabby little feet that allow them to creep up a native frangipani tree (Plumeria alba) eating all the leaves until it is completely bare. This one hooked onto her shoelace and seemed to be planning to move on up her leg. When she got a stick and tried to push it away, it reared up and threatened to bite her. Horrible.
The next day my husband pointed out that the leaves of the frangipani tree in our yard were also being eaten by five inch long caterpillars. Now they had come for us.
Still upset about the assault on my friend the day before, I was determined to defend my territory and shake these guys off. Not so easy, and there was some collateral damage to the leaves while I was whacking at the tree. Maybe a wall would keep them out.
Then one of the fallen leaves caught my eye, because underneath there was a cache of eggs. A sleeper cell? The eggs looked just about ready to hatch so I took them inside and placed them in an improvised detention unit – a bowl covered with foil (with small air holes).
The very next day, there they were – breaking out of their shells already, with black and white stripes like the tiny prisoners they were.
I soon released them back outside, but not in my yard.
Meanwhile I was wondering what the deal actually was with these caterpillars. They don’t seem to appear on any other types of trees, at least on St. John, so there must be some kind of special relationship with the frangipanis. What if the trees actually invite them? But what would make a tree want to be denuded by a swarm of caterpillars?
Some people say the frangipani trees benefit from it. Like pruning, it helps them grow. Others say the trees were going to drop their leaves anyway because the caterpillars appear just before the dry season, so no harm done. Certainly the caterpillars make the trees seem weak and look ugly. But unlike gardeners who say to pick off the caterpillars, maybe the trees have other concerns and don’t care very much about appearances.
The real deal seems to be between the moths and the trees. The brightly colored caterpillars turn into large, drab, grayish moths (Pseudosphinx tetrio), which are locally known as hawk moths, or giant sphinx moths. You don’t usually see them because they are active when it is dark, but you can sometimes find dried up dead ones on the ground.
The moth comes to the frangipani tree at night when its lovely flowers start sending out an enticing fragrance, offering the promise of nectar. But the deceitful frangipani fools the moth – it produces no nectar, just the scent. Meanwhile the frustrated moth spreads pollen from flower to flower as it vainly searches for food, unwittingly providing valuable pollination services.
Eventually the moth gives up and has to find its nectar elsewhere. It only lives a week or so, and has no time to waste. Then it comes back and lays its eggs on the leaves of the frangipani. Why not lay them on the tree where it got the nectar? Why bother with the annoying frangipani? Is it a form of payback for being fooled and sent away hungry?
No, the moth comes back because the frangipani tree offers a special form of protection to its offspring. The frangipani tree is a member of the Dogbane family and has poisonous latex in its sap. Most bugs avoid it. These caterpillars have adapted so they can eat the leaves and ingest the sap without being harmed, instead storing it up as a defense mechanism. Their bright coloring warns predators that they are carrying poison and should be avoided. Being viewed as toxic can be a powerful asset. Most birds will leave them alone and let them move on to the next stage of maturation (except for the local mangrove cuckoos and smooth-billed anis that know how to tear out the toxic part and eat the rest).
So, the tradeoff is that the frangipani tree gets pollinated by these moths due to its attractive fragrance (without having to use up additional resources producing nectar for the moths), and in exchange the frangipani tree feeds and protects the moth’s babies. Both gain a reproductive advantage. Maybe not such a bad deal really, even though from our perspective it may look like a hostile attack.