Ant-caterpillar symbioses

Being a small, squishy, slow-moving caterpillar is tough. Caterpillars are a protein-packed snack for carnivorous insects such as wasps and spiders, some ants, and birds.

For my post-doc, I’m studying an ant-caterpillar symbiosis.

A “symbiosis” is an interaction between two organisms of different species. Insect pollinators and plants, clownfish and anemones, us and the microbes in our gut.

As many things, symbioses are complicated. Typically, people think “mutualisms” when they think symbioses. But a mutualism, when both organisms help each other, is only one type of relationship.

There’s also commensalism, where one organism benefits and the other is neither helped nor harmed. An example is barnacles that hang out on whales. The barnacles gain a free ride, and the large whale isn’t too bothered by the hitchhikers.

Then there’s parasitism, which you’ve probably heard of. Parasitism is where one organism benefits (the parasite) and the other is negatively affected (the host). An example: you when you get sick. You get weak, tired, and achy. The parasites inside you—bacteria, viruses, etc.—are living their best life.

When Lycaenid (pronounced lie-see-nid) caterpillars sense danger, they send a signal for help. Some caterpillars use smell as their help signal; they mimic a pheromone that ants secrete when in danger. This alarm pheromone brings the “good” ants who protect the caterpillar. Other caterpillars use sound such as squeaking, clicking, or stridulation (sound made by rubbing two body parts together, like crickets). Like smell, the sound attracts ant protectors.

So, the caterpillar gets protection but what do the ants get out of it? In many cases, the ants get a snack. Lycaenid caterpillars have a special gland, the dorsal nectary organ, near their bottom. Once the danger has passed, the caterpillar secretes a droplet of sugar as a reward (see photo below). Sounds like a mutualism, right? The caterpillar gets protection, the ant gets a snack.

But, it may not be that simple.

From the caterpillar’s point of view, it likely takes a lot of energy to produce that sugary reward. Also, ants can possibly take advantage of the caterpillar (like a parasite) by coming around and reaping the rewards when there isn’t any danger.

In many cases, this ant-caterpillar relationship is an overall mutualism—caterpillar survival is usually much greater when the ants are around to protect them, and the caterpillar’s sugary secretion tends to be both nutritious and delicious. But the caterpillar I study, the Puget blue, has yet to be studied regarding its ant attendants.

This past field season, I spent time in the Pacific Northwest getting to know the Puget blue caterpillars and which ants are hanging around. Next year, I will do behavioral experiments to see which ants tend the Puget blue caterpillars and how they do it. Once we know more about the natural history of this relationship, I will examine how ant tending may affect caterpillar survival and thus, the population of the at-risk Puget blue butterfly (pictured above).

I already can’t wait to be back in the field!