Pic: Joyshree Chanam

For the love of honeydew

By Joyshree Chanam

Would you believe me if I tell you that ants tend cattle for milk, just as cowherds do? Well, that would be almost true, only that the animals that ants tend are much smaller than cows, and belong to a group of insects called Hemiptera. There are many ant-tended Hemipteran insects such as aphids, scale insects, mealy bugs etc. They are found as small colonies, usually on young branches of plants. Ants work hard to protect these juicy Hemipterans from predators such as beetles who love to eat them. Not only that, the ants also keep the area around the colony clean to prevent fungal infestation. Hemipterans have long syringe-needle-like mouthpart called stylets. They insert their stylets into the phloem tissue of the plant and happily suck the phloem sap, as you would drink juice with a straw. Then, even as they are feeding, they excrete out sugary droplets. These sugary droplets are called honeydew and the ants love it! They quickly sip up the honeydew droplets, often directly from the Hemiperan’s posterior, and store it in their crop (also called ‘social stomach’), and later take it back to their nest where they share it with their nest-mates.

Phloem sap is rich in sugars and amino acids. Since the Hemipterans are more interested in amino acids, they retain the amino acids in their body, and excrete out the excess sugars and sap as honeydew. If there are no ants to tend the Hemiptera colony, the honeydew might just accumulate there, leading to fungal infestation, which might kill off the whole colony.

Once, while walking through a forest, I saw a colony of mealy bugs that were being tended by some ants on a young branch. I felt a little naughty that day, and decided to disturb the peaceful scene by shaking the branch a bit. You won’t guess what happened. The ants ran helter-skelter of course, but not on their own. Each ant carried one mealy bug in its mouth and ran, to save them from the danger (that is me!). I felt guilty, but at the same time I couldn’t help imagining what a human cowherd would do if a giant comes after his cows. Surely, he couldn’t pick up his cows and run, right? But ants can lift many times their own body weight, so a measly mealy bug is hardly a problem.

The story of ants and Hemiptera is interesting indeed but it is not good news for the plants. You see, the phloem sap that the Hemiptera feed on is actually the precious food that the plant prepares for itself, by photosynthesis in the leaves. It is then transported to various parts of the plants via channels called phloem. Because of the hemipterans, the plants not only lose their phloem sap, but also become vulnerable to bacteria and viruses which enter the plant through the holes created by the stylets,

But some ants also help the plants on which they live by providing them security. When these ants come across trespassers such as caterpillars, which eat the leaves of their home plant, they bite and chase them away. Therefore, some scientists believe that even though having Hemipterans is a costly affair for the plants, the net effect might be beneficial because the Hemipterans attract ants, which in turn protect the plants from caterpillars. This kind of interaction between three kinds of organisms (here, the plant, the ants and the Hemipterans) belonging to three different trophic levels of the food chain is called a ‘tritrophic interaction’. I leave you with that to chew on!

This article appeared in the Hindu In School on 20 June 2014.

Picture: Ants tending aphids on a plant. Photo: Joyshree Chanam