Ants are amazing creatures, and we have known that for a while now. They can thrive virtually anywhere, they form large super-colonies,  and they herd aphids like cows. What we did not know was how much detail and planning goes into their Aphid herds.

Aphids produce a sugary fluid called honeydew that ants love to eat. To ensure a steady supply of honeydew, ants will protect aphids from predators, and in turn, aphids destroy plants. Hence, ant infestations are followed by aphid infestations. While studying  L. japanocus, a species of ants that tend colonies of the species M. yomogicola, a kind of aphid that feeds on the mugwort plant, scientists in Japan discovered something new.

The intriguing new discovery is that ants select their aphid population to have an optimal ratio of 65% green to 35% red. They do not blindly herd all aphids, instead they trim their herds for maximum efficiency. Color polymorphisms like this are usually due to balancing selection, from predation, yet the aphids were being protected by ants. According to the researchers, this system represents the first case of a balancing polymorphism that is not maintained by opposing factors but by a symbiotic relationship.

Puzzled by this, scientists in Japan set out to understand the underlying reasons. It turned out that green aphids produce more honeydew, while red aphids seem to prevent mugworts from flowering. In addition, aphid colonies on flowering mugworts went extinct.

To maintain a herd for more than a year, the ants were keeping red aphids around, to ensure long term food supply for the greens. This kind of selection is akin to farmers choosing the optimal number of meat and milk cattle to raise on a livestock farm. So, next time you see a ant herding aphids, remember it is not just a symbiotic relationship, but a carefully selected herd protected by Ant farmers who know how to plan long-term.

Article/Image Source – http://arstechnica.com/science/2016/09/ants-are-destroying-your-plants-by-nurturing-perfect-aphid-colonies/

Image 2 Source – http://phys.org/news/2016-09-picky-ants-polymorphism-bugs.html

Referenced Paper – Watanabe, S., Murakami, T., Yoshimura, J., and Hasegawa, E. (2016). Color polymorphism in an aphid is maintained by attending ants. Science Advances 2, e1600606.