Many homopteran species, including aphids, have evolved mutualistic relationships with ants. Such symbiotic relationships are beneficial to both species and will endure as long as the costs do not outweigh the benefits. The basic parameters of this mutualism are that the aphids provide the ants with a source of food – their honeydew secretions – and the ants protect the ants from predators and parasites; chasing off organisms wanting to eat, or lay their eggs on, the aphids. This rather simplistic view of the arrangement has however, been challenged by experiments in recent years, and as always in nature, the reality of the situation is more complex; the costs and benefits of aphid-ant interactions can vary and ants can have a negative impact on aphid reproduction for example, or even eat the aphids!

Nevertheless, it seems that where this relationship exists, the benefits for the aphids of supplying the ants with honeydew outweigh the costs, even though the latter can it seems, be high and exacting in certain circumstances!

I came across this little group of insects whilst walking along a trail in the forest on mount Doi Sutep-Pui, in northern Thailand, at an altitude of about 1,500m. A winged adult aphid – I do not know the species – had laid a small colony of nymphs on some newly emerged leaves. Again the species of plant involved is unknown to me. The tiny little colony of aphids is attended by three ants who are presumably imbibing the honeydew secretions produced by the aphids, which are ‘plugged in’ to the mid-vein of the leaf and feeding on the phloem produced by the plants. The aphids are therefore acting as a sort of living conduit for the sugar-rich plant sap to reach the ants.

Aphids produce an excess of honeydew because the phloem they are feeding on contains very low concentrations of amino acids and they need to imbibe a lot of it in order to obtain sufficient nitrogen. Nevertheless, in a mutualism they need to produce enough for their own needs plus that of the attendant ants. This burden can affect the growth rate of aphids in ant attended colonies, but presumably the benefits of being protected outweigh the costs of feeding the ants, as long as there are not too many formicid mouths to feed!

Despite the potential benefits to both parties, the majority of aphids do not enter into these mutualistic partnerships with ants (2). There are a very large number of potential ant-aphid possibilities, with over 4,400 species in the family Aphididae and an estimated 22,000 species of Formicidae (according to Wikipedia). Perhaps it takes a special situation for a mutualism to evolve; perhaps many such arrangements have formed and broken apart in the past. Certainly, a phenomenon with a huge scope for further studies.

1. Yao, I. (2014). Costs and constraints in aphid-ant mutualism. Ecological research, 29(3), 383-391.
2. Not all aphids are farmed by ants. https://simonleather.wordpress.com/tag/ant-attendance/