Some types of ants are aphid farmers. I have often observed their activity in my kalamansi tree. Aphids quickly destroy the new leaves of the tree.
Several trees of the plant family Rutaceae are growing in our back yard. (Rutaceae is the family to which orange and lemon trees belong.) Most of them are still immature, but a kalamansi tree is about thirteen or fourteen feet high. It has already produced some fruit for a few years.
For people living in Maine or Montana, the kalamansi is an exotic plant. Kalamansi is the name of the plant in Tagalog, and people in Vermont would probably look puzzled if you mentioned it. However, their eyes might light up if you mentioned lemonsito, its name in Cebuano. You can tell from the name that it is a little lemon. However, the fruit does not look like the bright yellow lemons that are sold in stores in Wisconsin.
Citrus microcarpa, its scientific name, also helps you understand the type of tree it is. The genus name “Citrus” reveals that it produces a citrus fruit, which is rich in vitamin C. The name “microcarpa” warns us that the fruit will not become as big as a grapefruit or even an orange. Microcarpa comes from the ancient Greek words micrós, which means “small,” and karpós, which means “fruit.” All the fruits that my tree has produced have been little green spheres smaller than a golf ball.
The tree periodically produces fresh, light green foliage, but almost immediately the emerging growth is attacked by aphids. The aphids do not seem to bother mature leaves, but they can quickly annihilate the new growth. I imagine that there is some spray that kills them, but I always remove them manually.
I do not know how they get up there. I read that some aphids ride the wind from one place to another. In addition, although aphids normally do not have wings, some types produce winged offspring when their current food supply starts getting low. The winged offspring then fly off to greener pastures.
Ants are another possible means of locomotion. Some ants are aphid farmers. They protect the aphids, and occasionally move them to their food supply. Of course, they expect that their efforts will be rewarded. The aphids secrete a sweet substance called honeydew which the ants consider a delicacy.
I never saw an ant carry an aphid up the trunk of my lemon tree, but I regularly see ants tending the aphids, apparently collecting honeydew. They are probably annoyed with me for regularly spoiling their snack.
If I see an ant running up or down the trunk of a tree, I cannot tell from its form whether or not it is an aphid farmer. Probably an expert with a magnifying glass could see the difference. However, when you see one or more eagerly applying themselves to a host of aphids, their activity is an infallible key to their identity.