I found these very common ants tending some very tiny (1-3 mm) root aphids one spring day. I stripped some bark from a very rotten log, and there they were. The ants immediately set about carrying their charges out of sight.

Recent research suggests chemicals on ants' feet tranquilize and subdue colonies of aphids, keeping them close by as a ready source of food. The study sheds new light on the complex symbiotic relationship between the ants (order Hymenoptera) and aphids, hugely destructive insects in the order Hemiptera.

It has long been known that certain types of aphids are herded and farmed by ants, and that the ants offer protection from other insect would-be predators (ladybugs and their larvae are perhaps the most prolific), in exchange for honeydew, a sugary secretion the ants eat. Ants have been known to bite the wings off the aphids in order to stop them from flying away with one of ants' staple foods.

Chemicals produced in the glands of ants can also sabotage the growth of aphid wings. The new study shows that ants' chemical footprints also play a key role in manipulating the aphid colony, keeping it sedentary.

The research, which was carried out by a team from Imperial College London, Royal Holloway University of London, and the University of Reading and published October 10, used a digital camera and specially modified software to measure the walking speed of aphids when they were placed on filter paper that had previously been walked over by ants. The data showed that the aphids' movement was much slower when they were on paper that had been walked on by ants, than on plain paper.

Furthermore, when placed on a dead leaf, where the aphid's instinct is to walk off in search of healthy leaves for food, the scientists found that the presence of ants significantly slowed the aphids' dispersal from the leaf. Lead author of the article, Tom Oliver from Imperial's Department of Life Sciences, explains how ants could use this manipulation in a real-life scenario:

"We believe that ants could use the tranquillizing chemicals in their footprints to maintain a populous 'farm' of aphids close their colony, to provide honeydew on tap. Ants have even been known to occasionally eat some of the aphids themselves, so subduing them in this way is obviously a great way to keep renewable honeydew and prey easily available." [3]

One of the larger aphids at 3mm - less than 1/8"

Aphids in the Suborder Sternorrhyncha are plant sap-processing machines. They feed by inserting their hypodermic needle-like proboscis directly into a plant's vascular system (phloem), which contains carbohydrate-laden sap under pressure. As a passive but very efficient process, sap flows into the insect's digestive system. [1] 

The sapsuckers must process a huge quantity of liquid, since the sap consists largely of water with only tiny amounts of nutrients. The excess (called honeydew) is excreted so copiously it has managed to fuel a symbiosis with ants. In return for the honeydew, ants will carry aphids or hopper nymphs to more vulnerable (and hence more productive) areas of the plant, and protect their charges by killing aphid-predators such as lady beetle and lacewing larvae. However, AMA and other ant species will also harvest the resulting "excess" aphids as a bonus protein source. [1]