Why are ants attracted to cherry plants?

16 October 2011

Part of the show Plant Pests and Plant Pathology

Question

This concerns a fruiting Cherry tree that attracts ants. It came from Tescos and is clearly a graft onto a root stock so unfortunately I don't have a species. The behaviour I observe is as follows: ants climb the tree and head to a leaf stem, at the base of which is two red buds about 1mm across. They pat the buds for a while with their antennae, from where they may move to another leaf and do the same before heading back down the plant. The buds produce no fragrence or fliud that I can detect. Their appearence is waxy. They don't eat the buds at all. I left the pictures full res. so you can zoom in.
My theory is that the Cherry produces these on purpose to attract the ants, who in turn defend the tree against insect attack. I tried to clear my peach tree of aphids once using ladybird larvae, and the ants that farmed the aphids would attach to a leg, until there was one on 4 of the legs, at which point the ants larvae would drop off the tree. The ants would move aphids around the tree to uninfested sections. Do ants pat aphids to stimulate sugars?
Regards,
Mike Garrard

Answer

John - Well ants of course are looking for food, particularly sugar. Though it would be nice to know which species and a little bit more about the botany but in general terms, some plants in fact will provide not just from the flowers, but they will provide false nectaries which give the ant some food and in return, the ants can stop other insects attacking the plant. Also, if there's any leakage of sap from the system whether you can see it or not, ants might be able to find that. If there are sucking insects feeding cryptically on the plants, you can't really see them, they may be producing honey dew. Aphids do this because they're simply looking for a bit of nitrogen. There isn't much in the sap that they feed on and so, their faeces is almost neat sugar, the honeydew, and of course, ants then come along, so the ants look after the aphids in that particular case. But by and large, the ants will look after the plant if it's offering them some reward. Ben - So you get ants that actually cultivate and encourage aphids? Surely, that's a huge problem for gardeners because you end up with an excess of aphids. John - Well, that's certainly true. But of course, the ant is in the business of keeping the aphid so that he can milk it. It doesn't really want it to destroy the plant otherwise there won't be any sugar coming in from photosynthesis. But some ants in fact have a very close relationship with plants and there are Secropia trees in Central America which provide the ants with a sort of living accommodation within the tree and when you pull a branch down, they all come out and attack you. So, these Azteca ants, so that's the genus. They really adapted the life to fit in exactly with the plant and it's of great benefit to that plant.