Lots of plants have glands that secrete nectar, the sweet liquid so attractive to bees and other insects. However, these glands aren’t always in the flower. When they’re outside the flower, they’re called extrafloral nectaries.
The function of these extrafloral nectaries has been conjectured for years and years, but still no one is really certain what their role is. Back when Charles Darwin was writing On the Origin of Species he noted that:
Certain plants excrete sweet juice, apparently for the sake of eliminating something injurious from the sap. … This juice though small in quantity is greedily sought by insects; but their visits do not in any way benefit the plant.
This remains one of the hypotheses for the role of the extrafloral nectaries – that they are helping with them elimination of undesirable compounds.
The other hypothesis, which is more popular these days, is that the extrafloral nectaries attract beneficial insects which help in defending the plant from “pests”.
I have to say, in my experience the glands attract mainly ants who, as Darwin noted, feed greedily from them. Meanwhile the aphids still come and begin feeding on the plant leaves. Then the ants, which are there in abundance having been dining at the extrafloral restaurants, set up their aphid farms, defending the aphids from would be predators like ladybirds. The ants then live a particularly happy life, lounging drunkenly around the extrafloral nectaries and milking their aphids.
There must be more to this than meets the eye though, otherwise it would be a bit of a suicidal strategy.
Today was the first day for a mass emergence of flying ants: that’s about three weeks later than it ‘usually’ happens here.
For a couple of weeks, the smaller male winged ants have been creeping out of cracks and crevices, only to be dragged back in by the regular worker ants. Today, the time was right and the boys got their freedom.
The ants are ‘supposed’ to mate in the air, but they’re quite happy to do it anywhere…
Once mated, the new queen sheds her wings and scurries around looking for a new place to start her own colony.
Happily, thanks to the birds and the spiders, not all the flying ants make it to start new colonies.
The Society of Biology have an interesting pdf factsheet about flying ants available to download here.
This young pine tree has a light infestation of pine aphids (Cinara spp – possibly Cinara confinis). The aphids are being “milked” by ants: basically, the ants are looking after the aphids and eating their excrement or “honeydew” to put it more politely!
At this low level of infestation (the aphids were only present on a couple of shoots throughout the whole tree), there isn’t a particular problem. The aphids haven’t been shown to spread any viruses, unlike many other aphids, and they provide a food source for many beneficial insects, like ladybird and hover fly larvae.
Ants tend to get a bad press in the gardening world: they farm aphids, they tunnel everywhere, and they sometimes bite or squirt you with formic acid.
But sometimes ants are beneficial, acting as pollinators to flowering plants. Othertimes, they are simple thieves – drinking nectar without transferring pollen. I think the latter is the case in this situation with the ant “stealing” nectar from a rosemary flower.