You wouldn't think there would be much to attract ants to a cactus. They have waxy stems covered with spines.
There is one structure in this photograph, however, that is highly attractive to ants.
Do you see it yet? No, it isn't the clusters of extrafloral nectaries.
It's a bud.
What are the ants doing?
In case you aren't familiar with the relationship between ants and cacti, the ants are visiting to obtain a sugary liquid produced by the plant. It would appear that the softer, quickly growing tissue of a bud is more susceptible to being eaten by plant-feeding caterpillars or beetles. It is likely the ants will defend their free "soda fountain" and chase these sorts of pests away.
Those of you who have seen extrafloral nectaries on cacti, do you agree that those yellow bulges between the spines above the bud are extrafloral nectaries?
Here's an ant visiting an extrafloral nectary on a barrel cactus.
You get to see a lot of surprising things in the Sonoran Desert of the southwestern United States. How cacti provide nectar for ants is just one example.
Most of us learned in school that flowers produce nectar, which is then collected by bees, butterflies, bats and/or moths. Extrafloral nectaries are structures on a plant that produce nectar, but they are not inside flowers. They may be located on petioles, leaves, sepals, or stems.
Extrafloral nectaries are more common than you would think. Extrafloral nectaries are found in a wide range of different plants from over 70 different families, from buttercups to violets. The primary visitors to all these extrafloral nectaries are ants.
Believe it or not, extrafloral nectaries are present in many cacti. Cacti have many specializations for saving water, and even have a different system of photosynthesis that is more water efficient.
Yet, certain species of cacti also have tiny extrafloral nectaries within patches of spines, known as thorn nectaries, which may literally drip nectar. The plant is losing water (albeit probably in very small amounts), to in attract ants. The orangish, waxy bumps in the following photo are the nectaries.
No one knows for sure why the cacti have extrafloral nectaries, and it is likely that different kinds of cacti have them for different reasons. Scientists have proposed that some cacti supply nectar to ants to keep them away from the flowers, where the ants might drive away pollinators. This seems unlikely since the extrafloral nectaries are active throughout the year or at different times of the year, not just when flowers are open. The cacti may supply nectar to keep the ants away from tending aphids, scales or mealybugs that might cause more problems for the plant, although ants don’t seem to tend some of the most prominent cactus-feeding species. The cacti may supply nectar to lure ants into the area because the ants’ activity improves the texture and/or nutritional value of the soil immediately around the cacti. This last idea makes a lot of sense given desert soils are often low in organic matter and nutrients, and ants are known to improve soil. Also, ants may provide a cleaning service, keeping down disease-causing fungal spores and bacteria, as well as chasing away or eating disease-carrying pests.
Ants may chase away seed-feeding bugs like these. On barrel cacti the nectaries are active when the plant has fruit, and the seed-feeding bugs are around. Can you see the ant and extrafloral nectary in the middle, between the fruit?
Not only do a variety of native ants take nectar from cacti, but introduced species may as well.
These tiny introduced Brachymyrmex gather nectar of the extrafloral nectaries of another barrel cactus. Do they perform the same duties as the native ants? Since we don't know exactly what the ants are doing in most cases, it is hard to know for sure.
Have you ever seen ants visiting extrafloral nectaries? What do you think?