Ants tending aphids

Ants have a sweet tooth. They gather nectar from flowers, buds and nectaries but also farm animals to obtain their sweet honeydew. These farmed animals are scale insects and aphids. Ants tend the aphids (above), protect them from their predators and parasites and keep them together for easy "milking". To obtain the honeydew produced by aphids, ants stimulate them with their antennae, after which the aphid will secrete a droplet of sugar rich liquid. These ants are tending groups of aphids on an Iris leaf. To soften the built-in flash light I put a piece of kitchen paper in front of it, holding it with my left finger.

Ants as scavengers and herders

So far I have overlooked two important aspects of garden ant foraging behaviour, mainly due to the lack of suitable photos. Yesterday, I spotted a strange large insect walking on the concrete yesterday, when I got closer I realised it was actually a dead bluebottle being dragged by a garden ant (Lasius niger). Another ant run frantically back and forth, but a single one seemed to do most of the work. The strength of an ant is quite formidable, able to lift or drag objects much heavier than themselves. This illustrates the role of ants as scavengers. Any dead animal will either be dragged to the nest or bits of it will.

The second aspect is one of the most surprising of ants - showing their adaptability -  but also a reason why they are hated by many gardeners: their role as aphid herders. Ants behave as farmers with their aphids and will try to keep the numerous aphid predators at bay as well as milking their honeydew. This is considered a mutualistic relationship and aphids also benefit from the behaviour. Even when aphid predators are excluded, aphids multiply faster in the presence of ants. The photo, taken today, shows two garden ants stimulating Elder Aphids (Aphis sambuci) to excrete their honeydew. Elder aphids form very compact groups on tender growing stems of Elder.

Ants in my plants

The Black garden ant (Lasius niger) is a very successful ant species. Part of their success lays in their adaptability. They are opportunistic in their choice of food: they feed on dead animals, nectar and pollen, seeds and honeydew. They are farmers, tending for aphids and scale insects and collecting the honeydew they secrete in exchange for protecting them from parasites. At the moment, ants are very active, feeding on plum and cherry nectar. They can actually enter the flower where the nectar is hidden. Although ants are known to pollinate a few plants species, they are mostly 'nectar thieves'. The reason the ants do not usually pollinate flowers are twofold. First, they are so small in relation to the size of the flower that they do not seem to touch the anthers or stigmas when entering the flower, and pollen does not adhere well to the ants surface. Second, individual ants  do not travel between different flowers in a single bout of feeding, but just get enough to travel back to the nest from a single flower - notice the distended abdomen of the ants leaving the plum flower on the photo above. They seem to feed on nectar on any cup shaped flowers lacking specific mechanical obstructions to them entering the flower (such as labiatae, the pea family). In fact, aspects of flower shape might be adaptations by plants to exclude these generalistic nectar thieves.

 Black garden ants find Paeony buds secretions irresistible, and it is hard to spot a paeony bud without its handful of feeding ants.

 The ants are now also "milking" scale insects. They encourage the scale insects to excrete the honeydew by 'tickling' them with their antenna.

More information:

C.M. HERRERA, J. HERRERA and X. ESPADALER (1984) Nectar thievery by ants from Southern Spanish insect-pollinated flowers. Insectes Sociaux, 31: 142-154. Here.