We hear so much about ants “farming” aphids that we thought we should answer the question, “Do ants farm aphids?”
It’s natural for human beings to try to explain an observation of the natural world in terms of human emotions and experience. While it is common knowledge that “ants farm aphids,” this personified view leads to some very confused beliefs and assumptions. The idea that one insect benefits from the husbandry of another is fanciful but misleading.
Of the nearly one thousand ant species in North America, some do harvest the excretions of other insects. Aphids, whiteflies, certain bugs, and other insects feed by sucking sap from plants. The sugary water (honeydew) these insects pass as waste may be collected by some ants. So the idea that ants farm aphids should really be, “Some ants harvest honeydew from some insects.” To compare this phenomenon to dairy farming implies that milk is a waste product, and does rather a disservice to both dairy farmers and the amazing evolutionary relationships between insect species.
In fact, these relationships can be extremely specific and quite strange. There is a whole category of beetles known as myrmecophiles (ant lovers), that take up residence inside ant colonies, and provide honeydew voluntarily in return for protection, food, and other benefits.
Among those ants that do take advantage of this sweet natural resource, some take steps to protect aphid colonies from predators. Certain ant species can be observed attacking predatory insects like ladybird beetles or their larvae, and knocking them off plants that host aphids. This is a more highly evolved relationship, but it is still not “farming.”
Some web sources, and even gardening books, suggest that ants “herd” aphids, which is once again simply confusing. Recent studies have discovered that some ant species are able to excrete chemicals from their feet that inhibit the growth of aphid wings, and tranquilize aphid colonies to prevent them from departing. Some ants have been observed biting off aphids’ wings, another means to prevent escape. This behavior and use of chemicals appears to be even more highly evolved and sophisticated exploitation of the aphids by the ants. But again, these inter-species relationships are particular, and still not well understood.
The notion that “ants herd aphids” has led to some very unfortunate conclusions. One is that ants actually carry individual aphids, one at a time, up into fruit trees for the purposes of extracting honeydew. This misunderstanding of ant behavior has led some gardeners to assume that all ants are bad pest insects because they cause aphid infestations. And this is simply untrue. Ants carry many things, from soil, to leaf matter, to their own entire colonies. But, despite what you may have heard, ants do not carry aphids for the purposes of “farming” them.
Indeed, ants are nearly all profoundly beneficial to gardens and the broader environment. They feed on pest insects and play an important role in soil health. Fire ants and carpenter ants are surely not welcome guests, but they do not cause direct harm to garden plants or crops. And they are but two of thousands of local ant species.
Aphid populations are impacted by heat, drought, rain, abundance of food, and so on. In 2015, the summer was so hot and so long that Black Aphids were able to reproduce up to ten generations in the season and this resulted in an officially acknowledged infestation in the City of Vancouver. Aphids are attracted to plants that are otherwise stressed by heat, soil chemistry, exposure, drought, etc. And they may also simply feed on healthy flowers and vegetables. But aphids do not rely on ants in any way to spread, reproduce, or infest our plants.
The complexity of relationships between organisms in the garden is truly wondrous. It should be marveled on its own, without attempting to understand it in purely human terms.