Ants in my Plants!

Ants are a very familiar and recognizable group of insects in our homes and gardens. Due to their common presence on or around our plants and garden pests, some people consider that they may be the cause of some of the issues that we see. For the most part, ants play important and diverse roles in their ecosystems and are usually beneficial to us in our garden settings. As with every situation, however, there are always exceptions.

Ants are in the family Formicidae, within the order Hymenoptera (making them relatives to bees and wasps). They are also eusocial, meaning that they share characteristic traits including a queen (though some species have multiple queens that peacefully share a nest) taking care of brood cooperatively (usually through workers), and reproductive division of labor (meaning that certain groups within a species play a role in reproduction while others do not). Eusocial insects are able to collect a large array of resources, store/share them within their colonies, and can have interesting and complex methods of communication through pheromones. Other eusocial insect groups include bees, wasps, and termites- though there are more eusocial ant species than all of these other groups combined with an estimated 12-20,000 species (and likely even more that are undescribed).

Ants also have very interesting and elaborate communication, movement, and mating behaviors. They send individuals to scout out ideal sources of food and nesting areas, and then use trail pheromones to navigate their way to these locations. When the mating season arrives, winged male and virgin female ants take nuptial flights and then go on to start new colonies. These newly mated queens store the sperm from these nuptial flights and will use these stores to selectively fertilize her eggs for the duration of her life.

Ants are omnivores, and feed on a variety of organic materials including fungi, nectar, seeds, plants, arthropods and other small animals (acting as predators or scavengers). Even though they do sometimes feed on plants, they rarely do enough physical damage to be very noticeable in most situations. With the exception being species of leafcutter ants (primarily found in tropical climates such as central and South America). These ants form complex societies and even farm their food. Like their name suggests, they cut leaves off plants and take them back to their large underground nests in which they cultivate and feed on the fungus that grows on these chewed leaves. They can take a significant amount of vegetation to accomplish this task, though plants often easily recover by producing new leafy vegetation. In temperate climates, we don’t need to worry about these ant species ruining our favorite ornamental tree or shrub.

There are also several ant species found in lots of different climates, which can be nuisance pests due to their nesting habits, behaviors and/or close proximity to humans – but I will not be discussing these ants in this post. The goal of this blog post is to discuss examples of common ants that can be found in our yards and gardens and what they might be up to. The Ants in our Plants, so to speak.  

Ants as Beneficial Garden Guests

Many ants are great generalist predators, especially when they work together! In fact, one of my first memorable entomological observations was a large white grub (the larva of a Scarab beetle) in my family’s lawn in Pakistan being swarmed by 40-50 ants that were working together to take down this sizeable opponent. I was 7 years old at the time, but I recall being endlessly fascinated by what I saw, and continued to observe the epic battle for nearly an hour.

If a large grub (or even slightly larger animals) didn’t stand much of a chance against a determined colony of ants, smaller soft-bodied arthropods would likely be no match! In fact, ants have been recognized as great biological control agents in agriculture, especially in tropical climates. That being said, they can also have a few behaviors that can make them detrimental to our agricultural (and garden) productivity, as I will explain in the next section.

Ants as companions to other insects

In some situations ants can act in a way that is contrary to our gardening goals by supporting, protecting, and partnering with other common pest insects. These are often referred to as ‘symbiotic relationships’ (where these organisms have a close association with one another, that may benefit one or both of these groups). You may be familiar with some of these associations, which often involve a honeydew producing insect (such as an aphid or a scale insect) being closely guarded and “farmed” by a group of ants.

The reason that several ant species associate with these honeydew producing insects is due to the fact that their waste (a sugary substance that is excreted from a diet rich in plant sugars referred to as honeydew) is an excellent nutrient rich food source. These ants will often “milk” these sap-sucking insects by manipulating their abdomens with their antennae in order to coax out more honeydew. Having such a great source of food in such close proximity also gives the ants an incentive to protect it – which means that these honeydew producing insects basically have ant bodyguards that can defend them from natural enemies (such as generalist predators and parasitoids). In some cases, ants will herd or move these sap-sucking insects to juicier plant tissues, and to safer locations. When these ants move to a new nesting site, they will bring aphid eggs with them in order to establish a new “herd”.

Seeing this in action can be quite a sight to behold. In fact, if you are seeing ants grouping around some of your garden plants in larger quantities, take a look to see if you can spot some of these honeydew producing pests as well.

This symbiotic relationship between ants and honeydew producing pests can also have significant economic and ecosystem impacts! As a meta-analysis by Anjos et al. (2022) demonstrated that although ants in various cropping systems can reduce the abundance of non-honeydew producing pests, their impact on honeydew-producing pests is reversed! This analysis showed a variety of instances in which ants decreased the number of natural enemies, and increase the abundance of these honeydew-producing pests (Anjos et al., 2022).

Ants in turfgrass

Since many ant species nest in complex underground colonies, they can move large portions of soil in order to create these dynamic living spaces. Ant nest mounds in locations where we don’t want to see them can often be a nuisance to us, and sometimes even detrimental to our plants. I receive calls about this in a turfgrass setting (especially if the turf isn’t very dense or competitive), where your plants are mowed low enough to make even smaller nests noticeable. Even though these ants feed on a variety of common turf pests like white grubs and cutworms, their nests can sometimes be unsightly. Although this usually isn’t a problem in home gardens (and raking small mounds, using a hose, and increasing your mowing height can be simple fixes which could cause the ants to relocate), in situations like golf courses where low mowing heights are an important component of play mechanics, this can be more of a problem.

As you would expect, larger ant nests that happen to be built in your home gardens can be an even larger problem, because piling large quantities of soil over turfgrass is not great for the health of the turf. Some of the ants that produce larger nests include Allegheny Mound Ants (found in the Atlantic Coast of the U.S.) which can build some large and very conspicuous nests that can be over a foot tall and multiple feet wide. Additionally, since ants like to locate their nest entrances in sunny locations, they can damage vegetation in close proximity that may be shading the nest entrance. They accomplish this by biting the plants and depositing formic acid into them and, when persistent enough, can even take down larger vegetation (such as trees) through a painstaking process of hundreds of these formic acid deposits (although this isn’t very common since they prefer to nest in more open spaces).

Ants on Peonies

If you grow peonies, you may have been waiting for this section of the Ants in Our Plants blog post, as you’ve likely seen ants on or around your peonies, especially around the flowers. This is another commonly observed mutualism that exists between some flowering plants and ants (where the flowers lure ants as a source of protection from other pests), the origins of which can be traced back to the Cretaceous Period. Peonies have extrafloral nectaries located on the base of their flower buds. These produce a honeydew-like material which is a rich source of sugars, lipids, and amino acids utilized by ants as a source of food. As ants track their way to these sugary food sources, they also protect the peonies from other flower-feeding insects such as thrips. These ants don’t harm your peonies at all so there is no need to worry about them: just marvel at this cool association between two different organisms next time you are enjoying your prized peonies.

Ants in Trees

Sometimes we can find ants in old tree cavities, and around logs and stumps. Although there are a variety of ant species that nest in these habitats, the group of ants most commonly seen in proximity to our home gardens are usually carpenter ants (Camponotus species). These ants primarily chew through dead wood, and create nesting sites in imperfections and cavities of older, often damaged and deteriorating hardwood trees. They excavate smooth nests within these cavities, and you can sometimes see a characteristic pile of sawdust around the entrance. In some situations these ants can be a structural pest in homes, especially if you have water damaged wooden structures (since damp wood is easier to chew).

They play an important role in nature by helping to break down dead and decomposing wood and cycling nutrients. In our ornamental trees, significant damage is rare and often indicates that a tree has other significant damage that is more of a concern than the ants themselves. Therefore, treatment is not usually recommended for the trees themselves, though some may choose to treat if the trees are located in close proximity to wooden structures that can potentially be damaged by these ants.

I’m hoping that this post illuminated some of the interesting and diverse roles that ants play in and around our gardens. More than anything, though, I hope that this inspires you to be observant and to go out and explore some of these interesting and complex associations between insects and plants that we can often see in our very own backyards!

Resources

Ants in Agricultural Systems
https://www.sciencedirect.com/science/article/abs/pii/S1049964420306058
https://royalsocietypublishing.org/doi/10.1098/rspb.2022.1316

Farmer Ants and their Aphid Herds
https://www.mcgill.ca/oss/article/did-you-know/farmer-ants-and-their-aphid-herds

Mound Ants
https://www.canr.msu.edu/news/mound_ants

Ants on Peonies
https://ipm.missouri.edu/MEG/2018/5/antsOnPeonies/

Carpenter Ants
https://www.extension.iastate.edu/news/2009/jun/060503.htm