“Sugar ants” is a common, non-scientific term used to describe small ant species strongly attracted to sweet substances, including pests like the Argentine ant (Linepithema humile), the odorous house ant (Tapinoma sessile), and the pavement ant (Tetramorium immigrans). These ants are a foundational part of the food web in the environments they inhabit. They exist within a complex ecological system where they are constantly hunted and controlled by a diverse array of natural enemies, maintaining a natural balance.
Insect and Arthropod Predators
The most immediate threats to sugar ants come from other arthropods, many of which are generalist predators. Predatory beetles, such as ground beetles (family Carabidae), are nocturnal hunters that use strong mandibles to capture and consume foraging ants. Both the adult and larval stages of these beetles are active in the soil and leaf litter where ants establish their nests.
Specialized ant predators also exist, including certain species of spiders and other ants. Jumping spiders and wolf spiders ambush worker ants on foraging trails, relying on speed and venom to overcome prey. Other ant species, like the native winter ant (Prenolepis imparis), actively defend their territory by attacking invasive Argentine ants. These winter ants use a venomous dab from the tip of their abdomen to neutralize foreign workers during conflict.
Parasitoid insects primarily target the invasive Argentine ant. Phorid flies in the genus Pseudacteon are known to hover over foraging ants, darting down to lay an egg on the ant’s body. Once the egg hatches, the larva migrates to the ant’s head capsule and develops, eventually causing the ant’s head to fall off. While these specialized flies are often absent in the introduced range of the Argentine ant, their presence in the native range significantly deters foraging activity, forcing ants to retreat underground.
Vertebrate Predators
Larger animals consume sugar ants, often as an opportunistic part of their diet in residential settings. Insectivorous mammals like shrews are high-metabolism hunters that consume their body weight in insects daily, including ants they encounter. These mammals actively search for prey in the soil and leaf litter, putting them in contact with foraging workers and shallow nests.
Many species of birds, particularly ground-feeding species such as robins, sparrows, and woodpeckers, consume ants when accessible. Woodpeckers will excavate rotting wood or turf to reach ant colonies, while sparrows snap up workers on the ground, especially during the spring swarming season when winged reproductive ants are abundant. Reptiles also contribute to ant control, with lizards and skinks readily feeding on them.
The presence of invasive ant species can negatively impact vertebrate predators that rely on native ants for survival. For instance, the invasive Argentine ant displaces native harvester ants, which are a preferred food source for the Coast Horned Lizard (Phrynosoma coronatum). This displacement of native prey contributes to the decline of specialized predators in the ecosystem.
The Role of Pathogens and Parasites
Beyond direct predation, microscopic biological agents limit ant populations through disease and colony-level infection. Entomopathogenic fungi, such as Beauveria bassiana, are naturally occurring soil fungi whose spores can infect ants. The fungal spores adhere to the ant’s cuticle, germinate, and secrete enzymes that dissolve the protective exoskeleton.
Once the fungus penetrates the cuticle, it grows inside the ant’s body cavity, eventually killing the host. Worker ants often exhibit “social immunity” behaviors, such as intensely grooming themselves and other workers to remove spores, or removing sick individuals from the nest. If an infection is established, the fungus can spread quickly through the crowded tunnels of the colony.
Nematodes, which are microscopic roundworms, can parasitize ants, particularly targeting the brood or queens within the nest. These parasites are an effective form of biological control because they bypass the worker caste to infect the most reproductively important members of the colony. Pathogenic viruses and bacteria are also present in ant colonies, and their rapid transmission is facilitated by the close social contact of feeding and grooming.
Ecological Impact and Natural Control
The constant pressure from predators and pathogens is a natural mechanism for maintaining the ecological balance of ant populations. When this balance is disrupted, ant species like the Argentine ant can reach supercolony status. They displace native ant species that serve as prey for other insects and small animals. Native predators are often not adapted to hunt the new invasive ant, leading to a breakdown in the local food web.
Human activities, such as the indiscriminate use of broad-spectrum pesticides, can inadvertently remove more predators and parasites than ants. This can worsen an ant problem by eliminating natural controls. For example, ground beetles and parasitic flies are highly susceptible to pesticides, and their loss can lead to a rebound in ant populations.
Supporting biodiversity, such as maintaining varied habitats with leaf litter and ground cover, helps sustain natural enemies like ground beetles and spiders. Preserving the natural food web ensures that a diverse group of organisms is present to exert continuous control over ant numbers. Understanding the complexity of this predator-prey relationship allows humans to better manage ant populations through conservation and targeted control methods.