How can an invasive species of ants travel across continents and end up defending other insects? Argentine ants have been doing exactly that for aphids, mealybugs, scales, whiteflies, and psyllids against the predators and parasites that would normally help keep their numbers manageable for growers.
That alliance turns a sugar-seeking ant into a hidden problem for citrus groves, vineyards, nurseries, and backyard plants. The main damage is usually indirect, yet the ants’ appetite for honeydew can increase pest pressure and complicate biological control.
A century-old invasion
Argentine ants are native to South America and were first detected in the United States around New Orleans in 1891. California recorded them by 1907, and researchers found that human transport created long-distance jumps far faster than the ants could spread by slowly budding into neighboring nests.
California’s mild coastal conditions gave them another advantage. Irrigated farms, landscaped suburbs, leaky pipes, and regularly watered gardens provide the moisture and steady food sources that allow colonies to persist in places where the surrounding landscape might otherwise be too dry.
The supercolony advantage
In their native range, neighboring colonies often fight. In California, a genetic bottleneck reduced variation in the introduced population, lowering aggression and allowing nests across a vast stretch from San Diego to Ukiah to cooperate rather than waste energy battling one another.
UC San Diego researcher Neil Tsutsui described the result as “one big colony”. In practice, it is a connected network of nests with multiple queens and workers that can move among sites, build enormous numbers, and overwhelm competing native ants.
Honeydew changes the battlefield
Aphids and other sap-feeding insects release a sugary waste called honeydew. Argentine ants collect it, then harass or drive away predators and parasitoid wasps that approach their food-producing partners, sometimes even moving the pests to more favorable parts of a plant.
The payoff for the pests can be substantial. The University of California, Riverside reports that parasitism of California red scale was reduced twofold to fivefold in the presence of Argentine ants, showing how a tiny insect can quietly disable an otherwise useful layer of crop protection.
Farms face a double problem
UC pest-management guidance identifies Argentine ants as especially disruptive in citrus and grapes because they protect scales and mealybugs from natural enemies. In citrus orchards, they can also interfere with irrigation by plugging sprinklers, adding another practical problem for growers.
In practical terms, growers may conserve beneficial insects and still see disappointing results while ant trails remain active. The evidence suggests that effective management must address the ants and the honeydew-producing pests as a linked problem rather than treating each one in isolation.
Native wildlife loses food
The damage does not stop at farm boundaries. Argentine ants displace larger native species, changing the menu for animals such as the coast horned lizard, which depends heavily on native ants and does poorly where its preferred prey has disappeared.
Field research in Southern California found horned lizards were more abundant where Argentine ants were absent and native prey remained. Laboratory work also showed that young lizards fed Argentine ants or prey typical of invaded communities failed to grow normally, although habitat loss and fragmentation remain important pressures too.
Why one nest is never enough
Argentine ant colonies are not tidy, isolated units. Their linked nests can be shallow, spread across a property, and contain multiple queens, which helps explain why removing one visible cluster rarely ends the invasion.
UC experts say completely eliminating ants from an outdoor area is generally unrealistic. Reducing water and food sources, sealing entry points, protecting valuable plants, and using species-appropriate baits are more practical than repeatedly spraying the workers visible on the surface.
New research complicates control
A 2026 UC Riverside study found that two common bait toxicants produced opposite behavioral effects in laboratory colonies. Boric acid increased clustering inside the nest and suppressed outside activity, while thiamethoxam sent more workers roaming through the foraging area.
The university reported that thiamethoxam killed all workers in about seven days, while boric acid killed roughly 90% in about two weeks under the study conditions. Queens may still survive, so a sudden surge in visible ants does not necessarily mean a bait failed, and temporary relief does not always mean the colony is gone.
California’s real lesson
Argentine ants show that an invasive species does not have to chew through leaves or fruit to cause serious damage. By protecting other pests, displacing native ants, and operating through resilient networks of nests, they have inserted themselves into farms, gardens, homes, and food webs across the state.
The full study on how boric acid and thiamethoxam alter Argentine ant behavior was published in the journal Insects on July 29, 2026.
