Invasive Argentine ants are fending off other pests in California citrus orchards and vineyards, worrying farmers and leaving natives hungry.
An invasive species of ants traveled across continents to America and is now defending other insects. According to a report from econews, Argentine ants have been protecting aphids, mealybugs, scales, whiteflies and psyllids against the predators and parasites that would normally keep their population in check. This unlikely alliance has turned ants into a hidden and threatening problem for citrus orchards, vineyards, nurseries and garden plants.
A centuries-old invasion
Argentine ants are native to South America and were first detected in the United States around New Orleans in 1891. California recorded their presence in 1907 and researchers found that human transportation created long-distance jumps much faster than the ants could spread by slowly sprouting to neighboring nests.Their presence was only facilitated by California’s mild coastal conditions. Irrigated farms, landscaped suburbs, leaky pipes, and regularly watered gardens provide the moisture and constant food sources that allow colonies to persist in places where the surrounding landscape might be too dry.In their native range, neighboring ant colonies often fight. But in California, a genetic bottleneck reduced variation in the introduced population, decreasing aggression and allowing nests across a vast stretch from San Diego to Ukiah to cooperate. UC San Diego researcher Neil Tsutsui described the result as “a big colony.” In practice, it is a connected network of nests with multiple queens and workers that can move between sites, form huge numbers, and overwhelm competing native ants.Since they have linked nests that can be shallow, spread across a property, and contain multiple queens, removing a visible group rarely has as much impact. Reducing food and water sources, sealing entry points, protecting valuable plants and using species-appropriate baits are more practical than repeatedly spraying visible surface workers, UC experts said.
A battle for molasses
Farms face problems
The UC Pest Management Guide identifies Argentine ants as especially harmful to citrus and grapes because they protect scale and mealybugs from natural enemies. In citrus orchards, they can also interfere with irrigation by clogging sprinklers, adding another practical problem for growers.In practical terms, growers can keep beneficial insects and still see disappointing results as long as ant trails remain active. Evidence suggests that effective management should address ants and honeydew-producing pests as a related problem rather than treating each in isolation.
Loss of food for native wildlife
New research complicates control
A 2026 UC Riverside study published in Insects found that two common bait toxicants produced opposite behavioral effects in laboratory colonies. Boric acid increased concentration inside the nest and suppressed outside activity, while thiamethoxam sent more workers wandering around the feeding area.The university reported that thiamethoxam killed all workers in about seven days, while boric acid killed about 90% in about two weeks under study conditions. The queens may still survive, so a sudden increase in visible ants does not necessarily mean that the bait has failed, and a temporary relief does not always mean that the colony has disappeared.
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