Argentine ants shield crop pests, threatening California orchards and vineyards

A century-old invasion of Argentine ants, first recorded in the United States in 1891 and in California by 1907, has evolved into a cooperative network that protects honeydew-producing pests such as aphids, mealybugs, scales, whiteflies and psyllids. By repelling natural enemies, the ants allow these sap-feeding insects to thrive, causing significant damage to citrus orchards, vineyards, nurseries and residential gardens.

The genetically uniform super-colony, described by UC San Diego researcher Neil Tsutsui as “one big colony,” spans from San Diego to Ukiah and resists control efforts that target only visible ant clusters. Effective strategies now emphasize limiting moisture and food, sealing entry points, and deploying species-specific baits, as conventional spraying proves insufficient. The ants also displace native ant species, reducing prey for wildlife like the coast horned lizard, which suffers reduced growth when native ants disappear. Recent UC Riverside research shows that bait toxicants such as boric acid and thiamethoxam have differing impacts on colony behavior and mortality, highlighting the need for integrated pest-management approaches.