Ant mediated disruption of Citrus Mealybug (Planococcus citri) biological control in California citrus orchards

Citrus mealybug, Planococcus citri, is a cosmopolitan pest of citrus that has been recorded in California’s Central Valley since the late 1980’s, but only caused minor and sporadic harm in the region due to the activity of biological control agents. Beginning in 2019 Central Valley citrus mealybug populations grew to economically damaging levels, requiring insecticide applications to avoid significant losses for lemon, navel, and grapefruit growers. The reason for increased mealybug activity is unknown, but is likely due to changes in climate, chemical registration, and ant management approaches that have increased overwintering success of mealybugs and ant tending behavior in commercial orchards. Ant tending is known to reduce biological control of citrus pests in Southern California, but there is no data regarding mealybug biological control nor tending ant species in the Central Valley. To address this knowledge gap, we completed a survey of citrus mealybug natural enemies across three Central Valley citrus orchards during the 2024 and 2025 growing seasons. During the same period, we performed ant and biocontrol exclusion trials to estimate the influence of ants and local natural enemies on citrus mealybug colony size, predation rate, and parasitism rate. Citrus mealybug colonies were primarily visited by generalist predators and Anagyrus spp. of parasitic wasps. Ant tending was positively correlated with increased mealybug populations, and alterations in predator/parasitoid activity suggesting that ant management is key to efficient citrus mealybug IPM. Research supported by Agricultural Research Institute Grant #24-02-106.