Invasional meltdown: Are Argentine ants facilitating the invasion of Asian citrus psyllid, Diaphorina citri (Hemiptera: Liviidae), in Southern California?

The Asian citrus psyllid (ACP), Diaphorina citri, is an invasive honeydew-producing citrus pest and a vector of the bacterium responsible for huanglongbing (HLB). This disease has devastated Florida’s citrus industry and the same threat is imminent in California as D. citri spreads throughout its citrus growing regions. California’s current ACP biological control program has focused on establishing the host-specific ectoparasitoid, Tamarixia radiata. Historically, biological control programs utilizing T. radiata have been very successful in ACP population reduction, often resulting in significantly stalled disease spread or disease prevention altogether. However, preliminary data suggests parasitism is not high enough to provide substantial control of ACP in Southern California. Perhaps one contributing factor is the invasive Argentine ant, Linepithema humile, which commonly tends ACP colonies. The presence of this mutualism may influence predation and parasitism, as ants prevent natural enemy access to D. citri nymphs. We sought to elucidate intraguild interactions among mutualists, predators and parasitoids of ACP with the goal of developing an action threshold for Argentine ant damage in this system; that is, the ant activity level that significantly interferes with biological control of D. citri. Specifically, we wanted to quantify the effect that ACP attendance by L. humile has on the parasitism rate of T. radiata, as well as predator composition and density, by manipulating ant activity in citrus orchards. Over the past 2 years we examined nearly a thousand flush samples from 98 trees at 5 different field sites in Southern California. Half of all experimental trees were baited with a thiamethoxam sucrose solution to reduce ant activity and the other half were left untreated. On all trees, half of the experimental flush were protected with a Tanglefoot barrier to completely exclude ants. Throughout the course of the experiment, we recorded and compared changes in ant activity, the presence of other honeydew-producing hemipterans, composition and density of predators, and parasitism by T. radiata. Results will be discussed in the presentation.