Predators of Asian Citrus Psyllid (Diaphorina citri) in Southern California

Asian citrus psyllid (ACP) threatens the viability of California citriculture because it spreads a bacteria, Candidatus Liberibacter asiaticus, that kills citrus trees. Preliminary observations suggested that predatory arthropods in southern California reduce the ACP population, but it was not known which predator species were important. The goal of this PhD project was to identify potentially effective natural enemies of ACP among southern California predatory arthropods for use as biological control agents. To accomplish this, we first measured the natural mortality of ACP in a southern California orange orchard by conducting a predator exclusion experiment monthly for seven months. Overall, ACP survival was nearly four times higher in cohorts protected from predators than in control cohorts. To detect which predator species contribute to ACP mortality, we conducted a year-long survey of predatory arthropods in two orange orchards. Among the predators collected, ten were abundant. We designed a molecular assay to detect ACP DNA in predator gut contents and used it to test field-collected specimens of these predators for recent predation of ACP. Three insects and all five spiders were positive for ACP predation in over 10% of specimens tested. Older C. comanche larvae and afternoon-collected adult D. pumilio were significantly more likely to test positive for ACP predation than conspecifics which were younger (C. comanche) and collected before dawn (D. pumilio). The two insects that tested positive most frequently, Diomus pumilio (Coleoptera: Coccinellidae) and Chrysoperla comanche (Neuroptera: Chrysopidae) were investigated further because of their potential for use in augmentative and/or conservation biological control. For each species, we conducted timed feeding trials to calculate a detectability half-life for the molecular predation assay analyte, and conducted prey choice assays with all life stages of ACP to determine which were most likely to be consumed after being probed by the predator. D. pumilio adults preferentially preyed upon ACP eggs and first-third instar nymphs, while C. comanche larvae most frequently consumed eggs and third-fourth instar nymphs. We recommend development of these insects as biological control agents of ACP, and suggest developing spider conservation tactics compatible with citrus IPM