Density dependent mortality, climate, and Argentine ants affect population dynamics of an invasive citrus pest, Diaphorina citri, and its specialist parasitoid, Tamarixia radiata, in southern California, USA.
The effects of climate and ants on population regulation of an invasive citrus pest, Diaphorina citri (Hemiptera: Liviidae), by its parasitoid Tamarixia radiata (Hymenoptera: Eulophidae), in southern California were examined over a four-year period. Densities of D. citri eggs, nymphs, and adults, and Argentine ant, Linepithema humile (Hymenoptera: Formicidae), a mutualist which defends psyllid nymphs from natural enemies, citrus flush growth patterns, and parasitism rates of T. radiata, were monitored every four weeks on grapefruit, lemon, lime, orange, and tangerine trees at 28 urban sites across three climate types (coastal, intermediate, and desert). Significant spatial and temporal effects on D. citri abundance and parasitism rates were observed over this four-year period. Highest D. citri densities and parasitism rates were found in the intermediate and coastal regions during peaks of flushing cycles of citrus plants over March-June and September-November each year. Over the course of this study, population densities of D. citri declined by over 75%. Parasitism by T. radiata was identified as a significant mortality factor often exceeding 60% during periods of peak parasitoid activity. Analyses indicated that parasitism resulted in delayed density-dependent mortality and subsequent reductions in D. citri densities lagged by ~ 1 yr. Trends in D. citri densities and parasitism rates over time were similar on grapefruit, lemons, limes, oranges, and tangerines. Presence of L. humile in citrus resulted in a 3-fold increase in D. citri densities and control of this pest ant is needed to maximize biological control of D. citri.