Exclusion experiments reveal relative contributions of natural enemies to mortality of citrus leafminer, Phyllocnistis citrella (Lepidoptera: Gracillariidae) in Alabama satsuma orchards

The citrus leafminer, Phyllocnistis citrella Stainton (Lepidoptera: Gracillariidae) is an important pest of satsuma mandarin (Citrus unshiu Marcovitch) in Alabama. A recent study identified several species of beneficial arthropods including spiders (e.g., Hibana sp.), ants (e.g., Solenopis invicta Buren), Chrysoperla sp., Harmonia axyridis Pallas, and two parasitoid species (Ageniaspis citricola Logvinovskaya and Cirrospilus sp.), as potential natural mortality factors of P. citrella in Alabama. Exclusion experiments with complete or timed (early or later) sticky or cage barriers were conducted in two satsuma orchards in southern Alabama during 2007 and 2008 to determine the relative contributions of key beneficial arthropods to natural mortality of P. citrella. Overall mortality of P. citrella on unprotected (control) satsuma tree branches ranged from ∼39%–52% depending on location and year. Results from both locations and years clearly showed that predation was the dominant natural mortality factor acting on P. citrella. Predation accounted for ∼87–96% of all deaths on unprotected (control) satsuma tree branches. In particular, predation by spiders was the single most important mortality element, which accounted for ∼50–70% of all deaths. Predation by ants was second, accounting for ∼10–19% of all deaths. Predation by predatory insect larvae accounted for ∼3–27% of all mortalities, while parasitism contributed the least (0–10%) to P. citrella mortality. Predation by spiders was excluded by a cage barrier, whereas a sticky barrier was more effective in excluding predation by ants. In general, the timed barrier treatments were not as effective in excluding the key predatory arthropods. These results are discussed in relation to the management of P. citrella in Alabama satsuma orchards.