This study examined the potential role of native parasitoids in suppressing pest populations of Argyrotaenia citrana (Fernald) in coastal California apple orchards. An initial survey of larval and pupal parasitoids of A. citrana found that the most common parasitoids of A. citrana were the braconid Apanteles aristoteliae Viereck and the ichneumonid Exochus nigripalpis subobscurus Walsh that together parasitized 33% of the A. citrana larvae collected. To identify which parasitoid species caused most host mortality, parasitoid-induced mortality was examined more closely using sentinel A. citrana larvae. Small batches of both newly hatched and older (third and fourth instars) larvae were released and recaptured to collect emerging parasitoids. The results suggested that most parasitoid-induced mortality occurred in young larvae attacked by A. aristoteliae. To determine whether A. aristoteliae would be likely to suppress A. citrana populations in apples, further release–recapture experiments were conducted to assess the parasitoid’s response to host aggregation. A. citrana larvae were released into small patches (individual clusters of fruit) and larger patches (an entire tree) at varying densities. In both the cluster and tree scale experiments the percentage of larvae parasitized by A. aristoteliae remained fairly constant at 40% regardless of the number of host larvae in a patch. These results indicate that A. aristoteliae attacks A. citrana in a density independent manner, which suggests that this parasitoid alone does not exert a strong regulatory effect on summer populations of A. citrana in apples.
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