Geographic origins and post-introduction hybridization between strains of Trioxys pallidus introduced to western North America for the biological control of walnut and filbert aphids

• Trioxys pallidus was introduced to North America as a biological control agent.
• Recent breakdowns in biological control services have been reported.
• The three introductions included three distinct genetic clusters of T. pallidus.
• Hybridization between strains was observed in the field, though at very low levels.
• Strains specializing on filbert and walnut aphid may represent cryptic species.

Strains of the exotic parasitoid wasp Trioxys pallidus were introduced on multiple occasions to the western United States resulting in successful classical biological control of aphids in walnut and filbert orchard systems. However, breakdowns in biological control services have been reported in recent years in both orchard systems. To aid in potential plans to stabilize these important biological control programs we investigated the individual origins of each strain that was introduced to and established in the western United States, and examined whether post-introduction hybridization between strains has occurred. We collected individuals from the native and introduced ranges of T. pallidus, performed genetic clustering analyses using 15 polymorphic microsatellite loci, and reconstructed a phylogeny and estimated haplotype networks based on the mitochondrial (mtDNA) locus cytochrome oxidase I (COI). Based on microsatellite loci we find that T. pallidus individuals can be assigned to one of three distinct genetic clusters, and based on mtDNA haplotypes, to one of two distinct clades/networks. The majority of individuals from California were assigned to the same genetic cluster as individuals collected in Iran, also with shared mtDNA haplotypes, while the majority of individuals from Oregon were assigned to a genetic cluster represented by only a single individual collected near Bordeaux, France. Although we found some evidence of hybridization between the strains, the rates were so low that hybrid-breakdown is unlikely to be responsible for recent failures of biological control services in the western United States. We did find elevated levels of inbreeding within established populations, however, suggesting that inbreeding depression may have played an important role in these recent breakdowns.

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