Comparative genetics of invasive populations of walnut aphid, Chromaphis juglandicola, and its introduced parasitoid, Trioxys pallidus, in California

Abstract

Coevolution may be an important component of the sustainability of importation biological control, but how frequently introduced natural enemies coevolve with their target pests is unclear. Here we explore whether comparative population genetics of the invasive walnut aphid, Chromaphis juglandicola, and its introduced parasitoid, Trioxys pallidus, provide insights into the localized breakdown of biological control services in walnut orchards in California. We found that sampled populations of C. juglandicola exhibited higher estimates of genetic differentiation (FST) than co‐occurring populations of T. pallidus. In contrast, estimates of both the inbreeding coefficient (GIS) and contemporary gene flow were higher for T. pallidus than for C. juglandicola. We also found evidence of reciprocal outlier loci in some locations, but none showed significant signatures of selection. Synthesis and applications. Understanding the importance of coevolutionary interactions for the sustainability of biological control remains an important and understudied component of biological control research. Given the observed differences in gene flow and genetic differentiation among populations of T. pallidus and C. juglandicola, we suspect that temporary local disruption of biological control services may occur more frequently than expected while remaining stable at broader regional scales. Further research that combines genomewide single nucleotide polymorphism genotyping with measurements of phenotypic traits is needed to provide more conclusive evidence of whether the occurrence of outlier loci that display significant signatures of selection can be interpreted as evidence of the presence of a geographic mosaic of coevolution in this system.

Genetics reveals shifts in reproductive behaviour of the invasive bird parasite Philornis downsi collected from Darwin’s finch nests

“…Therefore, our results may be biased towards a higher F IS ; although values of our 10% dataset are in line with those found on Floreana by Koop et al (2020;F IS = 0.19). Although high, our reported F IS was also consistent with estimates for P. downsi on other islands, on the South American mainland (Koop et al 2020), and with other invasive insect populations (Karsten et al 2013;Kirk et al 2013;Andersen and Mills 2018;Do et al 2022).…”

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“…However, in Korea C. juglandicola has been found also on other Juglans species, e. g. J. mandshurica and J. sinensis (Lee et al 2018). Presently, C. juglandicola and P. juglandis have Eurasian distribution, and both species were introduced in North America (Sluss 1967;Alford 2007;Andersen, Mills 2017;Wieczorek et al 2019). C. juglandicola has also been introduced in Africa (Algeria, Canary Islands and Tunisia) and South America (Argentina and Chile) (Salas et al 2016;CABI 2017).…”