Benefits of pre-release population genetics: a case study using Psyttalia lounsburyi, a biocontrol agent of the olive fruit fly in California

Authors: BON, MARIE-CLAUDE - European Biological Control Laboratory (EBCL); Smith, Lincoln; KENT, DAANE - University Of California; PICKETT, CHARLIE - California Department Of Food And Agriculture; WANG, XINGENG - University Of California; BLANCHET, ARNAUD - European Biological Control Laboratory (EBCL); GUERMACHE, FATIHA - European Biological Control Laboratory (EBCL); Hoelmer, Kim

Submitted to: International Symposium on Biological Control of Arthropods
Publication Type: Proceedings
Publication Acceptance Date: 7/3/2017
Publication Date: 9/11/2017
Citation: Bon, M., Smith, L., Kent, D., Pickett, C., Wang, X., Blanchet, A., Guermache, F., Hoelmer, K.A. 2017. Benefits of pre-release population genetics: a case study using Psyttalia lounsburyi, a biocontrol agent of the olive fruit fly in California. Proceedings of the 5th International Symposium on Biological Control of Arthropods, September 11-15, 2017. Langkawi, Malaysia. p. 3.

Interpretive Summary: The olive fruit fly (Bactrocera oleae) is an important invasive pest of olives in California that first appeared in 1998. A biological control project discovered the parasitoid, Psyttalia lounsburyi, which kills larvae of the fly. Two populations of the parasitoid, one from Kenya and one from South Africa, were released in California from 2005 to 2015, and it started to become established at sites since 2011. Molecular genetic analysis showed that these two populations can be distinguished because they have different haplotypes of the CO1 marker. Our results from populations sampled in California show that the South Africa strain is established at 20 sites whereas the Kenya strain is at only 5 sites of 25 sites where the parasitoid has been recovered. This was surprising because more than 96% of the parasitoids that were released came from Kenya. Thus it appears that the South Africa population is better adapted to most of California and future releases will use this population. Successful biological control will help reduce the need to apply insecticides, increase the profitability of olive production and reduce exposure of people and the environment to pesticides.

Technical Abstract: From a pest management perspective, limited knowledge on the genetics of released biocontrol agents has been repeatedly considered as one possible cause of failures in classical biological control. Introduced biocontrol agents are expected to experience a loss in genetic diversity as the result of severe population bottlenecks at different stages of biocontrol programs such as sampling during foreign exploration, laboratory rearing, and field releases. Diversifying the geographical origins of the biocontrol agent is one way to counterbalance this effect by sampling more pre-adapted genotypes and/or producing new genotypes through hybridization. In this context, conducting pre-release genetic studies to assess the genetic variation of the targeted agent at the population level is considered to be of prime importance for guiding the choice of the strains to be released. Psyttalia lounsburyi (Hymenoptera: Braconidae), is considered as the most promising parasitoid of the olive fruit fly (Bactrocera oleae), which is a pest in California threatening the state’s table and oil olive industries since its discovery in 1998. Two P. lounsburyi colonies were established with parasitized B. oleae collected from wild olives in Kenya (Burguret) in 2002, 2003, and 2005, and South Africa (SA) in 2005. A field release permit for P. lounsburyi colonies of both origins was granted late 2005 and releases started in northern California in 2005. From 2005 to 2015, a total of 51,600 females were released (Wang, unpubl. data). P. lounsburyi was recovered from field samples and is now established in coastal regions and hence one logical follow up was to identify the origin of the established populations. The natural populations of P. lounsburyi from Kenya and SA that were colonized in the lab belong to two different mitochondrial lineages, each population harboring only two haplotypes of CO1. Since the onset of the mass rearing of the quarantine colonies, and over the years, a sampling of each colony was preserved for genetic analysis. The Burguret and the SA colonies were characterized by one private haplotype providing a simple molecular marker for distinguishing between mass reared colonies. We analyzed the variation of CO1 in the 25 populations recovered in California and compared them to the colonies currently mass reared at EBCL. We found the haplotype corresponding to the SA colony in 20 out of the 25 populations recovered, and it was widely distributed from North California (San Anselmo) to South California (Buellton). The predominance of the SA haplotype in California was quite surprising considering that only 3.9% of the total number of female parasitoids released were of SA origin (Wang, unpubl. data). This result also suggests that the SA population was better adapted than the Kenyan populations to the ecological conditions found in Central and Northern California. Differences in life history traits might be considered as one factor; however, it is unknown what factors may improve fitness in this region. One consequence of the present study was to shift emphasis towards mass rearing and field releasing the SA colony in California, especially in Northern California.