Accidental introduction of Candidatus Liberibacter europaeus into New Zealand via a weed biocontrol agent from the UK

• Published in: Biological control 2021-09, Vol.160, p.104697, Article 104697
• Main Authors: Fowler, Simon V., Lange, Claudia, Beard, Sam, Cheeseman, Dagmar F., Houliston, Gary J., Paynter, Quentin, Peterson, Paul, Pitman, Andrew, Smith, Lindsay, Tannières, Mélanie, Thompson, Sarah, Winks, Chris
• Format: Article
• Language: English
• Subjects: Arytainilla spartiophila; Broom psyllid; Candidatus Liberibacter europaeus; Cytisus scoparius; Ornamental broom; Scotch broom
• ISSN: 1049-9644; 1090-2112
• DOI: 10.1016/j.biocontrol.2021.104697

[Display omitted] •NZ surveys with new primers show Leu absence coincides with broom psyllid absence.•Leu likely entered NZ in 1993 with psyllids released for biocontrol of broom.•Other entry routes/maintenance mechanisms for Leu in NZ were improbable.•We advise molecular screening of biocontrol agents that may vector plant pathogens. The bacterium, Candidatus Liberibacter europaeus (Leu) was reported in New Zealand (NZ) in 2011 in Scotch broom, Cytisus scoparius Link, and its biocontrol agent, Arytainilla spartiophila Förster (Hemiptera: Psyllidae). The aim of the study was to investigate the pathway by which Leu arrived in NZ using a retrospective risk assessment of Leu being accidentally imported with the broom psyllids in the 1990s. We developed species-specific primers for Leu detection and carried out end-point and real-time PCR analyses for Leu in potential host plants and broom psyllids throughout NZ to test two hypotheses: 1) Leu was accidentally released in NZ in broom psyllids in 1993; or 2) Leu was present in NZ pre-1993, and maintained in cultivated plants via cuttings/grafting, or in wild Scotch broom populations via shared parasitic plants. There was strong evidence to support the arrival of Leu with broom psyllids in NZ. Firstly, methods used to import broom psyllids from the UK provided a likely pathway for inadvertent release of Leu, which at the time was unknown to science, into NZ. Secondly, Leu in NZ Scotch broom plants was significantly associated with the presence of broom psyllids, with Leu absent from plants at sites without psyllids. In contrast, there was no evidence suggesting Leu was present in NZ before the release of broom psyllids: Leu was not found in ornamental brooms or pears, Pyrus communis L. (a Leu host in Europe), and parasitic plants were too uncommon to maintain Leu in wild Scotch broom. In addition, Leu in UK Scotch broom and broom psyllids has a matching partial 16S sequence to NZ Leu. It remains uncertain whether Leu is pathogenic in Scotch broom, or symptomless as in pears. Unpredictable impacts of Ca. Liberibacter spp. in different plant species probably implies that psyllids considered as future weed biocontrol agents will need to be free of these potential plant pathogens. This accidental introduction of Candidatus Liberibacter europaeus into NZ via a weed biocontrol agent happened because detection methods for unculturable organisms were not developed at the time. Molecular characterisation of insect microbi