Unexpected diversity in historical biological control programs: Population genetics of the nematode Deladenus siricidicola in Australia and New Zealand

•Australian and New Zealand D. siricidicola populations have unexpectedly high genetic diversity.•A novel lineage of D. siricidicola is dominant in Australia.•New Zealand and North American nematode populations share a common origin.•Genetic patterns may reflect original introductions and selection over biocontrol releases.•Newly found D. siricidicola diversity may improve biological control programs globally.