Successful spread of a biocontrol agent reveals a biosecurity failure: elucidating long distance invasion pathways for Gonatocerus ashmeadi in French Polynesia

Abstract

Invasive species are generally detected in new ecosystems long after their first arrival, making it difficult to elucidate pathways leading to successful invasion. In this study, the dispersal of a classical biological control agent, the mymarid egg parasitoid Gonatocerus ashmeadi, was monitored across ten islands in three major island groups in French Polynesia from the exact moment of its introduction into Tahiti to combat the invasive pest Homalodiscavitripennis. Within 10 months, the parasitoid spread quickly from Tahiti to widely separated islands (up to 1,400 km from Tahiti); presumably through the transportation of plant material containing parasitized H. vitripennis eggs. Gonatocerus ashmeadi thus functioned as a “biomarker”, providing an informal audit of the effectiveness of inter-island quarantine measures designed to curb the accidental spread of noxious organisms. Survey results suggest that invasive organisms, like deliberately released biological control agents, can be unintentionally and rapidly transmitted across vast distances by humans. Furthermore, even remote islands appear to experience relentless pressure from invasive propagules associated with human travel. Implications of survey work documenting the spread and impact of G. ashmeadi are discussed within the context of biological control programs, non-target impacts, and biosecurity initiatives.