Abstract
Scirtothrips aurantii is a generalist horticultural pest in its native African range and recently established quite widely in Australia on the invasive succulent weed Bryophyllum delagoense. Paradoxically, this thrips is not polyphagous in its incursive range. The issue is principally one of quarantine. Will the thrips in Australia shift, perhaps adaptively, to citrus, and should the primary focus be on containment around Australian citrus, or does the real quarantine risk exist offshore with thrips present on citrus in Africa? We examined the phylogenetic relationships between Bryophyllum-associated thrips populations in Australia and populations sampled from various host plant species in South Africa (including Bryophyllum) using both CO1 and 28s markers. Eight variable microsatellite markers were developed to assess the extent of gene flow between the thrips on different hosts in South Africa. The COI phylogeny resolved S. aurantii into three distinct clades with samples collected from B. delagoense in South Africa and Australia representing a single clade, a second clade associated with Gloriosa lilies and the third with horticultural hosts. The microsatellite analysis confirmed that the populations associated with citrus and Bryophyllum do not hybridize with one another in sympatry. We conclude that the citrus-damaging thrips are not currently present in Australia and remain a serious quarantine concern in relation to Australian horticulture.
Highlights
Effective quarantine is crucial to agriculture, trade and the environment
COI and 28S phylogenetic analyses The 28S phylogeny resolved two monophyletic clades, each with 100% bootstrap support within the species currently defined as S. aurantii (Fig. 2)
Thrips associated with Gloriosa superba formed a single clade with a mean pairwise nucleotide difference of 3.3% from the clade associated with B. delagoense and citrus (Fig. 2)
Summary
Effective quarantine is crucial to agriculture, trade and the environment. With respect to agriculture and trade, polyphagous species present extremely serious threats, especially if they have pest status (Boykin et al 2012). Though, that not all individuals within generalist taxa achieve the full ecological amplitude attributed to that taxon [e.g. Bemisia tabaci, Liriomyza trifolii and Copitarsia decolora (Simmons and Scheffer 2004; Scheffer and Lewis 2006; Dinsdale et al 2010)]. Understanding the source of such variation is crucial, to understanding the ecology and evolution of generalist taxa, but to understanding quarantine risk more completely. We examine the paradox of a pest species that is a putative generalist that has restricted host use in its invasive range. Our objectives are to (i) clarify the nature of the quarantine risk posed by the putative generalist and (ii) to assess the importance of an evolutionary approach to quarantine
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