Genetic assessment and performance evaluation of Lixus carinerostris Boheman (Curculionidae)

Abstract

Invasive alien plant species present a major threat to global biodiversity, ecosystem function, and economic stability. Cryophytum crystallinum L. (Aizoaceae) (Crystalline ice-plant), native to South Africa, has become invasive in California (U.S.A.), Mexico, Australia, and the Mediterranean, where it causes ecosystem degradation by increasing environmental salt concentration, displacing native species, and altering habitat structure. Classical biocontrol could offer a sustainable management strategy, but success depends on the proper characterisation of potential biocontrol agents. This thesis evaluated Lixus carinerostris Boheman (Coleoptera: Curculionidae), a stem mining weevil associated with C. crystallinum in its native range, as a potential biocontrol agent. This thesis addressed three main objectives: 1. Determine whether L. carinerostris represents a single oligophagous species or a cryptic species complex across its four known Cryophytum host plant species (C. crystallinum, C. guerichianum, C. pellitum, and C. barklyi); 2. Characterise the genetic structure of L. carinerostris populations; and 3. Assess weevil performance across all four host plant species. Cytochrome oxidase subunit I analyses (69 specimens, 11 localities, 4 host plant species) revealed that L. carinerostris was a single panmictic species with low genetic diversity (0.77% nucleotide diversity) and no host plant species-specific or geographic structuring. Nuclear Inter-Simple Sequence Repeats analyses revealed that weevils associated with C. pellitum are genetically distinct from the other populations, while populations collected off other host plants showed significant genetic admixture. Phylogenetic analysis confirmed L. carinerostris to be a single species while also highlighting taxonomic inconsistencies within the broader Lixus genus. The absence of isolation by distance and high gene flow demonstrates strong dispersal capabilities across the three Cape provinces. The field performance assessments revealed a mismatch between genetic homogeneity and developmental success across the four weevil/plant associations. Adult L. carinerostris demonstrated non-discriminatory feeding and oviposition behaviour. However, developmental outcomes varied considerably. Cryophytum crystallinum supported significantly higher pupal densities and adult emergence, C. guerichianum exhibited intermediate success, while C. pellitum and C. barklyi showed significantly reduced developmental success despite supporting larval establishment. This pattern exemplifies a partial evolutionary trap phenomenon, where oviposition efforts do not predict offspring performance. The genetic differentiation observed in weevils associated with C. pellitum suggests that source populations for biocontrol should be collected preferentially from C. crystallinum, with C. guerichianum-associated populations serving as an alternative if needed, while C. pellitum-associated populations should be completely avoided. The results suggest that C. crystallinum is L. carinerostris’s primary reproductive host plant species, supporting its potential as a biocontrol agent. The absence of other Cryophytum species in invaded regions minimises potential non-target risks to congeners. However, the resilience on a single host plant species in invaded regions necessitates careful consideration of plant phenology for successful weevil establishment. This research provides important baseline data for evidence-based biocontrol decision-making and emphasises the importance of integrating population genetics with ecological performance assessments in agent evaluation programs.