Does climate constrain the spread of Anthonomus santacruzi, a biological control agent of Solanum mauritianum, in South Africa?

• Climate analyses revealed that Highveld sites were poorly suited to A. santacruzi establishment.
• Cold stress to A. santacruzi adults is likely to occur at temperatures below 4.1 °C.
• Prolonged humidities below ∼47% will hinder the longevity of A. santacruzi adults.
• This study highlights the importance of climate assessments in biological control research.

The flowerbud-feeding weevil, Anthonomus santacruzi Hustache (Coleoptera: Curculionidae), was released in 2008 as a biological control agent against the South American invasive tree Solanum mauritianum Scop. (Solanaceae), in South Africa. The weevil has since established in two localities, namely along the KwaZulu-Natal South Coast and around the Sabie region of Mpumalanga. Recent releases on the Highveld region (Johannesburg and Pretoria) have been unsuccessful to date. This study aimed to test the hypothesis that climate, particularly low temperature and low relative humidity, restricts the survival and establishment of A. santacruzi in South Africa. The survival of A. santacruzi adults in relation to low temperatures and relative humidities was assessed in the laboratory. Climate analyses revealed that matches of <50% were associated with failure to establish and that minimum temperature and relative humidity displayed the greatest discrepancies between South American collection and South African release sites. Thermal assessments on A. santacruzi adults calculated the CTmin and LT50 as 4.1 ± 0.2 °C (n = 20) and −4.2 ± 0.3 °C (n = 90) respectively. The LH50 of A. santacruzi adults was calculated as 46.9%. The establishment of A. santacruzi at only the warm and humid release sites in South Africa advocates for the consideration of low temperature and low humidity as factors impeding the agents’ establishment, particularly on the cooler and drier Highveld.

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