Ecological Entomology (2020), DOI: 10.1111/een.12935 SHORTCOMMUNICATION Low-temperature physiology of climatically distinct south African populations of the biological control agent Neochetina eichhorniae DANIEL J. R O G E R S, 1 JOHN S. TERBLANCHE 2 and CANDICE A. OWEN 1 1 Centre for Biological Control, Department of Zoology and Entomology, Rhodes University, Grahamstown, South Africa and 2 Centre for Invasion Biology, Department of Conservation Ecology and Entomology, Stellenbosch University, Stellenbosch, South Africa Abstract. 1. Neochetina eichhorniae is the most widely established biocontrol agent on water hyacinth populations around South Africa. However, some N. eichhorniae populations have failed to adequately control their host population, specifcally those exposed to cold conditions. 2. The aim of this study was to determine whether two climatically distinct populations of N. eichhorniae in South Africa differ in their low-temperature physiology, which tests whether local-climate adaptation has occurred. 3. We estimated weevil CT min , LLT 50 , SCP, and SCP mortality using stan- dard approaches. Contrary to expectation based on climatic thermal profles at the two sites, weevils from the warm locality ((mean ± SE) CT min = 5.0 ∘ C ± 0.2, LLT 50 =-11.3 ∘ C ± 0.03, SCP =-15.8 ∘ C ± 0.6) were able to maintain activity and tolerate colder temperatures than the weevils from the colder site (CT min = 6.0 ∘ C ± 0.5, LLT 50 =-10.1 ∘ C ± 0.1, SCP =-12.9 ∘ C ± 0.8). 4. These contradictory outcomes are likely explained by the poor nutrient quality of the plants at the cold site, driving low-temperature performance variation that overrode any macroclimate variation among sites. The cold site weevils may also have adapted to survive wide-temperature variability, rather than perform well under very cold conditions. In contrast, the mass-reared population of insects from the warm site has likely adapted to the consistent conditions that they experience over many years in confnement. Key words. Climate change, cold adaptation, critical thermal limits, Curculionidae, water hyacinth. Introduction Water hyacinth, Pontederia crassipes (Martius) 1823 [≡Eichhornia crassipes (Martius) Solms-Laubach 1883] (Pontederiaceae), is South Africa’s most damaging invasive aquatic weed (Hill, 2003). Biological control can be an effcient long-term management strategy, although its use within South Africa has been met with varied success (Hill, 2003). Correspondence: Candice Owen, Centre for Biological Control, Department of Zoology and Entomology, Rhodes University, P.O. Box 94, Grahamstown, Eastern Cape 6140, South Africa. E-mail: c.owen@ru.ac.za Neochetina eichhorniae Warner 1970 (Coleoptera: Curculion- idae) is one of nine biocontrol agents released onto water hyacinth in South Africa (Hill & Coetzee, 2017). N. eichhor- niae is a small weevil (4–5 mm), native to Argentina, that causes severe damage to P. crassipes, reducing the plant’s ability to retain water and photosynthesise, and increasing susceptibility to pathogen infection (Julien, 2001). The weevil has, however, had diffculties establishing on a minority of P. crassipes infes- tations in South Africa, specifcally those associated with colder conditions and eutrophic waters, which may occur in isolation, or congruently (Hill & Olckers, 2000; Jadhav et al., 2007). To better understand the relationship between thermal physi- ology and environmental conditions, various metrics of thermal performance are employed (Sinclair et al., 2015). Key metrics © 2020 The Royal Entomological Society 1