Background <p>Banned or restricted pesticides in the Global North are still widely used in developing countries, notwithstanding their harmful effects on humans and the environment. A decade and a half ago, the South African government set out to phase out or impose severe restrictions on highly hazardous pesticides, yet many such substances are still traded openly in the country, endangering the lives of users and non-target organisms. The present study evaluated the effects of carbaryl and pymetrozine – two insecticides sold in South Africa – on the survival and behaviour of <i>Culex</i> sp. larvae.</p> Results <p>Mortality assays revealed that the tested concentration range of carbaryl (0, 12.5, 25, 50, and 100&#xa0;µg/L) caused significantly higher mortality compared to that of pymetrozine (0, 6.25, 12.5, 25, and 50&#xa0;mg/L). The calculated median lethal concentrations (LC_50s) for carbaryl (2 808&#xa0;µg/L) and pymetrozine (181.95&#xa0;mg/L) indicated that carbaryl could be several thousandfold more toxic than pymetrozine. Both chemicals, nevertheless, altered the natural behaviour of <i>Culex</i> sp. larvae by causing a significant decrease in breathing (<i>p</i> &lt; 0.05), leading the larvae to spend significantly more time swimming or floating within the water column (<i>p</i> &lt; 0.05). Although less toxic than carbaryl, pymetrozine showed significant potency, at the behavioural level especially, owing to its specificity.</p> Conclusion <p>Our results show why carbaryl, which is banned in the European Union and which exerts excessive toxicity, is an excellent candidate for phasing out, along with many other highly hazardous pesticides found on the South African market. The unrestricted trading of banned or controlled pesticides in markets such as South Africa presents an international environmental challenge that policymakers in developing economies must urgently address.</p>