| Summary: | Venom compositions include complex mixtures of toxic proteins that evolved to immobilize/dissuade organisms by disrupting biological functions. Venom production is metabolically expensive, and parsimonious use is expected, as suggested by the venom optimisation hypothesis. The decision-making capacity to regulate venom usage has never been demonstrated for the globally invasive Noble false widow <i>Steatoda nobilis</i> (Thorell, 1875) (Theridiidae). Here, we investigated variations of venom quantities available in a wild population of <i>S. nobilis</i> and prey choice depending on venom availability. To partially determine their competitiveness, we compared their attack rate success, median effective dose (ED<sub>50</sub>) and lethal dose (LD<sub>50</sub>), with four sympatric synanthropic species: the lace webbed spider <i>Amaurobius similis</i>, the giant house spider <i>Eratigena atrica</i>, the missing sector orb-weaver <i>Zygiella x-notata</i>, and the cellar spider <i>Pholcus phalangioides</i>. We show that <i>S. nobilis</i> regulates its venom usage based on availability, and its venom is up to 230-fold (0.56 mg/kg) more potent than native spiders. The high potency of <i>S. nobilis</i> venom and its ability to optimize its usage make this species highly competitive against native European spiders sharing the same habitats.
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