Animal venoms are fine-tuned by millions of years of evolution and dominated by potent and selective proteinaceous toxins that are metabolically expensive to produce. In contrast, zinc is a common environmental element and a non-selective allosteric modulator of various ion channels. We screened 85 arthropod venoms and found zinc content of up to 6.3% w/w. Zinc alone was weakly insecticidal to sheep blowflies but synergised with a range of insecticidal spider-venom peptides independently of their molecular mechanism. We propose that the weaponisation of zinc leads to a broadly weakened neurotransmission across multiple transmitter systems rendering insects more susceptible to a lethal blow by target-selective insecticidal toxins. This zinc/peptide co-application strategy provides an evolutionary advantage to venomous organisms that utilise zinc by reducing the metabolic cost of peptide-toxin biosynthesis and could be utilised in agriculture to enhance insecticidal peptide efficacy and cost-effectiveness.