Sabotaging milkweed by monarch caterpillars is a textbook example for disarming plant defence. By severing leaf veins, monarchs are thought to prevent toxic latex flow to their feeding site. Here we show that sabotaging by monarch caterpillars is not an avoidance strategy. Instead, caterpillars actively ingest outflowing latex to increase sequestration of toxic latex cardenolides. Comparisons with caterpillars of the related non-sequestering common crow butterfly revealed three lines of evidence supporting our hypothesis. First, monarchs sabotage inconsistently and therefore the behaviour is not mandatory to feed on milkweed, while sabotaging in crows precedes every feeding event. Second, monarchs eagerly drink latex, while crow caterpillars spit out latex during sabotaging. Third, monarchs raised on detached leaves sequestered more cardenolides when latex was supplemented artificially. Hence, we conclude, that monarchs converted the ″sabotage to avoid″ strategy of their relatives into a ″sabotage to consume″ strategy for acquiring toxins for defence.