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Many mandibulate insects that feed on milkweeds, or other latex-producing plants, cut leaf veins before feeding distal to the cuts. Vein cutting blocks latex flow to intended feeding sites and can be viewed as an insect counteradaptation to the plant's defensive secretion. Experimental vein severance renders milkweed leaves edible to generalist herbivores that do not show vein-cutting behaviors and ordinarily ignore milkweeds in nature.
Dussourd et al. (Fri,) studied this question.