1. We studied induced defences in common milkweed (Asclepias syriaca) to two specialist caterpillars (Danaus plexippus and Euchaetes egle) to test several hypotheses about co-evolution and defence allocation.
2. Latex, a potent defence of milkweed, showed nearly fivefold variation in the amount exuded among genetic families and more than doubled production in response to herbivory by D. plexippus; this induced latex response was fourfold stronger than the latex response to E. egle herbivory. In contrast, D. plexippus and E. egle equivalently induced foliar cardenolide concentration (an average increase of 26%).
3. We found broad-sense heritable variation for the induced plant responses, and there was a genetic correlation between the responses induced by the two herbivores for cardenolides, but not for latex. We found no genetic correlation between investment in latex and cardenolides in any of the treatments.
4. Using a bias-corrected Monte Carlo procedure, we found strong evidence for a trade-off between constitutive and induced cardenolides, but not for latex.
5. Synthesis. As natural selection by specific herbivores varies in space and time, we expect that these defence traits in A. syriaca will evolve independently, with the expression of cardenolides showing less specificity, as well as being constrained by a trade-off between constitutive and induced defence.