Leaves of Leucosceptrum canumharbor abundant toxic aromatic abietanoids, and they are rarely attacked by insect herbivores, except for the larvae of Nacna malachitis. The excrements of the insect that fed on L. canumleaves were investigated, leading to the isolation and identification of two unprecedented 11,12-seco-abietane diterpene polyenes: nacnabietanins A (1) and B (2). This discovery heralds a unique detoxification mechanism of plant aromatic abietanoids by insects through enzymatic cleavage of stable benzene rings into more easily degraded polyenes.