Larval Beetles Form a Defense from Recycled Host-Plant Chemicals Discharged as Fecal Wastes

Larvae of the leaf-feeding beetles Neolema sexpunctata and Lema trilinea carry feces on their backs that form shields. We used the generalist predatory ant, Formica subsericea, in a bioassay to determine whether shields were a physical barrier or functioned as a chemical defense. Fecal shields protected both species against ant attack. Larvae of both species reared on lettuce produced fecal shields that failed to deter ants. Commelina communis, N. sexpunctata's host, lacks noxious secondary compounds but is rich in phytol and fatty acids, metabolites of which become incorporated into the fecal defense. In contrast, the host plant of L. trilinea, Solanum dulcamara, contains steroidal glycoalkaloids and saponins, whose partially deglycosylated metabolites, together with fatty acids, appear in Lema feces. Both beetle species make modifications to host-derived precursors before incorporating the metabolites into shields. Synthetic chemicals identified as shield metabolites were deterrent when applied to baits. This study provides experimental evidence that herbivorous beetles form a chemical defense by the elimination of both primary and secondary host-derived compounds. The use of host-derived compounds in waste-based defenses may be a more widely employed strategy than was hitherto recognized, especially in instances where host plants lack elaborate secondary compounds.