Shield defense of a larval tortoise beetle

Larvae of the folivorous tortoise beetle, Plagiometriona clavata, carry shi elds formed from feces and exuviae above their bodies. We used an ecologica lly relevant predatory ant, Formica subsericea, in a bioassay to deter mine if shields functioned as simple barriers, as previous studies indicated, o r whether they were chemical defenses. Shields were necessary for larval su rvival; shield removal rendered larvae vulnerable. Shields produced by larv ae reared on a substitute diet failed to provide protection. Solvent-leache d shields also failed to deter ants, indicating the shield had a host-deriv ed chemical component likely located in the feces, not in the exuviae. Sola num dulcamara, the larval host plant, contained free phytol, steroidal glyc oalkaloids, and saponins. Shields contained partially deglycosylated metabo lites of host steroidal glycoalkaloids and saponins, a suite of fatty acids , and derivatives of phytol, which together formed a deterrent barrier agai nst ant attack. We compared the mobile shield of P. clavata to the stationa ry shield of another S dulcamara-feeding leaf beetle, Lema trilinea. Both l arval shield defenses were formed from a very similar array of host-derived compounds with deterrent properties. We concluded that convergent patterns of limited chemical transformation and selective incorporation of particul ar deterrent metabolites in shield defenses of two unrelated taxa represent ed responses to selection from invertebrate predators.