Do fecal shields provide physical protection to larvae of the tortoise beetles Plagiometriona flavescens and Stolas chalybea against natural enemies?

During their larval phase, Plagiometriona flavescens (Boheman, 1855) and Stolas chalybea (Germar, 1824) (Chrysomelidae: Cassidinae), carry masses of feces and exuviae on their back called fecal shield, and it is suggested to protect larvae against natural enemies. We investigated if the physical barrier provided by the shield plays any role in the defense of these larvae. We conducted a field experiment checking the mortality of larvae of both species with their natural shields substituted by an artificial shield, with shields removed and with their shields intact. Mortality controls for each of the 3 shield treatments were carried out on host plants protected against natural enemies. On both species we observed that larvae with their shields intact had a significant lower mortality proportion than larvae with artificial shields or without shields. Control larvae on protected plants had low mortality. Our results agree with literature data, showing that fecal shields do not provide a physical protection to larvae but are important in their defense, probably due to the chemicals present in them.