Later instars of two poplar caterpillar species excrete higher nitrogen content in frass

Herbivores consume large quantities of plant tissues while excreting excess elements to compensate for mismatches between the element content in their food plants and their physiological needs. This energetically costly excretion process decreases the fitness of herbivorous insects, yet how stoichiometric mismatches with food plants vary during their development remains poorly understood. In this study, we reared 4th- and 5th-instar caterpillars of Micromelalopha sieversi (Marumo) and Clostera anachoreta (Denis & Schiffermüller) (both Lepidoptera: Notodontidae) on fresh poplar leaves to investigate changes in carbon: nitrogen ratio in their bodies during development. We measured the weight of the caterpillars and analyzed the carbon (C) and nitrogen (N) contents in their bodies, frass, and consumed plants. Results showed that the 5th-instar caterpillars gained three times more body mass, absorbed and assimilated two times more N and three times more C than the 4th instar. C content in the caterpillar body increased, while N content decreased as the caterpillars developed. Consequently, a greater stoichiometric mismatch with host plants was observed in the 4th instar than in the 5th, as plant quality remained constant throughout development. In addition, N content in frass significantly increased from 4th- to 5th-instar caterpillars, indicating that caterpillars excrete more N in later development stages. Our study clearly revealed the decreasing extent of nutrient imbalance in the later stage of herbivorous insects, highlighting the need to incorporate changes in element contents during organismal development in studies of ecological stoichiometry.