Caterpillar guts and ammonia volatilization: retention of nitrogen by gypsy moth larvae consuming oak foliage

The gypsy moth (Lymantria dispar L.), a major defoliator of hardwood forests in the eastern U.S., has a highly alkaline midgut pH. We hypothesized that the high pH would cause high rates of ammonia (NH₃) volatilization as larvae consumed foliage, leading to potentially large losses of N from the ecosystem to the atmosphere during gypsy moth outbreaks. We measured NH₃ emission during the consumption of oak foliage by larvae in the laboratory. Surprisingly, we found very low amounts of NH₃ release of about 0.1% of the N consumed in foliage. We speculate that digestive mechanisms may limit NH₃ production in the midgut, and that the acidic environment of the hindgut traps most of the small amount of NH₃ that is produced, effectively preventing a potentially very large N loss from both larvae and ecosystem. The estimated rate of NH₃ emission from a defoliated forest is small compared to other inputs and outputs of N from the ecosystem, but could potentially enhance the neutralization of atmospheric acidity during the defoliation period.