Seasonal oophagy by generalist grasshoppers helps regulate the population of a specialist herbivore

Background/Question/Methods

Herbivore populations are often more limited by natural enemies than by food sources. While tritrophic interactions typically describe a relatively ordered world of a base food layer varying in quality and abundance, a layer of primary consumers feeding on the base, and a tier of predators picking among the primary consumers, omnivores can greatly add to the complexity of such interactions. The Solanaceae specialist herbivore Lema daturaphila primarily consumes Physalis spp. in northern Virginia and is the main herbivore of its host. Over the summer, as its host plant becomes more plentiful, Lema populations decline dramatically. While generalist predators and parasitoid flies exert increasing mortality on Lema larvae over the summer, there is also a substantial loss of Lema egg clutches, leading us to examine seasonal egg predation on Lema. During the summers of 2011-2012 we placed cluctches of Lema eggs on Physalis longifolia and monitored clutch survival with daily surveys and video cameras. We also carried out lab observations of grasshoppers presented with Lema eggs. 

Results/Conclusions

Weekly egg loss ranged from less than 10% in June to over 80% in mid July. Various egg consumers were witnessed in the field, including mites and lacewings, but the main consumers caught on video were grasshoppers (Melanoplus spp., including M. bivittatus) that are usually considered herbivores. In 2011-2012, 10 of 14 predation events caught on video were by Melanoplus spp., plus two additional orthopteran predation events (nocturnal Oecanthinae and diurnal Tettigoniidae). In lab experiments we found that both early and late instars of Melanoplus fed on Lema eggs, but that later instars were much more likely to consume eggs, potentially as an extra protein source for grasshoppers reaching sexual maturity. We found that Melanoplus consumed small amounts of Physalis foliage relative to clover, but that egg consumption was not related to either the amount of foliage consumed or to the presence of more preferred, higher nitrogen foliage. Overall, we found that Lema egg consumption in the wild increased as grasshoppers reached later instars and that grasshoppers, which do not particularly like Physalis foliage, appeared to be the main consumers of Lema eggs placed on Physalis foliage