Effects of genetic and environmental variation on resistance of willow to sawflies

Damage by insect herbivores to neighboring individuals of the shrub Piper arieianum in a neotropical wet forest varies greatly. This differential damage has a genetic basis and results in a 2-year decrease in growth, seed production, and seed viability, with larger plants recovering before smaller plants. The results provide evidence that leaf herbivores represent a potentially strong selective force for the evolution of plant defenses.

A hypothesis originally postulated to explain changes in abundance of sapsucking insects is here extended to an interpretation of changes in abundance of populations of geometrid defoliators.The hypothesis states that most herbivorous insects usually remain at a low level of abundance relative to the apparent abundance of their food because most of them die when very young from a relative shortage of nitrogen in their food.Only occasionally do their food plants become a sufficient source of nitrogen to allow a high proportion of the young insects to survive, and the population to increase to outbreak levels.The plants become a richer source of nitrogen when they are stressed by random fluctuations in the summer and winter rainfall, although other factors such as soil type and topography may contribute to this stress.The hypothesis may well have wider application to population fluctuations of other herbivores-both invertebrate and vertebrate.