SURVIVAL PATTERNS AND MORTALITY SOURCES OF HERBIVOROUS INSECTS - SOME DEMOGRAPHIC-TRENDS

Five hundred thirty life tables for 124 holometabolous, herbivorous insects were compiled from the literature and studied to uncover demographic patterns. Survival and sources of mortality of the preadult stages were correlated with their ecological setting, and several patterns emerged. Exophytic herbivores suffer a 5%-10% greater risk of preadult mortality than endophytics but are also nearly twice as large (pupal length, 11.8 mm vs. 6.4 mm; adult body length, 13.5 mm vs. 5.5 mm) and twice as fecund (206.1 vs. 99.3 eggs per female). Attack by natural enemies is the most frequent mortality source for immature herbivores (frequency = 48%). Plant factors are not nearly as frequent (9%) but have probably been underestimated in past life tables. The influence of plant factors is greatest in the early stages, and the influence of enemies is greatest in the later stages of development. Plant factors kill endophytic species more frequently than exophytic species (frequencies = 15% vs. 2%), whereas enemies kill exophytic species more frequently than endophytic species (frequencies = 51% vs. 44%). Most of the above patterns were consistent with conventional wisdom; the following were not. The frequency of competition and enemies did not vary with latitude or successional status of the habitat. Also, successional status, cultivation, and invasion status of the herbivore had no direct effects on herbivore survival. All of the patterns documented here should be viewed as guidelines for hypothesis testing with the understanding that much unexplained variation in survival exists among herbivorous insect species.