Predators, Parasitoids, and Pathogens as Mortality Agents in Phytophagous Insect Populations

We compiled life tables for 78 holometabolous herbivorous insect species to quantify levels of apparent enemy-induced mortality of immature insects. Enemies were classified by type (predator, parasitoid, or pathogen), and mortalities caused by each type in each herbivore immature stage were tested with Analysis of Deviance for differences associated with four ecological characteristics of preadult herbivores: feeding biology, invasion status, and the cultivation status and latitudinal zone of the habitat. Total enemy-induced mortality is higher in the late developmental stages, and overall, parasitoids kill more herbivores than do either predators or pathogens. Among the ecological variables, both feeding biology and latitude showed significant enemy effects in at least one late developmental stage, whereas neither cultivation status nor invasion status was associated with enemy-induced mortality in any stage. Bonferroni adjustment of probabilities for multiple comparisons resulted in few significant interactions between enemy type and the ecological variables. However, raw probabilities and comparisons across herbivore immature stages suggest several patterns that deserve attention in future studies: (1) endophytic herbivores suffer lower mortality by predators and pathogens than exophytics, and endophytic leaf miners suffer the greatest parasitoid-induced mortality, while endophytic gallers/borers/root feeders suffer the least; (2) overall enemy-induced mortality is similar in natural and cultivated habitats; (3) exotic insects do not suffer lower enemy-induced mortality rates than natives; and (4) predation and disease may be greater in tropical/subtropical habitats, whereas parasitism is greater in the temperate zone. These results identify several general patterns in insect demographics that should be useful for hypothesis testing.