Herbivorous insects that have similar means of exploiting plant parts for food can be classified into feeding guilds or functional groups. Groups of plant-feeders include chewers that consume foliage, stems, flowers, pollen, seeds, and roots; miners and borers that feed between plant surfaces; gall-formers that reside and feed within the plant and induce the production of abnormal growth reactions by plant tissues; sap-suckers that siphon plant fluids; and seed predators and frugivores that consume the reproductive parts of plants (Romoser and Stoffalano 1998). Some species, such as seed predators, seedling-eaters, and tree-killing bark beetles, are true plant predators, but most herbivores function as plant parasites because they normally do not kill their hosts, but instead feed on the living plant without causing death (Price 1980). These different modes of consumption affect plants in different ways. For example, folivores (species that chew foliage) directly reduce the area of photosynthetic tissue, whereas sap-sucking insects affect the flow of fluids and nutrients within the plant and root-feeders reduce plant capacity to acquire nutrients or remain upright.
Folivory is the best-studied aspect of herbivory. In fact, the term herbivory often is used even when folivory alone is measured because loss of foliage is the most obvious and easily quantified aspect of herbivory. The loss of leaf area can be used to indicate the effect of herbivory. In contrast, other herbivores such as sap-suckers or root-borers cause less conspicuous losses that are more difficult to measure. Nonetheless, Schowalter et al. (1981c) reported that calculated loss of photosynthates to sap-suckers greatly exceeded measured foliage loss to folivores in an early successional deciduous forest. Sap-suckers and root-feeders also may have long-term effects (e.g., through disease transmission or altered rates of nutrient acquisition or growth) (J.P. Smith and Schowalter 2001).
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