THE PLANT VIGOR HYPOTHESIS AND HERBIVORE ATTACK

Four sources of evidence are used to support the Plant Vigor Hypothesis that many herbivore species feed preferentially on vigorous plants or plant modules, as opposed to the Plant Stress Hypothesis arguing that stressed plants are beneficial to herbivores. Evidence includes patterns of within-plant species utilization by galling insects where females select large plant modules and larvae survive better than on smaller modules. Independent evidence concerns patterns of herbivory between plant species habitually growing in rich resource environments which are heavily utilized. compared with low-resource adapted species. A third source of evidence is the forestry literature in which many cases are known of most attacks by insect herbivores occurring on young and open-grown trees. Finally the similarities in exploitation of plants by moose and galling insects are compared and are accounted for by the common responses to vigorously growing plant parts. The Plant Vigor Hypothesis applies particularly well to insect herbivores most closely associated with plant growth processes, the endophytic gallers and shoot borers. Consequences of vigorous module utilization include resource regulation, or the utilization of resources such that resource quality is maintained, concentration of feeding in early successional vegetation, among insect herbivores an increased probability of competition among females for oviposition sites as opposed to larval competition, the evident lack of induced defenses in plants in some cases, and general problems with defending rapidly growing modules. Complications with the Plant Vigor Hypothesis include in certain instances the possible role of early induction of plant defenses and subsequent increased resistance with age and the pattern in some cases of herbivores preferring vigorous plant modules on plants in drier sites over those in adjacent wetter sites.