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The Plant Vigor Hypothesis (PVH) predicts that herbivorous insects will preferentially feed on large plants with more vigorous growth, or on more vigorous parts of the plant. However, we still need to understand how these predictions work throughout the ontogenic stages of plants and with other groups of natural plant enemies. We tested the PVH using gall-inducing insects, free-living herbivorous insects, and pathogenic fungi associated with Hymenaea courbaril seedlings of the same age cohort (six months) from seeds germinated in greenhouses. We classified the plants into three different sizes: small, medium, and large seedlings, and then measured above and belowground biomass, plant height, leaf area, and leaf area consumed by free-living herbivores. We also measured nutrient partitioning among roots, stems, and leaves and estimated hypersensitive responses. We found no differences in leaf area consumed by free-living herbivorous insects between plant size groups. However, we observed that larger seedlings were more vigorous and had greater nutritional value and, thus, represented adequate niches for colonization by pathogens and gall-inducing insects, confirming the PVH for both groups. The hypersensitive response was greater in medium and large seedlings, proving to be an effective defense mechanism against gall-inducing insects. Therefore, we highlight that nutritional quality, plant vigor and hypersensitivity response are factors that influence the incidence of pathogens and galling insects on H. courbaril seedlings. [ABSTRACT FROM AUTHOR]

Gall insects select vigorously growing plants and plant parts when initiating gall formation. Vigor is associated with rapid growth rate, and in turn, rapid growth confers competitiveness. Are there conditions under which the cost of vigor, in the form of increased susceptibility to attack, outweighs the benefit of competitive success? I present a simulation model to explore the interaction between susceptibility and competition on the selective advantage of increased growth rate. Assuming size-symmetric competition, the model shows that in general, vigor is favored (benefit > cost) at low to intermediate gall loads. At very high plant densities, however, plants with high gall loads may lose standing in the competitive size hierarchy from which they cannot recover. The details of this result, however, change somewhat when competition is size-asymmetric, that is, when a larger focal plant suppresses smaller neighbors, but smaller neighbors cannot exert a reciprocal effect on the focal. At low densities, the pattern of selection on growth rate is qualitatively similar to the size-symmetric case. At higher plant densities, however, fast-growing genotypes can suppress slow ones so much during the preattack phase that even at the highest gall loads they maintain their standing in the competitive hierarchy. Thus, heavy gall insect attack on vigorous plants can impose selection against high intrinsic growth rates under strong symmetric competition, but not strong asymmetric competition. While life history traits can evolve as a correlated response to selection on defensive traits that reduce susceptibility, this model reveals that susceptibility can evolve as a correlated response to selection on basic life history traits. [ABSTRACT FROM AUTHOR]

Gall-site selection by the aphid Kaltenbachiella japonica was evaluated in relation to leaf position in a shoot, and gall positions within a leaf. First-instar fundatrices induce closed galls on the midribs of host leaves, and several galls were often induced on one leaf. Leaves with many galls were often withered before emergence of sexuparae from the galls. Within a leaf, gall volume was positively correlated with the sum of lateral-vein length in the leaf segment at which the gall was induced. The observed pattern in gall volume among the leaf segments corresponded with that in the lateral-vein length. These results show that a foundatrix selects the most vigorous position within a leaf to produce more offspring. Although distal leaves grew faster than did basal leaves, gall density was highest on leaves at the middle order when a shoot has more than seven leaves. Optimal gall-site selection seems to be constrained by the asynchrony in timing between the hatching of fundatrices and leaf growth within a shoot. These results suggest that the observed gall distribution is affected by both the distribution of suitable galling sites within a leaf and the synchrony with leaf phenology of the host plant. [ABSTRACT FROM AUTHOR]