Nutritional quality of specific leaf tissues and selective feeding by a specialist leafminer

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Abstract
Many folivorous insects are selective feeders which consume specific leaf tissues. For specialist herbivores feeding on plants of overall low nutritional quality, selective feeding may allow consumption of a high quality resource. Selective feeding may also allow insects to avoid structural or allelochemical defenses. We examined the structure and chemistry of leaves of American holly, Ilexopaca Aiton, and the feeding site of its principal insect herbivore, the native holly leafminer, Phytomyza ilicicola Loew (Diptera: Agromyzidae), to test the hypothesis that the leafminer consumes tissues which are of greater nutritional quality than the leaf as a whole. Holly leaves have a continuous layer of palisade mesophyll, uninterrupted by fibers or vascular bundles. The leafminer feeds entirely within this layer. The palisade mesophyll contained 196 mg/g dry wt extractable protein, more than twice as much as the leaf as a whole, and 375 mg/g dry wt saponins, more than 9 times that of the leaf as a whole. The water content of the palisade mesophyll was 66% higher than that of the leaf as a whole. The palisade mesophyll is 3–4 cell layers thick in leaves grown in full sun, but only 2 layers thick in shaded leaves. Crystals, probably of calcium oxalate, are abundant in the abaxial cell layer. These may impose mechanical constraints on larval feeding in shade leaves, which are thinner than sun leaves. Selective feeding on the middle palisade mesophyll of sun leaves allows the leafminer to consume a resource which is lacking in mechanical barriers and is rich in protein and water, but which contains large amounts of saponins.