Reduced Performance of Two Specialist Herbivores (Lepidoptera: Pieridae, Coleoptera: Chrysomelidae) on New Leaves of Damaged Black Mustard Plants

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Abstract
This study addressed whether prior damage to black mustard, Brassica nigra (L.) Koch, would reduce growth, herbivory, or mortality of two specialist herbivores on new leaves. Plants received either no initial damage or 12 h of feeding by two third instars of Pieris rapae (L.) or 50 adult Phyllotreta cruciferae (Goeze) when plants had four leaves. Later, the seventh leaf of plants was either harvested for measurement of trichome density and glucosinolate concentration or enclosed in a mesh cage containing two neonate P. rapae or 10 adult P. cruciferae. Caged herbivores were measured for mass gain, leaf consumption, and mortality after 1 wk. Damage by P. rapae caused substantial increases in trichome density and sinigrin concentration, whereas damage by P. cruciferae had no effect. Larvae of P. rapae grew 30% more slowly on plants initially damaged by conspecifics than on control plants. Percent herbivory by P. rapae was 33% lower on plants initially damaged by either P. rapae or P. cruciferae than on control plants. Growth rate and percent herbivory by P. cruciferae were not generally affected by prior plant damage. However, mortality of P. cruciferae was 84% higher on plants previously damaged by conspecifics than on control plants. Together, the data demonstrate that induction responses negatively affected both Pieris and Phyllotreta and suggest that trichomes may be relatively important in the increased resistance. Test herbivores generally performed similarly on plants damaged by either herbivore, suggesting a low specificity of effect for the induction response.