Specificity of plant induction responses may be important to the interactions between mustards and insect herbivores. This study compared the effects of the cabbage white butterfly, Pieris rapae (L.), cabbage looper, Trichoplusia ni (Hubner), and the mustard flea beetle, Phyllotreta cruciferae (Goeze) on induction of leaf trichome density, sinigrin concentration, and nitrogen concentration in black mustard, Brassica nigra (L.) Koch. Plants were damaged for 12 h at the four-leaf stage, with effort made to standardize the damage applied. Induction responses were measured on the fifth, seventh, ninth, and 11th leaves counted from the cotyledons. Seventh leaves of plants damaged by P. rapae had 76% more trichomes per unit area than controls, whereas equivalent leaves of plants damaged by the other two herbivores exhibited no response. Ninth leaves of plants damaged by T. ni had 113% more trichomes per unit area than controls, whereas equivalent leaves of plants damaged by the other two herbivores exhibited no response. Trichome densities of fifth and 11th leaves did not respond to treatments. Leaf sinigrin and nitrogen concentrations were not affected by the damage treatments. Differential plant trichome response to P. rapae and T. ni may have been due to differences in location of feeding during the damage treatment. Other cues, such as salivary components, may also have differed between the two herbivores. This study is one of the first to document differential effects of two herbivores from the same guild on induction of morphological resistance.