The response of sunflower (Helianthus annuusL.) to feeding damage by insects and to mechanical damage was studied in the leaves of the plant. One class of chemicals found to accumulate after stress, the coumarins, was quantified by analysis with HPLC. Sunflowers were damaged by sunflower beetle,Zygogramma exclamationis(F.), flower thrips,Frankliniella tritici(Fitch), or by mechanical puncture. Scopoletin concentrations were 252μg/g fresh weight at 22 d and ayapin concentrations were 165μg/g at 25 d, in beetle-damaged sunflowers. The scopoletin concentration was three times higher and the ayapin concentration five times higher than that in control sunflowers (P<0.05). Thrips induced significantly higher scopoletin concentrations at 11 d and higher ayapin concentrations at 16 d after feeding began (P<0.05). Mechanical damage induced significantly higher scopoletin concentrations 11 d after the damage was inflicted. The initial type of induced response in sunflower was of the “halo type”; that is, highest in successive concentric circles approaching the wound site. A 3-mm puncture induced coumarins within a diameter of at least 1 cm at 1 wk. A systemic response may have begun after the first week. To study the effects of coumarin on host acceptability to the sunflower beetle, coumarins were applied to unstimulated sunflower leaf disks. Beetles chose between treated or control disks in a closed arena. The control disks were preferred over treated disks when 20μg/ml of scopoletin or coumarin and 100μg/ml of ayapin or scopoletin were sprayed onto leaves. The coumarins are apparently induced by insect feeding and slowly increase in sunflower tissue. Induced coumarins may subsequently deter additional feeding of sunflower beetles.