Twelve monoterpenes and five aromatic compounds were identified from H
esperis matronalis flowers in situ. The most abundant compounds were 1
,8-cineole, E-beta-ocimene, linalool, and benzyl acetate. These four c
ompounds were released asynchronously by the flowers during a photoper
iod of 15 hr light and 9 hr darkness: 1,8-cineole was released mainly
during the light period, E-beta-ocimene mainly near the switch from li
ght to darkness, and benzyl acetate mainly during the dark period. Lin
alool was not released rhythmically. Release rates of monoterpenes wer
e higher at 20 degrees than at 15 degrees, while release rates of the
aromatic compounds were not significantly different at these temperatu
res. The role of flower volatiles in host plant finding by a specializ
ed Hesperis feeding weevil, Ceutorhynchus inaffectatus, is discussed.