Tobacco hornworms (Manduca sexta) and brine shrimp (Artemia salina) were used as bioassays of the toxicity of leaves of post oak (Quercus stellata). Leaves from three post oak trees in the cross timbers of southern Oklahoma were ground in liquid nitrogen and stored at -70 C until the bioassays. Oak leaf material was incorporated into commercial hornworm chow at 1% and 3% concentrations. Ethanol extracts were used for brine shrimp LC50 analyses. Control hornworms grew much faster and larger than worms that ate chow containing oak leaf material. A significant decrease in toxicity of oak leaves occurred over the season from April to October. Leaves gathered in early, rainy months had a greater effect on bioassay organisms than did leaves gathered in later months during a prolonged drought. These results agree with the optimal defense theory, which says that younger leaves should be defended more than older leaves. However, leaf toughness (as measured with a penetrometer) increased over the season as chemical defense decreased. Therefore young leaves had more chemical, and older leaves more physical, defense from herbivory.

Key words: Artemia salina, bioassay, herbivore defense, Manduca sexta, phenology, Quercus stellata