RICE, STANLEY A.1*, ERICA CORBETT1, DIANA L. BANNISTER1, JENNIFER MARBLE1,2, IAN B. MANESS1, and JONATHAN V. EDELSON2. 1Department of Biological Sciences, Southeastern Oklahoma State University, Durant OK 74701-0609; 2Wes Watkins Agricultural Research and Extension Center, Box 128, Lane OK 74555. - Seasonal changes in herbivore defense in oak leaves: results from bioassays.
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