ABSTRACT
Photo-activated plant secondary compounds have been shown to be toxic to many organisms including insects. Insect defenses include behavioral mechanisms such as light avoidance, as well as specific biochemical defenses such as antioxidants and antioxidant enzymes. These antioxidant defenses eliminate or quench the deleterious singlet oxygen and free radicals formed by these phototoxins. In this paper we examined the role of dietary antioxidants in protecting the phototoxin-sensitive insect herbivoreManduca sexta. Elevated dietary levels of the lipid-soluble antioxidantsΒ-carotene and vitamin E resulted in a concentration-dependent reduction in the mortality associated with treatment ofM. sexta larvae with the phototoxic thiopheneα-terthienyl. Elevated levels of dietary ascorbic acid had no effect, whereas reduced levels greatly increased the toxicity ofα-terthienyl. Tissue levels of antioxidants were shown to increase substantially in larvae fed antioxidant-supplemented diets. The results suggest that the ability to absorb and utilize plant-derived antioxidants could be an important defense against photo-activated plant secondary compounds and may have allowed some insects to exploit phototoxic plants.
ABSTRACT
High levels of cyclic AMP were found in the corpora allata of adult female Diploptera punctata. The adenylate cyclase activator forskolin caused a rapid, reversible and dose-dependent accumulation of cAMP in the corpora allata in vitro. The sensitivity of the corpora allata to forskolin was low when juvenile hormone (JH) synthetic activity was high, and vice versa. Incubation of corpora allata with compounds which cause or mimic elevated intracellular cAMP levels (forskolin, 3-isobutyl-1-methylxanthine, 8-bromo-cAMP) led to a rapid and dose-dependent inhibition of juvenile hormone synthesis. Glands from day 5 virgin females were more sensitive to forskolin than glands from mated females of the same age. The results suggest that a cAMP second messenger system may be responsible for the intracellular transduction of inhibitory signals to the corpora allata of D. punctata.
