TOXICITY AND ANTIFEEDANT ACTIVITY OF LICHEN COMPOUNDS AGAINST THE POLYPHAGOUS HERBIVOROUS INSECT SPODOPTERA-LITTORALIS

Four frequently occurring lichen compounds, (-)- and (+)-usnic acid, vulpinic acid, and stictic acid were studied for toxicity and antifeedant activity against larvae of the polyphagous insect herbivore Spodoptera littoralis. Both enantiomers of usnic acid as well as vulpinic acid provoked strong mortality as well as significant growth retardation and a pronounced increase of the larval period in chronic feeding experiments with neonate larvae at concentrations comparable to or even significantly below their natural concentrations in various lichens. In these experiments the LD50 of the most active compound (-)-usnic acid was observed at 8.6 mumol g-1 dry wt whereas the LD50 of the (+)-enantiomer, or of vulpinic acid amounted to 90.8 and 111.0 mumol g-1 dry wt, respectively. Stictic acid caused no larval mortality. The deleterious effects of the usnic acid enantiomers and of vulpinic acid are probably due to the antifeedant properties, as well as to acute toxicity as demonstrated by injection into the larval haemolymph. In all experiments, (-)-usnic acid was always the most active compound indicating a remarkable correlation of antifeedant activity and acute toxicity.