Chemical variation and defense of Encelia farinosa

Chemical analysis of leaves from 12 different localities of Encelia farinosa (including var. phenicodonta and var. radians) collected on the peninsula of Baja California (Mexico) revealed the presence of various chemotypes that differed with regard to the concentrations of chromenes and sesquiterpene lactones. Localities of E. farinosa collected in the northern part of Baja California were characterized by high concentrations of the chromene encecalin (up to 252 μmol g^?1 dry wt.), whereas the sesquiterpene lactone farinosin was not detected. Localities of E. farinosa collected at the southern tip of the peninsula lacked encecalin, but were shown to accumulate farinosin (up to 85 μmol g^?1 dry wt.) instead. On the mainland of Mexico, as well as in Arizona (U.S.A.), farinosin concentrations varied from 18 to 44 μmol g^?1 dry wt. for 10 different localities analyzed. Chromenes were not detected or present only in minor amounts (up to 13 μmol g^?1 dry wt.), when compared to the samples from northern Baja California. Chemical variation within localities was small when compared to variation between different localities. Accumulation of encecalin and aridity seem to coincide at least on the peninsula of Baja California, as localities of E. farinosa that receive the least amount of rainfall contained the largest amounts of encecalin in their leaves. Leaves of E. farinosa that contained sufficiently large amounts of either encecalin or farinosin were both detrimental to neonate larvae of the polyphagous pest insect Spodoptera littoralis as shown by addition of the respective crude leaf extracts to artificial diet. Possible advantages of the observed intraspecific chemical variability of E. farinosa with regard to adaptation by generalist insect herbivores are discussed.