Detrimental effects of Cinchona leaf alkaloids on larvae of the polyphagous insect Spodoptera exigua

Young Cinchona ledgeriana plants contain two types of alkaloid: indole alkaloids in the leaves and quinoline alkaloids in the root. From Cinchona leaves, a crude alkaloid extract was made, containing the cinchophylline type of indole alkaloids and a small amount of 5-methoxytryptamine. The leaf alkaloid extract exerted a strong detrimental effect on the growth of larvae of the polyphagous beet armyworm, Spodoptera exigua (Lepidoptera). Feeding of larvae on an artificial diet containing the leaf alkaloids at the same concentrations as those found in the plant resulted in significant growth reduction, retardation in development, and mortality of the larvae. Cinchophyllines are composed of 5-methoxytryptamine coupled to a corynantheal unit. When incorporated into the artificial diet, 5-methoxytryptamine alone had no effect on the S. exigua larvae. Corynantheal, however, had a strong detrimental effect on growth of the larvae, its effect being comparable to that of the leaf alkaloid extract. In contrast to the indole-type leaf alkaloids, the quinoline-type Cinchona root alkaloids did not affect growth and development of the larvae. These results suggest that the indole-type alkaloids, which in Cinchona plants are present at the highest concentrations in the young, vulnerable leaflets, are involved in the chemical defense of the plant against herbivorous insects