Inhibition of herbivory on young holly leaves: evidence for the defensive role of saponins

Abstract

Response of the southern red mite, Oligonychus ilicis McGregor, to young and one year old leaves of Ilex opaca Aiton was studied on three dates during the period of leaf expansion in the spring. Young foliage, which is rich in nutrients but also contains high levels of saponins, was found to be unsuitable for colonization by this oligophagous herbivore until the leaves had matured and levels of saponins had declined. Mites preferentially colonized and had higher survival and reproduction on sclerophyllous, one-year old leaves than on young leaves in early spring. High levels of saponins in young, second-flush leaves of shoots that had earlier been damaged by frost were again correlated with low mite survival in June. Laboratory preference tests with the fall webworm, Hyphantria cunea Drury, and the eastern tent caterpillar, Malacosoma americanum (F.), indicated that young, saponin-rich holly foliage is especially unpalatable to these highly polyphagous caterpillars. Fifth instar fall webworms required significantly longer to complete their development, attained lower weights, and had greater mortality on artificial diet amended with low concentrations of purified holly saponins than on control diet. These results support the hypothesis that the high levels of saponins in young holly leaves provide protection from herbovores until the leaves have matured and their structural defenses are developed.

Keywords: Allelochemicals; Ilex opaca; Oligonychus ilicis; Plant defense.