Terpenes of Wild Sunflowers (Helianthus): An Effective Mechanism Against Seed Predation by Larvae of the Sunflower Moth, Homoeosoma electellum (Lepidoptera: Pyralidae)

Abstract

Sunflower moth, Homoeosoma electellum (Hulst), is a major pest of cultivated sunflower, Helianthus annuus L., in North America. Thirty perennial and 11 annual species of Helianthus and several cultivars of sunflower were evaluated for resistance to H. electellum larvae. Larvae feeding on sunflower inflorescences in laboratory and greenhouse bioassays performed significantly poorer on 14 Helianthus species than on the cultivar hybrid ‘894’. In in vitro bioassays, nonpolar extracts from H. annuus, Helianthus argophyllus Torrey & Gray, Helianthus ciliaris DC, and Helianthus petiolaris ssp. petiolaris Nuttall caused significant lengthening of larval stadia and retarded larval growth. Larvae feeding on purified sesquiterpene lactones and diterpenes from H. argophyllus, H. ciliaris, Helianthus angustifolius L., and Helianthus mollis Lambert in a wheat-germ diet at 0.1–1.0% concentration experienced high mortality, retarded growth, and an extended developmental period. The sesquiterpene lactone 8 β-sarracinoyloxycumambranolide isolated from glandular trichomes on anthers of Helianthus maximiliani Schrader was toxic to young larvae in bioassays. Two mechanisms giving Helianthus resistance to seed injury by larvae of H. electellum are terpenes in the florets and phytomelanin in the pericarp of seeds.