Effects of modifying the phytosterol profile of canola, Brassica napus L., on growth, development, and survival of the bertha armyworm, Mamestra configurata Walker (Lepidoptera: Noctuidae), the flea beetle, Phyllotreta cruciferae (Goeze) (Coleoptera: Chrysomelidae) and the aphids, Lipaphis erysimi (Kaltenbach) and Myzus persicae (Sulzer) (Homoptera: Aphididae)

Abstract

The sterol profile of canola, Brassica napus, was altered by treatment with 5 ppm of the systemic fungicides fenpropimorph and tridemorph. The usual Δ5-phytosterols sistosterol, campesterol, and stigmasterol were depleted in leaves and roots and replaced by unusual 4-alpha-methyl and 4-desmethyl sterols that were never observed in untreated plants. Growth, development, and survival of the bertha armyworm, Mamestra configurata, fed leaves of treated canola in the laboratory were affected adversely at specific stages in the life cycle. Larval survival was high and not significantly different in treated and control groups. Most of the mortality in the treated group occurred at pupation and during pupal-adult development. At ecolosion a high proportion of adults were deformed (crumpled wings, failure to exit the pupal case). The adverse effects of feeding on fenpropimorph-treated canola were not attributable to the fungicide itself because larvae fed an artificial diet containing a 10-fold higher concentration of fenpropimorph had normal growth, development, and survival.Although larvae of M. configurata fed canola with a modified sterol profile showed no obvious physiological symptoms, the larvae were more susceptible to the insecticide methomyl (Lannate). Phytosterol modification of the plant in combination with insecticide application may lead to a synergistic interaction in the pest insect.Fewer larvae, prepupae and pupae of the flea beetle, Phyllotreta cruciferae, were recovered from soil samples in field plots with canola treated with fenpropimorph or tridemorph in the laboratory. Adult beetles began to emerge from the soil later, emergence progressed more slowly and fewer were trapped in plots with treated canola.Fewer larvae and adults (up to 50% reduction) of the aphids M. persicae and Lipaphis erysimi, were produced on canola treated with tridemorph. Since aphids harbour sterol-synthesizing mycetocyte-symbionts and are not dependent on a dietary source of sterols, factors other than a deficiency of Δ5-phytosterols must be responsible for the reduction in aphid numbers on treated plants. Key words: Brassica, sterol, fungicide, insecta, development, survival