The effects of non-host plants on the ecology of Brassica lepidoptera

Dover, John William (1983) The effects of non-host plants on the ecology of Brassica lepidoptera. University of Southampton, Doctoral Thesis.

Abstract

The reputed repellent effects of certain Labiate herbs on the oviposition of Lepidopterous pests of brassicas were examined. Alcohol extracts of sage, Salvia officinalis L., thyme, Thymus vulgaris L., and white clover, Trifolium re pens L. reduced diamond-back moth, Plutella xylostella (L.), oviposition when presented on Brussels sprout, Brassica oleracea L., leaf sections in bioassays. A 1% alcohol-talc formulation of sage essential oil and a 10% alcohol-talc formulation of thyme oil also reduced Plutella oviposition. Individually caged, mated female Plutella were exposed to fresh sage, thyme and rosemary, Rosmarinus officinalis L. herbs, and also to their essential oils. The following metrics were recorded: longevity, fecundity, preoviposition period, termination of oviposition, postoviposition period and mature egg complement at death. The virgin mature egg complement was assessed in moths exposed to herb oils during pupation. Moths were strongly affected both by the herbs and herb essential oils. Brussels sprout plants intercropped with sage or thyme plants, under glasshouse conditions, had lower Plutella oviposition than those in monocrops. Sprouts intercropped with sage or thyme plants had equivalent oviposition to those intercropped with plastic model herbs. More eggs were laid on sprout plants with a thyme intercrop than on those with a white clover intercrop. Sprouts planted through green plastic sheeting had equivalent numbers of eggs to those planted directly into soil or through transparent plastic. A field experiment, with a Brussels sprout monocrop and sage, thyme and white clover intercrops, was carried out in 1980. Oviposition by the small white butterfly, Artogeia rapae (L.), and the cabbage moth, Mamestra brassicae (L.) was unaffected by cropping regime. The garden pebble moth, Evergestis forficalis (L.) was unaffected by cropping regime during first generation oviposition but the second generation had higher numbers of eggs laid on brassicas in thyme intercrops, compared with the monocrop. Survival of Evergestis egg and larval instars did not differ between cropping regimes. Predators, aphid parasites and adult whiteflies, Aleyrodes sp, recorded on plants were unaffected by differences in cropping regime. After a general mid-season population crash, the cabbage aphid, Brevicoryne brassicae (L.), infested more brassicas in sage and thyme intercrops than those in monocrop or clover intercrop plots. In 1981 a field experiment compared Evergestis oviposition on Brussels sprout plants in a monocrop, with those in green plastic mulch, black plastic mulch or thyme intercrop regimes. Evergestis oviposition was higher in both generations on thyme intercropped brassicas than those in a monocrop. Black plastic mulched sprout plants had more eggs than those in monocrops in the second Evergestis generation.

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