Pests of Cruciferous Crops

Abstract

Although members of the family Cruciferae are cosmopolitan, they are found chiefly in north temperate regions. Many are annual or ephemeral herbs which grow in dry open habitats and, generally, they are among the first plants to colonise disturbed soils. Their Latin name, Cruciferae (crux= a cross; fero =I bear) describes their flowers, which consist characteristically of four petals in the shape of a cross. The plants are also characterised by a wide range of secondary plant compounds, known as glucosinolates, which, with their breakdown products, give the plants characteristic tastes and often pungent odours. These chemicals are toxic to most insects but some other insects have overcome them, using the chemicals as essential cues in host location and selection. The insects that have adapted in this way are regarded as pests only when they attack Cruciferae cultivated by man. As many of the adapted insects can colonise successfully a wide range of the 220 genera of Cruciferae, many wild plants and garden flowers are reservoirs of pests of cultivated crops. The two genera of Cruciferae cultivated most extensively are Brassica L. and Raphanus L. Until about 15 years ago, cultivars of Brassica oleracea L. were the commonest crucifers grown on a field scale but, with the recent rapid expansion in the area of oilseed rape, the oil-bearing cultivars of B. napus L. and B. campestris L. are now commonest. Not only are these grown at a much higher density (80-120 plants/m 2) than are the B. oleracea cultivars (2-20/m 2), but also they occupy a much larger area than do other cruciferous crops and, hence, are the commonest host plants that pest insects are likely to encounter. This situation is exacerbated by seed shed before or during harvest giving rise to large numbers of volunteer plants in non-cruciferous crops in subsequent years. In temperate regions cultivated Cruciferae can be attacked by 50-60 insect species, of which about 20 are major pests of crop plants, damaging all the different stages of growth of the crop and all parts of the plants. The relative importance of individual insect species as pests varies from crop to crop and, although large numbers of some species may be present, the species are considered as pests only when the large numbers coincide with vulnerable stages of particular crops.