The intensity of aphid epidemics is regulated by the prevailing weather. In warm dry conditions, there is rapid and extensive colony growth, while cool damp weather inhibits population expansion (Blackman and Eastop, 1984; Minks and Harrewijn, 1987). Initial spring invasions rely on the proximity of overwintered crops to new plantations; consequently field hygiene by ploughing in or rotavating residues is of paramount significance to deprive these pests of sources of cover for hibernation. Spring-planted vegetable brassicas should not be sited close to overwintered oilseed rape crops. Attraction of new crops may be diminished by the use of inter-row mulches of transparent or blue plastic or straw.
Breeding for resistance to cabbage aphid is made difficult because of the large number of insect biotypes. There has been extensive selection and some breeding work in several parts of the world, notably in England and in France. It is suggested that good dominant resistance with some maternal effects is present in several cabbages. Some F1 hybrids between susceptible and resistant lines were susceptible (Metz et al., 1995), whereas Ellis et al. (1996) reported partial levels of antixenosis in red coloured accessions of cvs Ruby Ball and Yates Giant Red Brussels sprout cv. Rubine and a strain of 'Italian Red Kale'. Glossy accessions of cabbage and cauliflower possessed antixenosis and antibiosis resistance that lasted throughout the growing season of the crop. In kale, Ellis et al. (1998) found they were more likely than other B. oleracea to be resistant, while broccoli was a poor source of resistance. Several Portuguese kales carry resistance to the green peach aphid (Myzus persicae), especially selections of 'Crista-de-Galinha', followed by 'Joenes' and 'Roxa' (Leite et al., 1996). Seven Brassica species were tested by Ellis et al. (2000) who found that B. fruticulosa, B. spinescens, B. incana and B. villosa had high levels of antibiosis;
these were the same species reported to possess resistance to cabbage root maggot (cabbage root fly, D. radicum). There is a suggestion that light coloured foliage is associated with resistance as in some of the Australian cauliflower cultivars. Possibly this resistance results from disrupting the inability of these aphids to locate such coloured crops.
Early season control has often relied on the use of granular formulations of soil-incorporated insecticides. Later in the season, foliar sprays are used as the efficacy of the soil-applied materials declines. Strategies for the use of insecticides are dominated by factors such as the permitted interval between application of a particular chemical and harvesting, the declining availability of carbamate and organophosphorus formulations and the relationship between insecticide toxicity and its effect on natural predators.
There are subsidiary considerations such as the failure of systemic granular and foliage-applied formulations to provide control in periods of drought when translocation is slower within the plants. Irrigation may then be used to improve translocation by increasing the availability of soil moisture. Minimizing nitrogen fertilizers and the use of mixed cropping have also been advocated as means of diminishing the impact of these pests. Similar principles and problems apply to the control of M. persicae when using pesticides as for B. brassicae. The development of strains of these pests resistant to insecticides is a major problem, and with M. persicae is made more extreme by the wide host range and consequently increased exposure of aphid populations to sprays applied to many host crops. Integrated control systems using combinations of husbandry, and chemical and genetic host resistance offer the most environmentally attractive means for control, but this demands regular crop monitoring in order to evaluate the build up of pest populations (Finch, 1987). Crop monitoring is essential in order to target the use of insecticides as aphid colonies are established and before significant injury to quality or crop growth has developed. The concept of 'control thresholds' has been used whereby the build up of aphid colonies is monitored and applications varied according to crop growth stage. Thus, with Brussels sprout, fewer aphids are required to trigger the application of remedial sprays in the early stages of growth after transplanting than in the main growing season, while acceptable tolerance levels are reduced again as the buds form and zero tolerance is permitted close to harvest (Theunissen, 1984). Aphids are generally controlled in nature by parasites, pathogens and predators. These may provide opportunities for the development of biological control systems in the future, but so far none have been successfully used in vegetable brassicas.
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