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The principles which apply to variety mixtures can be applied to different crop species as well. A variety of approaches exists including border-cropping, strip-cropping, inter-cropping, mixed-cropping or polycultures and undersowing. In the case of border-cropping, plants of a different species are planted around the edge of the crop. Stripcropping refers to the use of strips of a different species at intervals within the crop, while inter-cropping is used to refer to alternate rows of the different species. Mixed-cropping or poly-cropping refers to the use of two or more species combined at random.
In terms of pest control, these techniques rely on the fact that pests recognize a suitable crop either by sight or by smell and that mixtures can be structured so as to confuse the pest. For example, thrips are white flies which are attracted to green plants with a brown (soil) background, but they will ignore areas of complete vegetation cover including mulches and weeds. Other pests may be attracted under the opposite circumstances. Even so, the size, planting density and shape of the crop can be significant in controlling pests.
Pests such as the carrot fly and the flea beetle do not rely, or rely to a lesser extent, on visual recognition. They depend instead on 'smell' or chemical signals from the plant. Trap crops such as wild mustard can be used as borders or in strips to attract flea beetles away from cultivated brassicas, because the wild species has significantly higher concentrations of the chemical allylisothiocyanote, a powerful attractant of flea beetle adults. This technique may not always be reliable or safe and could under certain circumstances exacerbate the problem and should therefore be used with caution. Flea beetles also rely to some extent on the recognition of bare soil and research has shown that their incidence can be reduced by increasing planting densities.
Mixing species can also assist in pest control in a number of other ways, including influencing the availability of light which can affect insect behavior; creating barriers, for example using tall non-host plants; increasing the distance between host plants; influencing the microclimate; reducing the likelihood of the selection of resistance breaking genes; and acting as alternative hosts for natural enemies. Polycultures generally contain a greater abundance of natural enemies.
Many crop combinations are possible and each can have different effects on insect populations. The choice of tall or short, early or late maturing, flowering or non-flowering companion crops can magnify these effects. Beneficial insects are more likely to be attracted to flowering plants, for example.
The mixed-cropping of different cereal species, such as wheat and rye, and the mixed-cropping of cereals and grain legumes has proved to be potentially successful, although trials on organi farms by Martm Wolfe and Elm Farm Research Centre indicate a need for further research to determine the optimal seed rates. In the case of cereals and grain legume mixtures, the harvested crop can be separated as part of the grain cleaning process. In some cases, markets may exist for cereal grain mixtures, but these need to be identified in advance.
The mixtures need to be selected so that they mature at the same time and can be harvested together. Combinations of winter wheat and winter field beans as well as spring oats or barley and peas will mature together . At Elm Farm, the winter wheat and winter field beans performed best when sown at 75% of normal seed rates for troth components. With the barley/peas mixture, the barley seed rate needs to be reduced to only 3% (i.e. about 5 kg/ha) of the normal seed rate in eastern counties of the United Kingdom, producing just sufficient barley to act as a support crop for the peas, but resulting in a 10% reduction in the area needed to produce the same total quantity of peas and barley separately. In the west and north of the United Kingdom, the quantity of barley may have to be increased.
The crops do not necessarily have to be mixed to gain these benefits. Martens (1983) found that aphid populations were significantly reduced when oats were cropped in strips with peas as compared with oats on their own. The lower populations were also associated with increased predator numbers. Grass strips dividing large fields of cereals can also be beneficial, allowing pest predators to migrate further into the field.
The use of species mixtures is more widely practiced in horticultural systems and in certain 'traditional' agricultural systems, particularly in developing countries. Altieri & Letourneau ( 1982) give several examples of crop mixtures, the pests controlled and the factors involved in their control .
Work at Cambridge University's Department of Applied Biology has concentrated on brassicas such as cauliflowers and cabbage grown with both broad and dwarf beans. Aphids are attracted by the silhouette of the plant and this type of companion cropping provides a form of camouflage.
Another common 'companion crop' is onions with carrots to mask the smell of carrots from carrot fly. In the early stages, this can lead to a reduction of up to 70% in fly numbers, but once the onions start to form bulbs, they are less effective in repelling the pests and numbers are only cut by 30%. The onions need to be sown at the same time as the carrots. Although they are only fully effective in the first generation of the fly, the second generation will be correspondingly smaller. Marigolds represent another possible option.
Undersowing of brassica crops with clover has also shown promisingresults. Dempster & Coaker (1974) report some success in the control of cabbage root fly in Brussels sprouts and cabbages with increased numbers of predators present, although there were significant difficulties still to be overcome with the competition between the crop and the clover. White clover is better suited than red and other species such as annual or even subterranean clovers may be worth considering. Much of the benefit of undersowing clover is the attractant effect of the flowers on beneficial insects. This principle can thus be extended to many other crops including maize and even permanent crops such as vines.
Although all these techniques show significant potential, they are often difficult to implement in practice, particularly in commercial situations, and they may considerably complicate the process of keeping crops relatively weed free. The future development of ecologically appropriate species mixtures will therefore depend on full consideration being given to their mechanical, labour and financial implications.
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