There are two sources of 'natural enemies' to pests, the local species and the exotic ones. Many pests of outdoor horticultural crops such as peach-potato aphid are indigenous (i.e. they are present in wild plant communities in the UK). Such pests are often reduced in nature by other organisms which, as predators, eat the pest, or, as parasites, lay eggs within the pest. These beneficial organisms, found also on horticultural crops, are to be encouraged and in some cases are deliberately introduced. A range of important organisms useful in horticulture is now described in some detail.
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