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Natural enemies in apple orchards and their enhancement

Apple and pear pests are hosts to a large complex of predators and parasites, some of which are important natural regulators of pest populations. Others generally contribute to the natural regulation of pest populations to a greater or lesser extent .

The natural enemies of pests fall into the following groups:

  • Predators
  • Parasitic wasps and flies (parasitoids)
  • Pathogenic fungi, viruses and nematodes

Predators

Some predators are specialised feeders attacking only one pest or a small group of pests and can respond quickly to increasing pest populations, effectively regulating them. The best example of this is the use of the orchard predatory mite Typhlodromus pyriagainst fruit tree red spider mite and apple rust mite.

Predatory flower bugs or anthocorids  are generalist predators and will feed on aphids, suckers, spider mites and the eggs and young caterpillars of codling and tortrix moth.

Several other groups of naturally occurring polyphagous predators, such as lacewing larvae, ladybirds  hoverfly larvae,   spiders , predatory midges  and ground beetles  prey on a range of pest species contributing generally to the reduction in pest populations. However, they are unlikely alone to prevent pest damage fully and reliably. An exception due to its abundance in orchards is the common earwig.

Parasitoids

These are natural enemies of apple pests whose larvae develop by feeding inside the bodies of their hosts.  There is a wide range of species of parasitic wasp, and some parasitic fly species, which offer some control of apple pests.

Pathogenic fungi, viruses and nematodes

Apple and pear pests are hosts to a wide range of microbial (virus, bacterial, fungal) and nematode pathogens, some of which are important natural enemies. Several have been exploited as biocontrol agents.

Key natural enemies

 

The orchard predatory mite Typhlodromus pyri

A few species of natural enemy are particularly important as they are resistant to broad-spectrum insecticides and are able to prevent serious outbreaks of one or more important pests. These are key natural enemies.

Key natural enemies are easier to exploit for pests that do not damage the fruit directly and can thus be tolerated at low to moderate levels in orchards. The most important key natural enemies identified to date for apple and pear pests are:

 

Adult common European earwig

  • Organophosphorus insecticide resistant strains of the orchard predatory mite, Typhlodromus pyri, which prevent pest mite outbreaks on apple.
  • The common European earwig, which is resistant to insecticides and prevents woolly aphid outbreaks as well as regulating populations of many other pest species
  • Parasitic wasps, which parasitise and regulate populations of leaf mining moths.  The parasitic wasp Platygaster demades attacks apple leaf midge.
  • Predatory flower bugs (anthocorids) which regulate pear sucker populations

Encouraging natural enemies

  • Persistent broad-spectrum insecticides, particularly synthetic pyrethroids, are harmful to a wide range of natural enemies and their use should be avoided if possible.
  • The best way of increasing natural enemies is to avoid the use of such insecticides and use only selective insecticides or cultural, biological and biotechnological control approaches.
  • Populations of natural enemies are increased by providing a diverse range of plants in and around the orchard.
  • To enhance biodiversity, alleyways should be sown with a mixture of annual and perennial flowering plants and grasses. The frequency of mowing should be reduced to allow taller species to flower and set seed.
  • Successive sowings of flowering herbs in and around the orchard will provide an alternative food source for some natural enemies, notably hoverfly adults, the larvae of which are important predators of aphids.
  • Artificial refuges should be provided, ideally one in each tree, to increase populations of some generalist predators, especially earwigs.