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Natural enemies – additional information

Potential effectiveness of natural enemies of apple pests

  • All the pests of apple and pear have a number of predators and/or parasites that attack them. These natural enemies can act as natural regulators of pest populations.
  • Some pests have a key natural enemy, or a complex of natural enemies, which effectively regulate populations and damage by that pest, preventing serious pest outbreaks and consequent economic damage, providing the natural enemies are allowed to thrive.
  • Other natural enemies are more polyphagous feeding on a wide range of prey including pest and benign species as the opportunity arises. These contribute generally to the reduction in pest populations.

Download ‘Potential Effectiveness of Natural Enemies’ word table

Predatory mites

The orchard predatory mite Typhlodromus pyri

Many species of predatory mites are found on unsprayed apple trees but, in selectively sprayed orchards, the most common and important species is the orchard predatory mite Typhlodromus pyri.

  • This species has developed strains resistant to organophosphorus and carbamate insecticides.
  • It is the key natural enemy of the fruit tree red spider mite and apple rust mite and effectively regulates populations of these pest mites such that outbreaks rarely occur.
  • This predatory mite is crucial to successful Integrated Pest Management. Unless the predator is established, regular outbreaks of fruit tree red spider mite and apple rust mite are inevitable and these can be very damaging and difficult and costly to control.
  • Once the predator is established and the biological equilibrium between the predatory mite and the pest mite has stabilised, fruit tree red spider mite and apple rust mite seldom cause problems, providing the equilibrium is not disturbed by the use of pesticides harmful to the predatory mite.

Typhlodromus pyri overwinters as a fertilised adult female underneath loose bark and in crevices etc. on fruit trees.

  • They emerge in spring and invade the foliage and commence laying eggs in May.
  • The development time from egg through larval and nymphal development stages to adult takes about 7 days at 26°C and 14 days at 20°C.
  • There are 3‑4 generations per year.

It is important to ensure that Typhlodromus pyri is present at adequate levels in every apple orchard.

  • This can be done by inspecting the undersides of fully expanded leaves for the presence of the mite in summer.
  • The predatory mite often occurs along the main vein often at the point where the main vein joins with a side vein, sheltering in the groove between the veins and the leaf lamella.
  • The predatory mite is a pale whitish-straw coloured, sometimes with red gut when a red spider mite has been consumed and is pear shaped with a smooth body with 17 pairs of setae. It often moves rapidly over the leaf.

The predatory mite will establish naturally in apple orchards but this can be a slow process. Pest mite infestations can develop rapidly on newly planted trees which do not have established populations the predatory mite (often because they have been sprayed with predator-harmful pesticides in the nursery e.g. with frequent sprays of carbendazim to control canker).

  • Where the orchard predatory mite is absent, e.g. in newly planted orchards, it should be introduced in summer by transferring extension shoots from established orchards where the predatory mite is abundant.
  • Summer prunings may be used.
  • Ideally, at least one shoot should be placed amongst the foliage in each tree of the orchard where the predator is to be introduced.

Although Typhlodromus pyri is resistant to many insecticides, other insecticides and some fungicides are either very, moderately or slightly harmful.

  • Synthetic pyrethroid insecticides (bifenthrin (Talstar), cypermethrin (various products), deltamethrin (Decis), lambda-cyhalothrin (Hallmark), are particularly harmful, especially those products recommended for mite control.
  • Use of synthetic pyrethroids on apple should be avoided except as a last resort.
  • Several fungicides are harmful or moderately harmful to the orchard predatory mite, especially when programmes of multiple sprays are applied. Carbendazim (Bavistin), dinocap (Karathane), mancozeb (Karamate), maneb (Manex) and sulphur fall into this category.
  • Use of these products should be avoided if possible but where use is necessary the number of sprays applied should be minimised.

 

Lacewings

Green lacewing adult

The larvae of several species of lacewing occur on apple but the green lacewing Chrysoperla carnea is the species most likely to be encountered.

  • It overwinters as an adult (often in artificial refuges if provided) and emerges early in spring. Adults are attracted to the honeydew produced by aphid colonies.
  • Eggs, which are borne on stalks, are laid on leaves. Larvae feed principally on aphids.
  • The provision of artificial refuges is useful for increasing numbers early in spring.

Lacewing larva

Lacewing larvae show a greater tolerance to several pesticides than many other predators, though they are susceptible to insect growth regulators.

  • There is little evidence that natural populations of lacewings have a major regulatory effect on aphids or other pests.
  • However, they are available from biological control suppliers and it is possible to make inundative releases, which have been shown to give at least partial control of aphids and spider mites in some experiments.
  • Such an approach is prohibitively costly for commercial apple production.

 

Hoverflies

Adult hoverfly

The larvae of several species of hoverfly (syrphid) are common predators of aphids on apple and pear.

  • The slightly flattened, maggot like larvae feed amongst aphid colonies and are voracious predators.
  • They can have a significant regulatory effect on aphid populations, especially later in the season, but they appear too late for rosy apple aphid which can rapidly cause damage early in the season.
  • Hoverfly adults are attracted to flowering plants and these can be beneficially sown round orchard margins or in the ‘weed-free’ strip under the tree to increase orchard biodiversity.

 

 

Hoverfly larva

 

  • Unfortunately, hoverflies and their larvae are adversely affected by broad-spectrum insecticides including by most aphicides.

 

Ladybirds

Adult ladybird

Ladybird adults and larvae are the commonest predators of aphids. The two, seven and ten spot ladybirds have red wing cases with prominent black spots and are common in orchards. The conifer ladybird (small, black with 4 red spots) is an important early season predator of woolly aphid.

Unfortunately, ladybirds appear sporadically and are not reliable natural enemies.

They are generally considered to have comparatively poor prey-searching ability.

The prey is detected by contact, larvae spending most of their time searching uninfested leaves. High thresholds of prey density are required to stimulate egg laying.

 

Ladybird larva

Eggs are laid in batches on the leaf surface.

Newly hatched larvae often remain on the egg batches cannibalising unhatched eggs and siblings.

Predatory midge larvae

Larva of the predatory midge Feltiella acarisuga

The larvae of some midges (e.g. Aphidoletes sp.) feed exclusively on, and are common predators of, aphids and some species (e.g. Feltiella acarisuga) prey on spider mites.

  • Newly hatched larvae have limited mobility and perceive prey by contact, starving when the prey density is not high.
  • Natural populations probably have only limited impact on aphid populations.
  • Mass rearing methods have been developed for some predatory species but these have only been used successfully on protected crops.

Ground beetles

Predatory ground beetle larvae (carabids)

Many species of predatory ground beetle (carabid) dwell in or on the soil surface in orchards. Most do not climb the tree in search of prey but some may foray onto tree trunks occasionally.

  • Larvae and adults feed on a wide range of soil-dwelling insects, mites, molluscs etc.
  • Different ground beetle species have different spectra of prey.
  • There are few significant soil pests of apples and pears in the UK.
  • However, several important pest species spend part of their life cycle in the soil, usually the pre-pupal and pupal stages.
  • Important examples are apple sawfly, apple leaf midge and Blastobasis moth.
  • The impact of predatory ground beetles on these pests has not been studied adequately, but it is probable that it is significant.
  • Populations of predatory ground beetles can be enhanced by the provision of ground herbage over as great as possible a proportion of the ground surface.

Spiders

Spiders are polyphagous predators. They display a variety of prey-capture tactics. Some spiders spin silk webs to ensnare prey, others actively hunt prey.

  • Approximately 40 species can be found in apple orchards in the UK.
  • They do not show specificity towards their prey and generally attack their prey relative to the rate of encounter.
  • They can have significant effects on pest populations.
  • Web-spinning spiders have been shown to significantly reduce numbers of rosy apple aphid returning from its summer host in the autumn.
  • Unfortunately, spiders are adversely affected by pesticides and the number and range of species present in commercially sprayed orchards is much lower than in unsprayed orchards.

Pathogenic fungi, viruses and nematodes

The baculoviruses are the most important group of virus diseases of insects. They have to be ingested to act and are usually very host specific and are mainly important in moth caterpillars. Several have been exploited as biocontrol agents, notably the granuloviruses of codling and summer fruit tortrix moths.

Bacillus thuringiensis (Bt) is the most important pathogenic bacteria but it normally is only found in orchards in association with sprays of Bt products.

  • The pathogenicity of Bt, which has to be ingested to act, is due mainly to the production of a crystal protein toxin.
  • Bt can be mass produced by fermentation and has long been available as a microbial biological control agent.
  • The most widely used strain in orchards is active against caterpillars and has been widely used for control of winter and tortrix moth caterpillars.

Insect pathogenic fungi are common in the environment particularly in soil. The flora is dominated by a small number of species.

  • They are known to be important mortality factors in overwintering larvae for codling and tortrix moths.
  • There have been several attempts to exploit them for control of orchard pests but they have a requirement for high humidity and adequate temperatures for spore germination.
  • They generally infect the insect through the cuticle.
  • They may be adversely affected by sprays of fungicides applied for scab and mildew control in orchards but such effects have not been adequately investigated.

Insect pathogenic nematodes are also important natural enemies of some pests, notably overwintering codling moth larvae. Nematodes are soft-bodied invertebrate animals that need surface moisture to move and survive. Suitable conditions for them occur only very transiently on the aerial parts of fruit trees.

  • They have been exploited for control of pests which live in galleries (larvae of apple clearwing moth), on the trunk of the tree close to the soil (e.g. codling moth) or in the soil.
  • They can be mass produced in culture on artificial substrates and several biocontrol formulations are commercially available principally for control of soil pests of protected crops.
  • They are not generally used for biocontrol on orchards.

Earwigs

The common earwig is very widely distributed and abundant on apple trees. Most trees have a resident earwig population. Earwigs are omnivorous feeding on plant material including apple fruit and a wide range of insects and mites.

Adult common European earwig

  • Earwigs mate in late autumn and the female then excavates an underground nest in which the pair overwinter.
  • Eggs are laid in the late winter and early spring, the first batch typically containing 30‑40 eggs.
  • The female then ejects the male from the nest as males eat the eggs.
  • By late spring most males have died. Some females lay more than one batch of eggs but all eggs are fertilised before oviposition.
  • Females display maternal care for their young.
  • The young larvae leave the nest after moulting to the second instar. There are four larval instars.
  • Larvae from the first batch of eggs become adult around mid-July and those from the second batch in September.

A phenological model has been developed in the Netherlands to predict earwig life stages. Earwigs from the first egg batch reach the adult stage after 880 day degrees above a threshold temperature of 6°C measured from 1 January. Orchard populations are highest between July and September. Earwigs forage at night and shelter by day so populations in orchards are often underestimated. They tend to shelter in narrow crevices.

Earwigs are important predators of many pests of apple and pear. They feed on aphids (particularly woolly aphid), apple and pear suckers, Blastobasis caterpillars, codling and tortrix moth eggs and larvae, scale insects and spider mites.

  • Reports from European funded research suggest that thiacloprid (Calypso), spinosad (Tracer), indoxacarb (Steward) and flonicamid (Mainman) could harm earwigs.
  • To test these claims, AHDB funded Project TF 196 to investigate the effects of a range of commonly used products on different earwig life-stages.
  • In the project’s first year, laboratory tests suggested that earwigs were far more sensitive to chlorpyrifos than to the other products tested. Spinosad (Tracer) and thiacloprid (Calypso) were also harmful. Methoxyfenozide (Runner) reduced the growth of nymphs. Indoxacarb (Steward), spirodiclofen (Envidor) and acetamiprid (Gazelle) were only moderately toxic. Abamectin (Agrimec), chlorantraniliprole (Coragen) and flonicamid (Mainman) were not harmful in the laboratory experiment. This initial study did not take into account the repeated exposure that would occur in a commercial orchard.
  • In the second year, the effects of products applied in spring or summer in a Discovery orchard were assessed. In trees treated with chlorpyrifos, numbers of earwigs fell over time. Significantly fewer earwigs were found in areas treated with thiaclorpid (Calypso) and flonicamid (Mainman). It was thought that thiacloprid may have a direct effect on the earwigs.
  • Conventional spray programmes were also compared to ‘earwig-safe’ programmes on two commercial farms. Numbers of male earwigs were unaffected by either spray programme, but significantly more females and nymphs were found on trees treated by the ‘earwig-safe’ programme.
  • At present, growers will be forced to continue to use some of the products harmful to earwigs, to control a range of other pests. AHDB has commissioned Project TF 220 to further research the effects of these products on earwig nymphs and adults, and to assess the effects of different spray timings.

Unfortunately, earwigs feed on plant material including the tender leaves in shoot tips (causing characteristic marking along either side of the main vein) and on the flesh of ripening apple.

  • It is believed that they mainly cause secondary damage by excavating pre-existing damage. This can be serious on soft skinned varieties like Discovery. Up to 20% of fruits may be damaged but injury is usually much less.
  • AHDB funded research (Project TF 185) used a form of ‘genetic fingerprinting’ to identify the species whose remains were found in the guts of earwigs caught in orchards, and to see whether they had a preference for eating any orchard pests. The prinicpal contents were found to be lichen and fungi, but the DNA of apple pest species including rosy apple aphid and apple leaf midge was also detected. No apple DNA was found in gut samples, confirming that the earwigs had not been feeding directly on the fruit.
  • Overall therefore, the benefits of earwigs outweigh their disadvantages as pests.
  • They should be encouraged by the provision of artificial refuges and the use of pesticides that are harmful to them should be avoided from June to September.

Predatory bugs (anthocorids, mirids and nabids)

 

Adult predatory flower bug (anthocorid)

Predatory bugs are generalist predators and will feed on and thus help to control many pests including aphids, suckers, spider mites, codling and tortrix moth eggs and young caterpillars. Predatory flower bugs (anthocorids) are often the most common predatory bug species in apple and pear orchards.

  • They overwinter as adults and emerge in March and April congregating on the catkins of willow and sallow before dispersing to other plants.
  • They have two generations per year but the broods are not distinct and all stages are present from April to September. There are five nymphal stages.
  • Populations can be enhanced by avoiding the use of broad-spectrum insecticides (especially synthetic pyrethroids or chlorpyrifos), by providing willow and sallow in the vicinity of orchards for adults in spring and by tolerating low populations of certain pests such as suckers and apple grass aphid to act as food sources to increase populations.

Parasitic wasps and flies (parasitoids)

There is a large number, and a diverse range, of species of parasitic wasp and some parasitic fly species which are natural enemies of apple and/or pear pests. They are commonly known as parasitoids, a term referring to any insect whose larvae develop by feeding on the bodies of other arthropods, usually insects.  The larva or larvae eventually kill their host which provides all the parasite needs for its development.

Many parasitoids play only a minor part in regulating populations of their host. However, some species are important natural regulators of populations of their host. Insufficient attention is devoted to them.

  • Adult parasitoids tend to attack a particular life stage or range of life stages of their host. Parasitoids are classified by the life stage of the host which is attacked (egg, larval, pupal or adult parasitoids).
  • At oviposition the host is either killed or permanently paralysed or the host is not or only temporarily paralysed and continues feeding whilst the parasitoid remains quiescent as an egg or first instar larva until the host is big enough to support full development.
  • Parasitoids are either solitary (one egg per host) or gregarious (two or more eggs per host).

Practically all apple and pear pest species are host to one or more parasitoids and it is probable that there are many unknown parasitoids. Some parasitoids are highly specialised attacking one or small group of closely related pest species. Others have a broader host range. They themselves are host to hyperparasitoids.

Exploitation of existing populations of parasitoids is an important task in Integrated Pest Management. The orchard habitat has to be made as suitable as possible for the particular species.

  • For highly specialised parasitoids, adequate populations of the pest may have to be present. For this reason, exploitation of parasitoids is a more practical prospect for pest species which do not attack the fruit directly and which do not cause serious damage at low population densities and thus can be tolerated at low to moderate levels.
  • Examples are such pests where parasitoids could or do act as key natural enemies include leafhoppers, leaf miners and apple leaf midge.
  • Other important resources which contribute to the success of a parasitoid are suitable overwintering or shelter sites and or alternative hosts or food sources.
  • Sensitivity to pesticides is also an important factor. Many species are likely to be highly sensitive to broad-spectrum insecticides and ways of avoiding harmful affects are crucial to the success of exploitation.
  • Adult parasitoids are often the life stage most exposed to insecticides.
  • Another option is the introduction of parasitoids into the orchard. This may be done by transferring plant material infested with parasitised insects from another orchard or by using parasites reared in mass culture by biological suppliers. However, this is usually uneconomic.

Artificial refuges

Many species of natural enemies in fruit trees seek refuges as a daily sheltering place or an overwintering site. The sites that they seek are often cracks or crevices in the bark, but young fruit trees with smooth bark offer far fewer refuges of this kind than large old trees. This deficiency can to some extent be counteracted by the provision of artificial refuges. Unfortunately, artificial refuges can provide winter shelter for some pest species (e.g. apple blossom weevil adults). However, their benefits greatly outweigh their disadvantages.

 

Bottle refuge

  • Bottle refuges may be made from 1 or 2 litre plastic drinks bottles. The bottom of 1 litre bottles is removed. 2 litre bottles are cut in half.
  • Each bottle or bottle half is filled with a roll of corrugated cardboard made from a strip 15 cm wide by about 1.3 m long.
  • A short wire is pulled through the plastic near the base to secure the cardboard.
  • The bottles may be suspended from branches or attached tightly to trunks, branches or tree stakes with wire (open orifice facing down so they don’t fill with rain water).

A wide variety of other types of refuge may be used.

  • One simple and inexpensive method is to tie extra lengths of hollow plastic tree tie round the stake. The open ends are favoured refuges for many predators.
  • Sick bands tied round the tree trunk provide excellent refuges for many overwintering natural enemies including importantly predatory mites.
  • Unfortunately, they also provide overwintering sites for several important pests, notably for codling and tortrix moth larvae. For this reason, trunk bands have to be cleaned out of pests in the winter which makes them less than ideal as refuges for overwintering natural enemies.

Ideally, every apple tree in the orchard should be provided with a refuge. However, the labour required for provision of bottle refuges in intensive orchards is likely to be prohibitive.

  • The refuges should be left on the tree continuously.
  • In summer they will provide daytime shelter for numerous earwigs as well as spiders, predatory flower bugs (anthocorids) and ladybirds.
  • In winter, earwigs are in the soil but the refuges are occupied by predatory bugs, lacewings, spiders, ladybirds and many other beneficial insects.

Enhancement of biodiversity

Flowering plants

Many wild flower species attract insects that are predators or parasitoids of fruit pests. The plants may provide a food source in the form of nectar or pollen, or plant feeding insects feeding on them may constitute alternative prey or hosts. Adult hoverflies are the main natural enemies attracted, often in considerable numbers, by such wild flowers. Their larvae are important predators of aphids. Examples of flowering plants are:

Birds foot trefoil                     Lotus corniculatus

Buckwheat                              Fagopyrum esculentum

Coriander                                Coriandrum segetum

Corn camomile                        Anthemis arvensis

Cornflower                              Centaurea cyanus

Corn marigold                         Chrysanthemum segetum

Field poppy                             Papaver rhoeas

Phacelia                                   Phacelia tanacetifolia

Wild carrot                              Daucus carota

  • Flowering herbs should be sown repeatedly during the season, ideally to ensure a continual succession of flowering from May to September.
  • They should be sown round the headland of the orchards or even in the strip under the trees if competition with the tree for moisture or nutrients is not a problem.
  • This latter option is likely to be impractical in most situations.

Species diverse alleyways

The alleyways in orchards should be sown with a mixture of grasses and annual and perennial flowing plants.

  • If white clover is included, this will help provide nitrogen for the tree.
  • The use of broad-leaved herbicides in alleyways should be avoided.
  • The frequency of mowing should be reduced as much as possible to allow taller species to grow, flower and set seed.
  • The species composition should be chosen to suit the soil type.
  • A wide range of mixtures is available from specialist seed suppliers (e.g. Emorsgate Seeds, King’s Lynn, Norfolk).
  • Avoid species that are likely to harbour important pests (e.g. docks harbour dock sawfly, mayweed harbours common green capsid) or which are likely to seed in the weed-free (herbicide) strip under the tree and be difficult to control with herbicides.

Hedgerows and windbreaks

Woody species in hedgerows and windbreaks and their understory vegetation can act as important sources of natural enemies of apple pests but can also harbour important pests of apple.

  • They can provide alternate prey for predatory insects or other food sources such as pollen and nectar.
  • They can harbour various apple pests such as apple blossom weevil (Anthonomus pomorum) and winter moth (Operophtera brumats).
  • Several moderately important apple pests occur on other woody plants in the family Rosacae, including hawthorn (Crataegus sp), rowen (Sorbus aucuparia) and whitebeam (Sorbus sp).
  • In general, it is probably best to avoid hawthorn because it does harbour damaging pests such as the apple fruit rhynchites weevil as well as the bacterial disease fireblight, to which several modern apple varieties are very susceptible.
  • Willows (especially Salix caprea and S. cinerea), hazel (Corylus avellana), common alder alder (Alnus glutinosa) and birch (Betula sp.) as well as nettles (Urtica dioica) provide food sources for anthocorid bugs (Anthocoris nemoralis and A. nemorum) which are important predators of aphid pests of apple.
  • In general, insufficient is known about the best woody species for hedgerows and windbreaks but probably a diverse range of species avoiding species that harbour important apple pests, such as hawthorn, is probably the best strategy.

Avoiding the harmful effects of pesticides

Broad-spectrum, persistent insecticides do great harm to many natural enemies, either eliminating them or reducing populations so severely that they have little affect.

  • However, there is a considerable range of more selective insecticides.
  • Most fungicides and plant growth regulators are comparatively harmless to predators and parasites though there are some important exceptions.
  • The effects of pesticide groups on natural enemies are summarised in the table below.
  • When choosing a pesticide for control of a pest or disease, the least harmful to natural enemies should be selected.
  • It is particularly important to avoid using products which are harmful to key natural enemies.

Insecticides, acaricides and fungicides which are applied as foliar sprays to apples and/or pears and known or possible effects on  important groups of natural enemies – Download