Natural enemies, cultural and biological control (Woolly aphid)

Cultural control

Physical destruction

  • It is possible to destroy colonies physically e.g. with a scrubbing brush in May and early June while they are localised on the trunk and before spread to young shoots may occur. This approach is only likely to be practical on a limited scale or where tree density is low.

Cultural practices to foster natural enemies

  • The main cultural control approach is to foster populations of natural enemies, especially predators. This can be done in several ways.
  • Artificial refuges should be used to foster earwigs, which are important natural enemies of woolly aphid, as well as other natural enemies such as lacewings.
  • Ideally, a refuge should be provided in each tree. This may simply be some extra lengths of hollow tree tie round the stake. In orchards with high tree densities, it is likely to be impractical to provide more elaborate refuges such as half of a plastic drinks bottle containing a roll of corrugated cardboard.
  • Flowering plants (e.g. corn marigold, corn camomile and mayweed) can be established in or around the orchard to provide alternative food sources, mainly nectar and pollen, for adult hover flies. These may then lay their eggs in aphid colonies.
  • Ground herbage under the tree may also become infested with other aphid species (e.g. grasses can become infested with bird-cherry oat aphid) which can provide an alternate food source for aphid predators (e.g. ladybird adults and larvae) and parasites.

Natural enemies

Natural enemies play an important part in naturally regulating woolly aphid populations. If natural enemies are encouraged and not harmed by broad-spectrum pesticides, woolly aphid is seldom a serious pest of apple.

Insect predators

  • Many insect predators prey on woolly aphid.
  • The common European earwig, Forficula auricularia, is an important predator of aphids and often prevents damaging infestations developing.
  • Sprays of diflubenzuron (Dimilin) in summer, especially applied at night, reduce earwig populations and have been shown to cause outbreaks of woolly aphid.
  • Adult ladybirds like Exochomus quadripustulatus are important predators early in the season.

Parasitic wasps

  • The parasitic wasp Aphelinus mali is an important natural enemy of woolly aphid.
  • It was deliberately introduced into Europe during the 1920s and 1930s (see Biological control below).
  • The adult wasp is 0.7-1.0 mm long and mainly black, with the antennae, each hind femora and the base of the abdomen yellow.
  • First adults emerge during bloom, about 120 day-degrees above a threshold of 9.4oC from 1 January.
  • They are active and may be seen running about in the close vicinity of the host colonies.
  • Females lay a single egg in each host with a preference for third instar nymphs. An average of 85 eggs is laid by each female.
  • The parasite develops within the host aphid. Attacked aphids cease to produce wax so that blackish, naked parasitised individuals soon become obvious and may be exposed by blowing away the wax from the colony with a sharp puff of breath.
  • Rates of parasitism vary with environmental conditions. The parasite is not favoured by humid conditions.
  • There are 4‑5 parasite generations each year. The parasite overwinters as larvae or pupae in dead host mummies.
  • In general, the parasitoid is rarely able to control the aphid alone in the field (see biocontrol below) but does contribute to natural regulation of the pest as part of a natural enemy complex.
  • The adult parasite is sensitive to broad-spectrum insecticides (e.g. chlorpyrifos (Dursban etc.) or synthetic pyrethroids). It is probable that residues of such insecticides on bark are harmful to the adult parasite for a considerable period after spraying.

Biological control

  • Natural populations of the important predators and parasites of woolly aphid should be fostered (see ‘natural enemies’ and ‘cultural control’ above).
  • Artificial introductions of predators or parasites from biological control suppliers are unlikely to be economic.
  • The parasitic wasp, Aphelinus mali, could be introduced from other orchards if it is absent.
  • This is done by collecting branches bearing parasitised aphids and placing them near woolly aphid colonies in the orchard where they are to be introduced.
  • This is best done in good weather in early summer.