Woolly aphid – additional information | Apple Best Practice Guide

Woolly aphid – additional information

Life history

The entire life cycle is passed on the host tree. Young aphids overwinter in sheltered positions such as in cracks or under loose bark. They are not covered with wool so are inconspicuous at this time.
In March or April they become active and start to secrete wool from their wax glands.
Breeding colonies are present by the end of May. In spring and early summer, colonies are found mainly on spurs and branches, especially round pruning wounds.
Later, the infestations may spread to the young growth, particularly water shoots. They are found on the axils of leaves at first then spread out along the whole length of the shoot.
A few winged aphids are produced in July and may fly off to infest other trees. Other winged aphids occur in September and produce egg-laying females.
Although a single egg is produced by each female, there is no further development which can only occur on American Elm (Ulmus amiricana). Thus the life cycle consists essentially of wingless female aphids producing living young.
Breeding slows down in the autumn and adult aphids die during winter.
Depending on yearly temperatures, there are approximately 8-12 generations per year.

Pest status

An important pest of apple which periodically increases and in some seasons in some orchards.

Other hosts

Also attacks Japanese quince (Chaenomales sp.), Cotoneaster sp., Hawthorn, Malus sp. and Pyracantha sp.

Varietal susceptibility

Apple varieties vary somewhat in their susceptibility to woolly aphid (e.g. Bramley is more susceptible than Cox) but commercially-grown apple varieties are not resistant. The relative susceptibility of varieties has not been characterised adequately.

Resistant rootstocks have been bred by crosses between Northern Spy, which is highly resistant, and various Malling rootstocks which resulted in the Malling-Merton (MM) series of apple rootstocks which are highly resistant to woolly aphid. However, the rootstock does not confer resistance to the scion.
The Malling-Merton rootstock MM106 is a semi-dwarfing woolly aphid resistant rootstock which is widely grown in the UK.
The MM rootstocks have been very valuable in countries where root infestation occurs (Australia, South Africa and North America) but the resistance of rootstocks is of limited benefit in the UK where root infestation does not occur.
In other countries, isolated cases of infestation by local races of woolly aphid occur on resistant rootstocks.

Distribution

Common and widespread in the UK.
Woolly aphid originated in the eastern part of North America and was first noticed in Britain in 1787.
It now occurs in all countries where apples are grown having been distributed on nursery material.

Damage

The colonies occur on bark and aphids do not infest the foliage or fruits directly.
Galls often form on the branches at the point where aphids have fed.
The galls often split open allowing entry for diseases such as canker or Gloeosporium sp.
The main damage is caused by contamination of fruits and foliage with honeydew, wax, dead aphids etc..
Although woolly aphid colonies are conspicuous, the amount of injury to the established tree is probably less than the appearance suggests.
At harvest, infestations can be a severe nuisance to pickers.

Other pests with which woolly aphid may be confused

Woolly aphid infestations and the damage they cause are unique on apple and are unlikely to be confused with other pests.

Monitoring

Woolly aphid colonies are readily visible and visual inspection of the orchard is the main monitoring method.

The aphid is collected by beating and this can indicate when low populations are present which have otherwise been missed.
A sample of at least 25, preferably 50, trees should be inspected for the pest in early June.
A specific inspection for this pest should be made.
It is too easy to miss the obvious.
The presence of colonies on the current year’s extension growth is the critical factor and the economic threshold is normally considered to be 2 trees in a sample of 50 with aphids on the current year’s growth.

Forecasting

Useful forecasting models for woolly aphid have not been developed.

Chemical control

Attacks of woolly aphid tend to be sporadic and it is important to watch for sign of infestation moving onto young shoots in June (see ‘Monitoring’) and treat only when necessary.

A high volume spray of an approved insecticide should be applied in spring or early summer as soon as potentially damaging infestations are detected.
Successful control depends on efficient wetting and good distribution of spray.
High volume spraying is likely to be more effective as the spray has to penetrate the protective woolly covering of the aphid and reach colonies living in cracks and on the undersides of branches.
Flonicamid (Mainman) is specifically recommended for control of woolly aphid on apple.
Flonicamid (Mainman) is a good choice because it is selective, has low toxicity to natural enemies and is partially systemic.
However, in recent years, growers have found flonicamid and all other products to be less effective against woolly aphid. Spirotetramat (Batavia) now offers best control. It must be applied after flowering and works best when pests are moving from brown wood to green tissue. It will prevent population build-up but does not offer pest ‘knockdown’.
Fatty acids (Flipper) has an EAMU approval on apples for controlling aphids and has increased the control options available should growers prefer to reserve other insecticides for control of pests later in the season. It is known to complement the use of Batavia as it provides quick ‘knockdown’.
Use of synthetic pyrethroids, which are harmful to natural enemies, should be avoided.

Natural enemies, cultural and biological control

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. S
prays 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 9.4^oC 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.

Apple Best Practice Guide