Woolly apple aphid - Natural enemies and diseases

A complex of natural enemies attacks woolly apple aphid in New Zealand, and these parasites and predators provide very effective control in the absence of toxic pesticides. For this reason, woolly apple aphid is often difficult to find on neglected wild apple trees. This contrasts with the situation soon after this pest first arrived in New Zealand, when there were reports of extremely high populations and apple trees festooned with large amounts of wool. These massive infestations caused large and severe galls on the stems, wood, and roots of the trees. The introduction of the aphelinid wasp, Aphelinus mali, was one of the most successful classical biological control programmes undertaken in New Zealand. It made a major contribution to long term reduction of woolly apple aphid populations. However, it is not the only important natural enemy of woolly apple aphid and it receives significant assistance from predators, particularly earwigs and lacewings. In some parts of the world, such as Washington State, U.S.A., predation is considered more important in the control of woolly apple aphid than the activities of A. mali, and predators may play an equally important role in some parts of New Zealand. Only one fungal pathogen of woolly apple aphid has been reported, Verticillium lecanii (Zimm.). Studies in Australia have shown that 1-14% of aphids may be attacked, but the occurrence of the fungus is localised and spasmodic. It has not been recorded on woolly apple aphid in New Zealand.

The known and possible natural enemies in New Zealand are:

Aphelinus mali (Haldeman) is a parasitoid wasp of the family Aphelinidae. In the absence of toxic pesticides, it is commonly found among woolly apple aphid colonies laying its eggs (parasitising) in the nymphal stages. It overwinters as a fully fed larva within the aphid host body which stops producing wool and turns black. These blackened aphid 'shells' are called mummies. The adult wasps (0.7 - 1.0 mm long) emerge in spring, leaving distinct round holes in the empty mummies. A. mali completes several generations over the summer and parasitism rises to high levels (50-100%) by February and March. Black aphid mummies are easily seen amongst the woolly apple aphid colonies, and the tiny adult wasps have a characteristic rapid jumping movement as they move among the aphids. A few other parasitoid wasp species have been reported overseas but information about them is scanty.

Earwigs are recognized as important predators of woolly apple aphids in several countries. This refers particularly to the European earwig, Forficula auricularia L., which is common in New Zealand orchards. Toxicity of some pesticides to earwigs has been linked to outbreaks of woolly apple aphid overseas and the same pesticides have also caused outbreaks here. The available evidence indicates that earwigs are probably major predators of woolly apple aphid in New Zealand apple orchards.

Hover flies (Syrphidae) are predators of many species of aphids and their larvae are reported to feed on woolly apple aphid overseas. It is probable that several species of hover fly contribute to predation of woolly apple aphid in New Zealand but this has not been studied.

Lacewings are predators of woolly apple aphids and two species are known to feed on this pest in New Zealand, the brown lacewing, Micromus tasmaniae Walker, and Drepanacra binocula Newman. Both were accidentally introduced to New Zealand. The importance of their predation is not known. The brown lacewing is common in organic orchards and in orchards using integrated fruit production (IFP) without broad-spectrum pesticides. This species can often be found feeding in woolly apple aphid colonies. Outbreaks of woolly apple aphids have occurred where pesticides have been observed to kill large number of these lacewings, and their contribution to control may be underestimated.

Ladybirds: Thirty five species of ladybirds comprise 48% of all the predator species known to feed on woolly apple aphid worldwide. The elevenspotted ladybird, Coccinella 11-punctata (L.), and the twospotted ladybird, Adalia bipunctata (L.), have been found feeding on woolly apple aphid in New Zealand, and it is probable that others of our ladybird species also feed on woolly apple aphid. A ladybird of the genus Rhyzobius (which occurs in New Zealand) has been recorded as a predator of woolly apple aphid in Australia. The importance of ladybird predation in the control of woolly apple aphid in New Zealand is not known.

Praying mantis, Orthodera ministralis (F.), is known to feed on aphids but it is not known whether this includes the woolly apple aphid.

Predatory beetles, such as ground beetles (Carabidae) and rove beetles (Staphylinidae), may feed on woolly apple aphids as they move up and down apple trees and between the roots and the aerial parts of the plant. However, this has not been recorded.

Predatory bugs of various families are known to feed on aphids and it is probable that woolly apple aphid is part of the diet of a number of species, as it is overseas. The predatory bugs found in apple and pear orchards include the pirate bug, Orius vicinus (Ribaut) (Anthocoridae), a mirid bug, Sejanus albisignata (Knight), a damsel bug, Nabis kinbergii (Nabidae), the fragile assassin bug, Pleioiaria antipoda, (Reduviidae), and soldier bugs, such as Cermatulus nasalis (Westwood) and Oechalia schellenbergii (GuÈrin-MÈneville) (Pentatomidae).

Predatory midges (Cecidomyiidae) are reported to feed on woolly apple aphid overseas but none have been recorded in New Zealand.

Silvereye, Zosterops lateralis Latham, is known to be a predator of aphids, including woolly apple aphid. It feeds rapidly on aphid colonies, consuming many aphids, including those parasitised by Aphelinus mali.

Spiders: The role of spiders as predators of woolly apple aphid is unknown.