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Woolly apple aphid - Description |
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| Adults - Eggs - Nymphs - Damage |
Woolly apple aphid produces long cottony-white wax filaments which give it a distinctive woolly appearance. The aphids live together in dense colonies and the associated wool is more easily seen than the aphids beneath. They also produce copious quantities of sticky honeydew which may be visible as spherical droplets amongst the colonies. Brushing and crushing the aphids releases their body fluids which cause a purple stain on hands and clothing. Most of the population consists of wingless females which reproduce asexually and produce live young, like many other aphids. These young stages of woolly apple aphid are similar in appearance to the adults and differ only in size. Winged aphids are also produced, mainly in late summer.
Adult The adult female aphid can occur in winged and wingless asexual forms. Wingless aphids predominate and they have a reddish-brown to purple appearance beneath the white cottony wax filaments that cover the end of the abdomen. The wingless adult aphid is about 2.0 mm long. It does not lay eggs but produces up to an average of about 120 live young (all females) during its adult life. Winged adult female aphids are dull blue-grey to black with a banded abdomen, and slightly larger than the wingless form. Their clear (translucent) forewings have characteristic strong dark veins at the front. The winged forms are often found amongst woolly apple aphid colonies between February and April. In North America, these winged aphids migrate to elm to continue the sexual part of the life cycle. However, this does not occur in New Zealand. Like the wingless females, the winged females here produce live young but, in contrast, these are few in number (about 3 young) and a mix of sexual males and females. On reaching adult, these males and females mate and the females each produce a single egg, but the eggs do not survive and make no contribution to continuing the woolly apple aphid population. The winged aphids are not, therefore, important in the spread of this pest in New Zealand.
Egg The eggs of woolly apple aphid are produced only by the sexual forms in late summer, and they have rarely been seen in New Zealand. Those that have been monitored have been sterile.
Nymph Newly emerged nymphs (first instars) of woolly apple aphid are called crawlers. They are oblong and somewhat flattened and have a mealy grey to brown appearance. Their conspicuous stylets (piercing mouthparts) are at least as long as their bodies. Crawlers have two important roles, dispersal and overwintering. As their name implies, they are mobile and mainly account for the dispersal of woolly apple aphid in New Zealand. In addition, the crawlers of the final summer generations move into cracks and crevices on the bark of the tree and are mainly responsible for overwintering the aphid population. These overwintering first instars are long-lived and in a dormant (diapause) condition, whereas in summer, the crawler stage is much shorter. Although dispersive, many crawlers settle to feed close to their mothers and this is how dense colonies form. Others walk over the wood, stems, leaves and fruit of the host plant and over the ground to colonise new areas or hosts. Some may infest the roots. After finding a suitable feeding site, the crawler inserts its stylets into the phloem and settles at that site. This takes about 3 days. The preferred sites include nodes on 1-3 year old wood and water sprouts, pruning scars, and damaged bark on main limbs and the trunk. Later nymphal stages grow larger with each moult and produce the characteristic white wool. These nymphs are similar to the wingless adult form only smaller. They retain their legs and are mobile, but do not disperse as much as crawlers. The fourth stage (instar) moults to adult.
Damage Woolly apple aphid infests above- and below-ground parts of apple trees and, in spring, small colonies are typically found in old pruning wounds and scars on major branches and the trunk. These can grow into large colonies. As the aphids disperse and reproduce over the summer, colonies are formed on actively-growing terminals and watersprouts, beginning at the nodes and spreading up and down the stems. The aphid feeding causes gall formation wherever colonies establish and persist on the wood of the tree or the roots. Cell division is induced and woody outgrowths appear which may seriously disfigure young trees and nursery stock. The root galls may be very large and continued feeding can kill roots, stunt tree growth, and even kill young trees. The galls are prone to split at about 0oC or below, and this can allow the entry of fungal infection, such as Gloeosporium. The dispersing aphids may also reach the fruits and establish small colonies in the stem end, the calyx or even the core, particularly on varieties with an open calyx (e.g. 'Red Delicious'). These infestations result in downgrading of the fruit. White waxy filaments on the fruit can also cause a white waxy finish, while beads of honeydew which drop on to fruit can result in downgrading due to russetted or blackened areas, associated with the formation of sooty mould. Severe infestations of woolly apple aphid create unpleasant working conditions and can stain clothing during hand thinning and harvesting.
Root galling by woolly apple aphid is found most commonly where large colonies have formed on rootstocks susceptible to this pest (e.g. M9, MARK, M26). This occurs particularly on soils with an open structure or prone to cracking. Root infestations can provide a substantial uncontrolled reservoir of aphids from which the dispersing crawler stages infest the above-ground parts of trees, especially in spring. From time to time small colonies may be found on aphid-resistant roostocks in the Malling-Merton series (e.g. MM106 and MM793).