Pest 3 - Wooly Aphid

I found this infestation of wooly aphid on an apple tree walking down the street in North Hobart.

Wooly aphid is of the Hemiptera order and its scientific name is Eriosoma lanigerum.

It is a serious pest of apple trees. They have piercing and sucking mouth parts. 

Appearance : Adults are usually always female and are mostly wingless, although some do have wings particularly in autumn, but are mostly insignificant in Australia. Globular in shape and purplish in colour, they are usually hard to see because of the white cottony threads they secrete from their pores which gives them a protective covering over the colony. They also produce a white powder which covers their bodies. Nymphs are also usually female and are smaller, initially not so globular and paler. 

Life Cycle : Gradual metamorphosis (egg - nymph - adult). Most aphids are wingless females that can reproduce asexually. The young are mostly females. Winged forms appear in autumn and can migrate to neighbouring trees. More migration occurs from tree to roots in early winter with a return to above ground parts in spring. Survival of population is usually by aphids hibernating in cracks and crevices of old wood (overwintering). 

Damage : Wooly aphids are sap suckers and target new growth where it is thin, such as lateral growth), damaged areas such as pruning areas or areas of broken bark. Over pruned and unhealed trees are particularly susceptible. Affected wood takes on a lumpy, gnarled appearance due to the sap sucking action; laterals become deformed, cracked and buds are destroyed by the woolly material secreted. Fruit can prematurely fall off or be disfigured, and the woolly secretions can make it very unpleasant to harvest fruit in heavy infestations. Roots can be gnarled and growth can be stunted in young trees. In addition, the honeydew secreted by the aphids makes conditions favourable for the growth of the fungus, sooty mould. 

Distribution on plant : aphids congregate on thin, new or damaged growth in spring and autumn but then over winter usually remain in cracks or underground in root system. Prefers cool, moist conditions and therefore will favour shaded areas of the tree, particularly in the interior of dense trees. Low humidity and high temps (> 27 degrees celsius) are not tolerated. 

Their spread is mostly by being wind-borne or by humans on clothing.

Host plants : mostly apple trees, rarely pears. Also affects hawthorn, cotoneasters and liquid amber. 

Controls :  

cultural : The use of resistant rootstock such as "Malling-Merton" and "Northern Spy."

In ornamentals, woolly aphid could be scrubbed off but this will only reduce populations.

Preventive measures include not mowing around trunk, pruning in summer when cuts heal quicker, covering wounds with an antifungal sealant paint.

biological : The wasp Aphelinus mali was introduced into Australia in the 1920s and parasitizes the wooly aphid. These aphids appear black and have the wasp larvae feeding within the aphid body, killing it. Small holes, visible to the naked eye, appear on such aphids. The wasp can overwinter as pupa within the aphid bodies and then appear as adult wasps in spring at the same time that aphid populations become active. Conserving these wasps is an important method of keeping the woolly aphid populations under control can can be achieved by not using non-specific pesticides and allowing occasional colonies of woolly aphid to exist on surrounding hosts eg. hawthorn. Twigs with parasitized aphids can be stored in sheds over winter away from birds and introduced into infested trees in spring. 

There are also natural predators of woolly aphid such as ladybirds, lacewing larvae, syrphid fly larvae. 

chemical :  organophosphate : dimethoate (but this is toxic to Aphelinus mali). Winter oil in dormant periods, maldison (another organophosphate but non-systemic), pirimicarb (carbamate).

References :  Kerruish, A148-150; Judy McMaugh, pg 213;  Natural control of garden pests,  Jackie French, pg. 79-80.