Woolly Apple Aphid

Appearance

The adults of "Eriosoma lanigerum" are small to medium-sized aphids, up to 2mm long, and have an elliptical shape, are reddish brown to purple in colour but the colour is normally hidden by the white cotton-like secretion from the specialised glands in the aphid's abdomen which gives it the common name of woolly apple aphid. The wax is produced after each moult so newly moulted individuals lack the wax coating, the main purpose of which is thought to be to prevent the honeydew secreted by the aphids to contaminate them but it may also produce a shelter from the weather and from parasites and predators. It has sooty-brown antennae has six segments and the colour of the tibias varies from dark brown to yellowish. When the adults are crushed they leave a blood red stain. The presence of this wooly substance distinguishes "E. lanigerum" from any other aphid occurring on apple trees. In many populations reproduction is wholly asexual and nymphs are produced by parthenogenesis. The nymphs are salmon pink in colour with dark eyes and circular cornicles which are slightly raised from the surface of the abdomen. The nymphs go through four instar moults before becoming an imago. The earliest stages are known as crawlers and they do not produce the waxy filaments until they settle to feed. The hibernating nymphs are very dark green, almost black, although they may be paler and can be dingy yellowish-brown and lack the secreted white waxy covering.

Distribution

"E. lanigerum" is native to North America but it is now found in all of the regions of the world where apples are grown. It was first recorded from Great Britain in 1787.

Behavior

In cooler areas "E. lanigerum" spends the winter months as a nymph on the roots of its host plant or in the more sheltered above ground portions of the host such as under bark on the trunk or main branches. Where sexual reproduction occurs they will also overwinter as eggs and this occurs when elms are prevalent with the eggs being laid into crevices in the bark. The eggs hatch out into wingless "stem mothers" who begin to give birth to nymphs by parthenogenesis. Nymph colonies wintering above ground may be wiped out by severe winter weather. In the Spring, April in Great Britain, the colonies begin to produce young which infest the host tree and if there are no above ground colonies they move up the tree until almost the whole tree is covered in aphid colonies, which prefer to be sited at the axils of leaves on terminal shoots. Where the population levels are high almost every leaf on the tree will have a colony at its base. The third generation of young produced grow into winged adult females which are capable of sexual reproduction. each female producing a single egg, but these can only develop on the American elm "Ulmus americana". The males are wingless. Each aphid may give birth to up to five live young a day allowing the rapid growth of colonies, with a total of over 100 nymphs in its life. The aphids feed on sap by piercing the outer integument of the host where it is thinnest and excrete a substance known as honeydew which contains a high proportion of sugars. There can be between eight and twelve generations in a year, depending on the summer temperatures.

In its native range in north eastern North America the winged adults, or alates migrate at the end of the summer to overwinter on elms but in regions where there are no elms the fate of these alates is not known and all observed reproduction is by parthenogenesis. Elm is the primary host species and it appears that egg production on other species, such as apple trees, is rare and that eggs laid on apple always fail to hatch.