Woolly Apple Aphid
The aphids’ bodies are nearly covered by a woolly mass of long waxy fibers that gives them a whitish, mealy appearance and that are much shorter on the root-inhabiting aphids. Photo by G. Krawczyk.
Description and life cycle
Some apple varieties, such as Northern Spy, are resistant to this pest. Elm trees in the vicinity of orchards increase the migration of the aphid to apple trees. Infested nursery stock is also a source for spreading aphids.
Injury caused by the woolly apple aphid consists of gall-like formations and swollen enlargements on roots and shoots. Once started, these galls increase in size from year to year as a result of aphid feeding. Galls form favorable places for fungi to attack.
Aboveground colonies of aphids may develop around leaf axils on sprouts or on new growth, particularly at abrasions or cuts, and they prevent injured bark from healing. They are often found on the crowns of trees just above the roots. They may also develop in large knots on roots and underground parts of the trunk. Infested trees often have many short fibrous roots. The underground forms are more damaging, while the aboveground forms cause little damage, especially on larger trees. The foliage of infested trees takes on a yellowish appearance. Young trees are easily uprooted when infested.
The aphids’ bodies are nearly covered by a woolly mass of long waxy fibers that gives them a whitish, mealy appearance and that are much shorter on the root-inhabiting aphids.
The aphid spends winter in two stages: the egg stage and the immature nymphal stage. Nymphs hibernate underground on the roots of the tree. When elms were prevalent, eggs were usually laid in fall in the cracks or crevices of bark. Eggs hatched in the spring into wingless, parthenogenetic, viviparous stem mothers. These fed on elm buds and leaves for two generations during May and June, causing the elm leaves to curl into a rosette. They then produced a winged third generation that migrated to apple, hawthorn, or mountain ash. After establishing new colonies the migrants produced repeated generations during the summer. They fed in wounds on the trunk and branches of the tree. In fall, winged aphids developed in both the aerial and the root colonies. They flew back to the elm, where they gave birth to males and females. Both males and females were wingless. A few days after mating, the female laid a single, long, oval, cinnamon-colored egg almost as large as her body. The egg was laid in the crevices of bark.
With the disappearance of elm trees, the woolly apple aphid lives primarily on apple trees throughout the year. Each group of aphids, small or large, is termed a colony. Aphids are present year-round on the roots. Females in the aerial colonies may give birth to crawlers at any time in spring, summer, or fall.
Newborn nymphs are very important in the distribution of woolly apple aphid. They spread either through some mechanical agent or directly by crawling. Birds and insects can also transport aphids. Crawlers are generally more abundant in spring and fall. They are not able to work into and through the soil. In orchards the swaying back and forth of trees by wind and the presence of organic matter, clods, stones, and other factors may provide pathways to the roots. Crawlers begin to infest the roots early in the season. Their downward motion begins any time the crawlers are numerous, especially in early summer and fall. Infestations by aerial colonies are not a true indication of root infestations, since trees can have aerial infestations over a season without their roots becoming infested.
Effective biological control agents are number of syrphid flies (Heringia calcarata and Eupeodes americana) whose larvae are predators and the tiny wasp Aphelinus mali.Some pesticides, such as certain carbamates and pyrethroids, encourage outbreaks by killing parasites and predators. These products should be used sparingly when woolly apple aphids are present. An application of a summer aphid treatment should control woolly apple aphids. There are presently no control methods for underground aphids.
The best control of woolly apple aphid is genetic. Plant resistant rootstocks like M.111. The Malling Merton series of rootstocks was bred specifically for woolly apple aphid resistance.