The woolly apple aphid, Eriosoma lanigerum, is one of several species of aphids that can infest apple trees in Ohio. Other aphid species found on apple trees are rosy apple aphid (see Extension Fact Sheet HYG-2207-94) and green apple aphid (see Extension Fact Sheet HYG-2206-94). Woolly apple aphid occurs sporadically; it is not usually found in most orchards in most years. It may be found either in commercial orchards or in home plantings. Woolly apple aphid is native to eastern North America, and while it feeds mainly on apple, it is also found on elm, pear, quince, hawthorn, mountain ash, and cotoneaster. When elms were common, this insect alternated between elm as a winter host and apple as a summer host. Now that elms are rare, woolly apple aphid usually lives on apple throughout the year.
Woolly apple aphid is an indirect pest that weakens the tree by its feeding on bark and roots, which reduces tree health, prevents wounds from healing, and transmits perennial apple canker. Woolly apple aphid is also a direct pest when it infests fruit cores of some cultivars. It can also be a nuisance pest during harvest when its waxy covering brushes off the tree and onto clothing of pickers.
Colonies of woolly apple aphid form at wound sites on trunks, limbs, and twigs, where they feed on tender bark. As populations grow, aphids can be found around the axils of leaves on water sprouts or on terminal shoots. Swollen galls form on stems where aphids have fed. Foliage turns yellowish on infested branches.
Another way woolly apple aphid damages apple is by contributing to the development of black sooty mold. As aphids feed, they excrete excess sap in a form known as honeydew. Honeydew on leaves and fruit provides a medium for growth of black sooty mold. Sooty mold on leaves can affect photosynthesis and may reduce fruit yield, while sooty mold on fruit can lower fruit quality and marketability.
In addition to feeding on small branches and wounds, woolly apple aphid may be found year-round on roots where they often go unnoticed. Mature trees usually suffer little damage. Yellowish foliage is a sign that woolly apple aphid may be infesting roots. The root systems of nursery stock can be damaged, and severe root infestations can stunt or kill young trees. Infested trees often have short fibrous roots, which predisposes them to being easily uprooted. Swollen galls also form on roots; galls increase in size from year to year and are sites where fungi can attack. Aphid feeding on the root systems also disrupts the nutrient balance of root tissue, which can affect growth of other parts of the tree. The underground form of woolly apple aphid is more damaging than the above-ground form. Trees can have above-ground infestations of woolly apple aphid but no root infestations.
|Galls on roots of apple tree |
infested with woolly apple aphid
(from Lovett and Fulton 1920,
Oregon Agric. Expt .Stn. Circ. 22)
Like other aphids, woolly apple aphid is a small, soft-bodied insect with piercing-sucking mouthparts and two cornicles (or 'tailpipes') projecting from the back of the abdomen. Woolly apple aphid in the wingless adult stage is dark brown-to-purplish and 1.8 mm long. Its cornicles are very short and look like elevated rings. Woolly apple aphid is so called because of its fuzzy appearance; aphids living above ground produce and surround themselves with long white waxy strings, while the underground form has a bluish-white covering of shorter rod-like wax particles. In the nymph stages, woolly apple aphid is reddish-brown and develops a bluish-white waxy covering as it grows. Nymphs are 0.6 mm when born, and reach 1.3 mm in their last stage. Eggs, which are rarely produced, are oval, 0.3 mm long, brown-to-purplish in color, and covered with a waxy substance.
Woolly apple aphid usually overwinters in the nymph stages underground on apple tree roots, one to two meters beneath the soil surface. Nymphs and adults may be able to survive above ground in sheltered crevices of the bark during mild winters. In areas with many elm trees, woolly apple aphid overwinters in the egg stage in the cracks and crevices of elm bark.
In the spring, wingless females give birth to live nymphs. The first-stage nymphs are called crawlers because they are the most active of the four nymph stages. Crawlers allow colonies to disperse from roots to above-ground parts of the tree. Crawlers can be carried by the wind, birds, or other insects from tree to tree within an orchard or nursery. Crawlers also move downward to infest the roots. Once aphids complete four nymph stages, they reach the adult stage. Woolly apple aphid reproduces without mating during the spring and summer; female aphids give birth to large numbers of nymphs. Winged adult females are produced when colonies become crowded. There are several generations per year.
In the fall, some nymphs develop into wingless males that mate with wingless females. Each mated female then lays a single egg nearly the size of her body. Sexual reproduction is believed to occur only when elm grows near other hosts; sexual forms of woolly apple aphid and eggs are rarely produced on apple trees. Eggs on elm hatch in the spring into wingless females that, without mating, produce two generations that feed on elm. Such feeding causes clusters of stunted leaves to form at the tips of elm twigs. A winged third generation migrates to secondary hosts including apple, hawthorn, and mountain ash.
|Wooly apple aphid; wingless female (left), winged female (right)|
(from Lugger 1900, Iniv. Minnesota Agric. Expt. Stn. Bull. 69)
Small parasitic wasps attack aphids; they lay their eggs in aphids by stinging with their ovipositor (egg-laying organ). The wasp egg hatches within the aphid, and the young wasp larva consumes the aphid. Parasitized aphids turn brown or black. In time, the wasp larvae emerge as adults from the aphids, leaving behind empty aphid skins. These skins, called "aphid mummies," can be found attached to leaves. Aphelinus mali is a tiny wasp native to North America that frequently parasitizes woolly apple aphid. This wasp is susceptible to insecticides; it can reduce woolly apple aphid populations in abandoned orchards where insecticides are not used, but usually cannot survive in commercial orchards where insecticides are used, particularly pyrethroids or carbamates. Other natural enemies of apple aphids include predators such as hover fly larvae, lacewing larvae, lady beetle larvae, and lady beetle adults. These predators feed on many different aphid species in addition to other insect pests. A cool, wet spring favors aphid development because these conditions are unfavorable for the aphid's natural enemies.
Resistant varieties must be used to prevent underground infestations. The Malling-Merton (MM) rootstock series provide resistance to woolly apple aphid attack. Some apple varieties such as Northern Spy are resistant to this pest.
Removal of suckers at the base of trees will create conditions that discourage development of woolly apple aphid populations in early-spring. Summer pruning of water sprouts also contributes to woolly apple aphid suppression.
Pruning cuts and water sprouts should be examined in late-spring and every few weeks throughout the summer for the presence of new colonies of woolly apple aphid. Specific action thresholds have not yet been developed.
An insecticide can be applied if woolly apple aphid is detected at damaging levels on above-ground parts of trees. Insecticides are most effective if applied when the aphid is in the active crawler stage and is just moving up into the tree. This may occur in late-spring or not until mid-summer. Thorough coverage of the canopy is needed for insecticide to be effective. Because of the aphids' waxy covering, high volume application is needed to get thorough spray coverage. A second application may be needed two weeks after the first if aphids continue to be detected.
Insecticides used to control woolly apple aphid in commercial orchards are dimethoate (Cygon), endosulfan (Thiodan), chlorpyriphos (Lorsban), or methyl parathion (Penncap-M). Home gardeners can use diazinon or insecticidal soap.
Woolly apple aphid infestations on rootstocks cannot be controlled by insecticides.
NOTE:Disclaimer - This publication contains pesticide recommendations that are subject to change at any time. These recommendations are provided only as a guide. It is always the pesticide applicator's responsibility, by law, to read and follow all current label directions for the specific pesticide being used. Due to constantly changing labels and product registration, some of the recommendations given in this writing may no longer be legal by the time you read them. If any information in these recommendations disagrees with the label, the recommendation must be disregarded. No endorsement is intended for products mentioned, nor is criticism meant for products not mentioned. The author, The Ohio State University and Ohio State University Extension assume no liability resulting from the use of these recommendations.
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