Woolly Apple Aphid, Eriosoma lanigerum (Hausmann)

I. Introduction: This gall-forming aphid is distributed worldwide. Woolly apple aphid (WAA) colonies may occur aerially or on roots. Root injury has been reported as being most severe in the Virginia to Maryland region.

II. Hosts: Apple, pear, hawthorn, and mountain ash.

III. Description: Adults and nymphs are reddish brown or purple, although usually covered with white waxy filaments. Adults are about 1/8 inch (3 mm) long. Aerial colonies are usually located at pruning cuts or the base of sprouts.

IV. Biology: There are still questions regarding the basic life history of WAA.  Until recently, elm has been regarded as the overwintering (primary) host.  However, WAA is apparently anholocyclic (developing without eggs) on apple, spending the winter either aerially or on roots.  Those on elm are now considered a separate species, E. herioti Börner (this species may also develop on apple, however). The primary host of WAA, if one exists, is unknown. A few overwintering eggs are produced on apple but these do not survive.  Live aphids also overwinter in colonies on apple roots.  Crawlers can move to roots at any time they are active, mainly in June and July, and in the fall. WAA may move from tree to tree, either by winged migrants or by crawlers.  Crawler dispersal between trees is favored by close tree spacing, and clean, smooth soil surfaces.  High soil temperatures, weed cover, and distant spacing of trees inhibit spread between trees.

Frequency of infestation in orchards 2 to 25 years old is correlated with orchard age.  Orchards more than 25 years old may have 79 to 100% of trees infested. Severity of root infestation per infested tree ranges from 24 to 50%, with individual trees as high as 84% infested.  The Malling Merton (MM) series of rootstocks (derived from 'Northern Spy') was bred for resistance to WAA.  Some reports indicate that this resistance may be breaking down.  Although MM rootstocks do not provide total resistance, infestation levels are still lower than on other rootstocks and they are a recommended control tactic.  However, growers may need to consider a greater degree of dwarfing than offered by MM rootstocks.  Some more recent Geneva rootstocks have high levels of resistance to WAA root infestations.  Several natural enemies [e.g. syrphid larvae, lady beetles and parasitoids such as Aphelinus mali (Haldemann)] provide biological control; outbreaks can be induced in the canopy by pyrethroids, or the continual use of methomyl (Lannate) or spinosyns over a two- to four year period.

V. Injury: Root injury is the most important form of injury, consisting of galls (globular swellings) at feeding sites.  This injury can reduce the growth of young apple trees, but the effect is often not a strong one.  Increased tree losses in nurseries have been noted.  Mature trees are affected to a lesser degree.

VI. Monitoring: Watch for an abundance of colonies on branches, especially around healed pruning cuts or wounds in the bark, beginning in mid-June.  Examine five pruning cuts on each sample tree. The University of Massacusetts has established a provisional threshold of 50% of pruning cuts infested.

This is taken primarily from a chapter by D.G. Pfeiffer, L.A. Hull, D.J. Biddinger, & J.C. Killian on apple indirect pests, reprinted with permission from Mid-Atlantic Orchard Monitoring Guide, published by NRAES, 152 Riley-Robb Hall, Ithaca, New York 14853-5701.

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