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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