Woolly apple aphid
Excerpt from Publication 310, Integrated Pest Management
Table of Contents
Woolly apple aphid, Erisoma lanigerum (Hausmann), is native to North America and found in all apple growing areas in the United States and Canada. This aphid has historically been considered an occasional apple pest in Ontario, but is now seen more frequently in orchards. There are several generations a year, and the woolly apple aphid hosts include apple and elm as well as pear, mountain ash and hawthorn.
Aphid colonies are first observed in June on pruning cuts, around wounds of limbs and trunks and at the base of young shoots. Infestations increase and spread to growing twigs and leaf axils of water sprouts. Eggs are cinnamon-colored, oval shaped and 0.3 mm in length. Nymphs are 0.6-1.3 mm in size, reddish brown or purple and often covered with a white waxy coating (Figure 4-20). Nymphs pass through 4 instars. Adults are about 3 mm long. Aerial colonies are usually located at pruning cuts or the base of sprouts (Figure 4-21).
Figure 4-20. Woolly apple aphid nymph
Figure 4-21. Terminal infested with woolly apple aphid
There are many unanswered questions about the life history of woolly apple aphids. It was believed they overwinter only on elm trees as immature nymphs or eggs, with mature winged adults moving into orchards during the spring and summer. Recently, it was determined they can complete their lifecycle on apple, overwintering aerially or on roots. The aphids on elm are now considered to be a separate species called E. herioti (Börner) that can also develop on apple. The primary host of woolly apple aphid is unknown.
Crawlers can move to roots any time they are active, mainly in June and July, and in the fall. There are winged aphids, and crawlers, and both move from tree to tree. Close tree spacing, and clean, smooth soil surfaces favour crawler migration. High soil temperatures, weed cover and distant tree spacing inhibit aphid movement between trees.
Research shows the frequency of infestation in orchards is correlated with orchard age. Orchards 25 years or older may have 70% to 100% tree infestation, with severity of tree infestation from 24% to 50% on each tree. Malling Merton (MM) series rootstocks are bred for resistance to woolly apple aphid, however some reports suggest this resistance may be breaking down.
Woolly apple aphid feeding forms knots or galls on twigs or roots. Galls are more visible on water sprouts than on tree wounds. Areas damaged by these aphids are more sensitive to frost and winter injury. Underground colonies also form galls on the roots. Aphid colonies feed on healing tissues of limb and trunks wounds, and where canker disease has established, aphid feeding may spread the pathogen.
Woolly apple aphids excrete a sticky material called honeydew that drips on fruit and leaves. Honeydew causes russet spots on the fruit, and a black, sooty fungus may establish on the honeydew, downgrading fruit quality. Honeydew is also a nuisance to harvesters because it is sticky and stains cloths on contact.
Look for characteristic cotton, waxy covering around pruning cuts and water sprouts in the spring and mid to late summer. There are no thresholds for woolly apple aphids. Consider management when infested twigs become swollen and galls form at feeding sites. Management is also necessary if colonies infest areas near fruit clusters or on young trees and nursery stocks. Damage is minimal when colonies remain on water sprouts and limbs away from the fruit clusters.
In organic orchards, and orchards minimizing use of broad-spectrum
insecticides, control woolly apple aphids by natural enemies. Most
important natural enemies include Aphelinus mali (Haldemann),
lacewing larvae, lady beetles and syrphid fly larvae. Flowering
plants in or on borders of orchards provide nectar and pollen to
maintain and attract natural enemies.
When necessary, early summer chemical management is better than
late summer controls. Chemical management is also better when nymphs
are young and colonies start to form - as pesticides are able to
penetrate the waxy coating protecting aphids. For best results with
pesticides, use higher volumes of water for thorough coverage.
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