Woolly Apple Aphid

Table of Contents

1. Introduction
2. Description
3. Damage
4. Biology
5. Monitoring and Management

Introduction

The woolly apple aphid, Erisoma lanigerum (Hausmann), is an occasional pest of apple in Ontario. It has several generations a year and feeds mainly on apple and elm, but also on pear, mountain ash and hawthorn.

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Description

Aphid colonies are first observed on pruning cuts, around wounds of limbs and trunks and at the base of young shoots (Figure 1). The insect can increase and spread to growing twigs and the leaf axils of water sprouts.

Figure 1. Woolly apple aphid colony on pruning cut.

The woolly apple aphid has a distinctive, white, waxy covering. The aphid colonies resemble small tufts of wool or cotton batting. Beneath this waxy covering, the aphids are reddish-brown to purple in colour (Figure 2). The aphids are soft-bodied and stain a characteristic dark red colour when crushed. Immature aphids are brown and have no white, waxy covering.

Figure 2. Colony of woolly apple aphids.

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Damage

Woolly apple aphids do not directly feed and damage apple leaves. Their feeding forms knots or galls on twigs or roots (Figure 3). 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 a canker disease has established, aphid feeding may spread the pathogen.

Figure 3. Swelling of "gall" on apple twig as a result of aphid feeding.

Woolly apple aphids also excrete a sticky material called honeydew that drips on fruit and leaves. Honeydew can cause russet spots on the fruit and a black, sooty fungus may establish on the honeydew thereby, downgrading fruit quality (Figure 4). This is also a nuisance to harvesters because of stickiness and staining of clothing on contact.

Figure 4. Woolly apple aphid on fruit.

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Biology

The life cycle of the woolly apple aphid involves two host-plants: the elm and the apple. The insect passes the winter as either an egg or a young nymph. The over-wintered eggs are light brown and are found in cracks and under the bark of elm trees. Eggs hatch in early spring and the young nymphs feed on elm buds and developing leaves.

There are generally five nymphal stages. Two to three generations develop on elm and then winged females appear and migrate to apple or other hosts to establish colonies. On apple, several summer generations will develop before adult aphids migrate back to elm in early fall. The decline of native elm due to Dutch elm disease may be partly responsible for the relatively minor pest status of the woolly apple aphid in Ontario orchards.

Young nymphs overwinter in various protected locations on trees. During the growing season they are attracted to the base of water sprouts growing from the tree crown, and beneath loose or dead bark around wounds and cankers on limbs and trunks. Large numbers of aphids may die in severe winters, but enough individuals usually survive to infest trees in the spring. In more southern climates immature aphid colonies also overwinter on apple roots one or two metres beneath the soil. These nymphs develop and mature in the spring. Several generations can develop on the roots each year. This underground form is rare in Ontario.

In the spring, eggs hatch and overwintered nymphs complete their development. The adult female produces live young which, as first-stage nymphs, migrate throughout the tree. These nymphs are called crawlers. They are brown and have no waxy covering. When the nymphs locate a suitable spot, they congregate, settle, secrete a waxy covering and form colonies. Colonies develop more rapidly on shoot bases than on tips. Reproduction is continuous during the summer and colonies build rapidly in favourable season. In mid- to late summer, some winged females establish new colonies in the upper tree canopy. Wind, birds and other insects can also spread nymphs to other trees.

In early fall, may winged adults migrate back to elm trees where mated females lay eggs which overwinter on the bark.

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Monitoring and Management

Look for the characteristic cotton, waxy covering around pruning cuts and water sprouts in the spring. Monitor for them again in the leaf axils and growing shoots in 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 near fruit clusters or on young trees and nursery stocks. Growers are encouraged to monitor the migration of woolly apple aphid in their orchards. When colonies remain on water sprouts and limbs away from the fruit clusters, damage is minimal

Orchard management has an impact on woolly apple aphid populations. The following guidelines may help prevent a build-up of aphids, and reduce the population without pesticides.

• Remove suckers at the base of the tree trunk. This will eliminate a favoured establishment site for these aphids. Early generations which develop on these suckers will migrate up into trees.
• Remove sucker and water sprouts on major scaffold limbs. These are the preferred sites of aphids and "handsuckering" early in the season (June) improves spray coverage inside the tree.
• Paint large pruning cuts with a commercial pruning paint to discourage aphid colonies.
• Summer pruning in August will remove larger developed colonies.

When necessary, summer chemical management is better than in late summer. Chemical management is also better when nymphs are young and when colonies start to form. See the Pest Management Decision Guidelines in Chapter Five of Publication 310: Integrated Pest Management for Ontario Apple Orchards, for insecticide choices . For best results, use higher volumes of water for thorough coverage. If aphids migrate up into the tree after the first spray, a second spray may be necessary in 14. The white, waxy covering protects the aphid from insecticides and a full mature leaf canopy prevents adequate spray coverage. Therefore, later season sprays are discouraged.

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