Woolly aphids are sap-sucking pests that are bluish-black in colour that produce a covering of fluffy white wax that resembles wool. They can be found clustered around the bases of new shoots on branches, wounds on trunks and branches, and on exposed roots of apple trees.
These pests also attack various other shrubs and trees, including alder, elm, mountain ash, hawthorn, serviceberry (Amelanchier), and firethorn (Pyracantha) trees. Infestations of woolly aphids tend to be sporadic, and vary depending on time of year, location and tree species.
The sap loss caused by aphid feeding is usually not significant enough to reduce the vigour of healthy trees, but with heavy infestations the leaves may droop, or dry out, shrivel and drop prematurely. The biggest problem with sap-sucking insects is that they can spread diseases from tree to tree.
Management of Woolly Aphids
It’s easy to control woolly aphids using safe, environmentally friendly control methods.
- Wash aphids off from trees and shrubs using with a moderate jet of water. Even heavy rainfall can reduce their populations.
- Wipe them off by hand or prune back selected unwanted new growth to control small, localized aphid infestations.
- To control large infestations, spray the aphids with horticultural oil or insecticidal soap. Follow the application rate shown on the label. For aphids, 5ml/litre of horticultural oil is typically used, and if the waxy coating is repelling the spray, use 10ml/litre as is used to control the more resistant scale insects.
- Prevent the problem by using natural slow-release fertilisers, avoid synthetic high-nitrogen fertilisers as high levels of nitrogen encourage aphid reproduction.
- Use glue-banding on tree trunks to keep out ants that harvest the sugary honeydew that aphids excrete and in turn protect the aphids from natural predators.
- Encourage natural predators such as ladybirds, lacewings, syrphid (hover) fly larvae, and parasitic wasps by planting companion plants that have shallow flowers such as sweet alyssum and calendula to provide an alternative nectar (food) source for these beneficial insects.
- Avoid use of broad-spectrum insecticides as they kill these beneficial insects which are more greatly affected than the pests, and take longer to recover their populations, leading to explosions in pest populations!
Hover fly larvae are voracious predators of aphids, and they consume them in great numbers. A study by Washington State University in 2013 found that planting sweet alyssum flowers attracted hover flies which did a good job of reducing woolly aphid populations. The study’s findings were that:
- Sweet alyssum flowers had the highest attractiveness to syrphids (hover flies).
- Faster suppression of woolly apple aphid occurred on trees closer to alyssum flowers.
- Higher densities of natural enemies were observed near sweet alyssum plantings.
- Natural enemies were found to move between sweet alyssum and adjacent apple trees.
This study can be found at – https://doi.org/10.1016/j.biocontrol.2013.03.007
Using any of these methods or even better, a combination of them, can effectively control woolly aphids in fruit trees.
- Controlling woolly aphids on apple trees, University of Washington, Botanic Gardens
- Woolly Aphids on Trees, University of Maryland Extension. Extension.
- Woolly aphids, Iowa State University, Horticulture and Home Pest News. Hortnews.
- Lessando M. Gontijo, Elizabeth H. Beers, William E. Snyder, Flowers promote aphid suppression in apple orchards, Biological Control, Volume 66, Issue 1, 2013, Pages 8-15, ISSN 1049-9644, <https://doi.org/10.1016/j.biocontrol.2013.03.007>