We have a corkscrew willow in our yard, 20 ft tall, around 15 yrs old. Around 6 weeks ago, willow aphids (when crushed, looks like a small blueberry stain) have taken over, and because of their infestation, yellow-jacket numbers have also increased significantly (which is not ideal with young children playing). What to do?
The corkscrew willow (Salix matsudana) is a lovely and interesting specimen tree, but it is unfortunately highly susceptible to pests and diseases, the willow aphid being one. These insects feed by sucking the sap from leaves and stems and in doing so excrete a syrupy substance called honeydew which is very attractive to wasps, especially at this time of year.
Many sources suggest that the most effective solution is to try to reduce the amount of honeydew on the surfaces of the tree by spraying it thoroughly with a strong spray from a hose. This should also reduce the aphid population by forcing the aphids off the tree, and may help to avoid a re-infestation since we are nearly at the end of the growing season. If you see aphids again next year, begin washing your tree as soon as possible to avoid an extensive infestation. Avoiding the use of insecticides helps to ensure a healthy population of predator insects (including those wasps) to help keep the aphid population down.