We have a couple of european prune plum trees in our yard and they’ve been attacked by a small white bug. It almost looks like dust/ash until you come closer and see… they are leaving an oily film on the leaves and making holes in the them… any idea what they are and how to get rid of them? We have been spraying them with insecticide soap. The trees are in full sun all day and there is nothing planted around their base.
Thank you for contacting the Toronto Master Gardeners with your inquiry.
It is difficult to tell from your photo exactly what pest it is however, from your description it appears that you could have an infestation of white fly or aphids. Whiteflies like aphids use their piercing, needlelike mouthparts to suck sap from phloem, the food-conducting tissues in plant stems and leaves. Large populations can cause leaves to turn yellow, appear dry, or fall off plants. Like aphids, whiteflies excrete a sugary liquid called honeydew, so leaves may be sticky and shiny.
One of our earlier posts details the proper control of white fly:
“If the white specks are observed on the under side only of the leaves, the specks could be whiteflies. Insecticidal soap solutions (made up according to product label instructions) sprayed on both sides of the leaves may be helpful. This method eradicates the adult insect only, so repeated applications will be necessary to catch all the adults maturing through eggs, nymph, pupa then adult stages. Another method is to buy sticky yellow pad traps in garden stores. Place the traps close to the leaves and the whiteflies, attracted to the colour yellow, will land on the traps and get stuck on the glue. Thirdly, earthworm castings, when added to the soil of whitefly-infested plants has been found to repel the flies within a few weeks. Earthworm castings can also be purchased in garden stores.”
The control method for aphids is similar. A strong blast of water from your garden hose can dislodge the adults along with repeated application of insectcidal soap to catch the nymph stages can help control the population of this leaf sucking insect. When spraying with insecticiadal soap, make sure to spray both the tops and underside of the leaves.
Natural enemies such as lacewings, and parasitic wasps may help to control aphids, but may not appear in sufficient numbers until aphids are abundant.
If the infestation is severe, you may which to contact a certified arborist to apply the soap. To find a certified professional arborist to help you with a tree problem, visit the Ontario branch of the International Society of Arboriculture here.
We also have a Gardening Guides on Controlling Insects without Pesticides and Integrated Pest Management which you might find of use.