THIS summer, wingless aphids have appeared in very large numbers on the leaves of green ash tree leaves and to a lesser extent on black ash leaves. Where I live, there are a great many ash trees displaying the obvious signs of heavy feeding by aphids on the leaves.
This aphid is called the ash curl aphid. Entomologists have given this aphid the scientific name, Prociphilus fraxinifolii. In my experience, I have not seen this species of aphids for years. It constantly perplexes me why they suddenly show up now. When I examine the trees, I do not see the major predatory insects of aphids — ladybugs — whose larval stage looks like a miniature dinosaur feeding on the aphids. I suspect the ladybug population has been significantly reduced by insecticides. I find this sad situation to be common on many property trees I inspect.
The aphids feeding in the ash trees were brought to my attention by many people, including my neighbours.
I have a Mancana ash — a cultivated variety of Manchurian ash — which has not been affected by the aphids, and yet many Mancana trees are.
This year, the aphids were so numerous that they caused the leaf tissue to shrink and curl into a ball-like structure enclosing and protecting the aphids while they fed on the leaves. This is the main distinguishing feature of the leaf curl aphids. The other very prominent feature of these aphids are the fine strands of waxy filaments they produce to completely surrounding their bodies.
How do aphids reproduce?
Interestingly, in the early stages of their development, all aphids are female. New aphids are produced by unfertilized females through a process called parthenogenesis, not uncommon among many insects. Males do show up in much fewer numbers later in the summer to repopulate the species for the following year.
There are a very large number of different species of aphids. The ones that are now preying on green and black ash trees can do much damage to those trees. Curled leaves, often appearing like balls, are very common features with advanced populations of these aphids.
On a daily basis, I am examining many ash trees to see if there are any predatory insects attacking the aphids. I have examined dozens of ash trees and I have yet to see one ladybug. I would welcome comments from readers who have observed active ladybug-feeding this year. Yet while examining trees and shrubs at the Manitoba Legislature grounds with some master gardeners this past June, we saw numerous ladybug larvae doing their predatory work on aphids.
Where and why have they gone?
Controlling aphids in the summer on mature trees is very difficult and next to impossible for property owners who do not have access to the appropriate spray equipment and chemicals.
Summer spraying for the leaf curl aphids is too late, in my experience, for effective control. In addition, a great deal of bird species and desirable predatory and pollinating insects — especially wasps, bees and butterflies — can be injured or killed by pesticide sprays at this time.
Aphids are best controlled in the egg stage in April before the leaves emerge from the winter buds. The product that can be used at that time is dormant oil — a horticultural product that can only be sprayed on woody plants where buds have not opened. Never spray dormant oil on leafy vegetation. It will damage the leaves. If you have woody shrubs, especially dogwoods and European cranberries that have been susceptible to summer-feeding aphids — and most of these shrubs are — I recommend you use dormant oil as well, but in April. This product is far more environmentally friendly than the pesticide spray compounds.
If you have been plagued by aphids the summer — and a great many people have been — make a note on your calendar to purchase dormant oil for treatment on your shrubs and trees next April. Buy the product this summer so it’s ready for use next year. These treatments can also be done successfully on large trees by licensed pesticide spray applicators. In the Winnipeg area, we have had more than our share of rainfall and strong winds, which are, of course, significant factors that limit pesticide spraying.
Please remember that summer is not necessarily the best time to spray pesticides on urban trees.
Sometimes it is difficult to enjoy our gardens when we are plagued with pests. With luck, the pests will not be so bad next year.
Michael Allen M.Sc.F., RPF (ret’d) is a consulting urban forester, tree diagnostician and certified arborist. He owns Viburnum Tree Experts. He can be reached at 204-831-6503 or 204-223-7709. His website is treeexperts.mb.ca