Have you noticed your cottonwood, aspen and other poplar leaves turning yellow earlier than what you think is normal?
During the mid to late summer of this year, large cottonwoods, aspens and other poplars have been attacked by poplar petiole gall aphids (Pemphigus populitranversus), which enlarge as they feed on the sap of these poplars.
This activity is most noticeable on North West poplars as the aphid feeding causes the leaves to turn a crisp, bright yellow and then drop prematurely in large numbers from the trees by mid-summer. This behaviour is caused by large numbers of very small aphids.
The petiole is the name given to a leaf stem that is attached to a woody twig. A gall is an unnatural type of swelling and in this instance, it is located at the base of the leaf blade, which is another name for the flat leaf surface.
In the case of the swelling produced by this gall aphid insect, it is unique to our trees in Manitoba.
The aphid adult lays eggs in the petiole at the base of the leaf blade. The eggs hatch to become aphid larvae and feed on the tree’s sap.
As the larvae grow, the base of the petiole swells into a distinctly round shape called a gall. The larvae are maturing in this gall.
The rapid growth of the aphids eventually causes the transverse splitting of the gall. This is a unique positive sign that the aphids have attacked a poplar tree.
The mature aphids are then able to fly off through the split to lay eggs either in the tree from which they originated or in other trees. Let’s start at the beginning of this unusual insect life cycle.
Of all the tree-feeding aphids, the poplar petiole gall aphids are among the most complex.
The aphids have an unusual one-year life cycle that alternates between two groups of hosts. Only one host is a tree. A tree "host" is the parent tree on which the aphids initially develop. The female aphids lay the eggs during the fall season in bark cracks on preferred poplar trees.
In spring, the eggs hatch into nymphs (a name given to aphid larvae) at about the same time as the appearance of new poplar leaves just emerging from the buds.
Now, here is an unusual fact about this species of petiole gall aphid. All of the developing aphids will become asexually reproducing females. These females therefore cannot produce new broods of aphids.
Nymphs feed on developing leaf petioles through tubular, sucking mouthparts. This frenzy of feeding induces the attacked host poplar to produce a swollen growth that is called a "gall." The gall envelops the developing aphids.
As this overwintering asexual form of the petiole gall aphid matures, it produces young that remain within the gall until full-grown. These new adult females develop wings.
As the galls grow, they split open in a transverse manner during the course of the summer. The newly developed winged adults fly during late June and July to their summer hosts through these slits.
Here is another strange fact about these aphids: The second new hosts are not poplar trees, but the roots of plants in the cabbage family. The aphids are able to find cracks in the soil where they will locate and find those roots.
Colonies of nymphs — not just those in the poplar family — secrete a waxy substance that is believed to protect them from excess moisture. I believe they are also protected from predators. I have seen these waxy secretions on ash tree leaves, for example, produced by the ash leaf curl aphids. Several asexual aphid female generations may be produced on these summer host underground plants, namely the cabbage family roots.
At the end of summer, winged adults are produced that fly back to their winter host poplar trees where they give birth to small, mouthless males and females that mate and then the males die. This is a brief sexual generation. After mating, each female lays one egg that is almost as large as she is and then dies.
As mentioned, the poplar petiole gall aphid forms a spherical green gall with a transverse slit on the petiole of large poplars. Usually you find the yellow leaves on the ground showing this obvious feature. This is a key feature in identifying this pest and its feeding behaviour. It is also a root-infesting pest on cruciferous crops in summer, where it is known as the cabbage root aphid.
I find it amazing that such a tiny creature as this aphid can have one of the most complex insect reproduction cycles in our area. In my nearly 47 years of experience in the tree business, the families of aphids are by far the most complex in the insect world here in Manitoba.
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