Some of you might have noticed the fact that there are Pine trees, and even Cypress trees all over Cape Town, and all over the Western Cape, that are getting discolored.
This seemingly small problem can evolve into something extremely serious if we do not take action soon. These trees are the victims of aphids, something that previously was not very common around this area, but now seems to be spreading rapidly. In all the research I have done on aphids, and in my personal experience, they rarely kill trees, but for some reason, the new infectors are killing trees at a rapid pace. They make quick work of destroying a tree. In the space of time it takes to mow the lawn or make a phone call, these little creatures can already inflict a harmful effect on even big, old trees. It is rare to see such a damaging outcome from tiny aphids. This is a very big concern, as it can evolve into a full blown pest.
There seem to be 2 likely suspects, one being the White Pine Aphid (Cinara strobe) and the other Monterey Pine Aphid (Essigella californica). Both these aphids feed on pines and both use the same feeding habits:
The aphids feed by piercing the stem of the pine with a stylet mouthpart. This is then used like a soda straw to draw "sap" and nutrients from the plant. As the sap has an excess of sugar from the aphids' perspective, they excrete a sugary syrup, called honeydew. This is the mist of droplets that make windshields of cars sticky and create a problem for car paint. You may notice "wet" patches on sidewalks under pine trees where this syrup accumulates. Besides being a nuisance for owners of cars parked under pines or people sitting on chairs or benches under the trees, the aphids are robbing the trees of the sugars and amino acids that they have synthesized from sunlight, water, and carbon dioxide. These materials were made to feed the tree, not the aphids. So, the trees are injured as they have lost some of their food supply. As this honeydew accumulates on the leaves and twigs of the tree, a fungus, sooty mold, begins to grow, using the honeydew as a food. The sooty mold doesn't hurt the tree except when heavy amounts develop and block sunlight from the chlorophyll of the leaves. In this instance, the sooty mold can then reduce photosynthesis and the ability of the tree to synthesize the food it needs. (Sourced from New Mexico State University Website).
Not all of the above mentioned signs of aphids are visible in the trees that are infested here at home. Researching aphid infestations in New Zealand and Australia brought me closer to what is happening on the home front. The aphids seem to feed off the pine needles rather than the stem of the tree. This causes much more severe damage, which in turn causes the yellowing, and then dying of the needles, and eventually the dying of the infested branch, resulting in the dying of the whole tree. As I said before, normally aphids should not kill their hosts, but I have witnessed it in several locations around Cape Town. This also explains the lack of signs that would normally accompany aphid infestation, as mentioned above.
I suspect the aphids to be Monterey Pine aphids. They have been known to have sexual reproductive stages, but this has only been observed at high altitudes in North America. In Australia the life cycle is without sexual reproduction, as no males have been found and thus it is totally parthenogenic.
The aphids are most active in summer, and then spend winters in egg form on the needles. The adults can be winged or wingless. They feed singly, rather than in groups, which makes them hard to spot, but may sometimes confer at the base of needles.
There are several methods for approaching aphid removal, having varying rates of success. There are natural solutions that do not always work, mainly, spraying the tree with a forceful water spray, to dislodge aphids. Insecticidal soap is also a low impact option; look for active ingredients listing potassium, or sodium salts of fatty acids. Horticultural oil has also been known to help get rid of aphids. Lady Bugs and certain wasps seem to be good natural predators of aphids, and if the ants are not in a symbiosis with the aphids, ants can be good predators too. The best known chemical to treat aphids with is Metasystox, although I have heard conflicting reports about its availability.
In my personal experience, the most effective way to get rid of an infestation, is to remove the infected branches. This is best done in the winter, as it is easy to see which branches are already dead, and then also remove the branches with eggs on them. It is very important that this gets done safely, and by a professional, as climbing big trees, and cutting limbs is not an ordinary job. It is also important to cut the right branches.
Written and researched by:
Contact: 083 267 96 39
Willem Avenant - Tree Specialist - Western Cape, Cape Town