Aphids are an insect with a soft body that utilizes their mouthparts to pierce and suck the sap from plants. These colonies of insects can usually be found on the underside of newly growing terminal growth. If a leaf becomes heavily infested it will appear yellowish and will eventually wilt. This is caused by the colony removing large quantities of the leaf's sap. Much of the damage caused by aphids is cosmetic, meaning it looks unsightly to the homeowner. Generally speaking a healthy tree or shrub is able to withstand several years of aphid attacks. While consuming the sap of trees and shrubs, aphids produce a liquid, sugary waste product commonly known as "honeydew". If you have ever parked your vehicle under a tree and noticed a sticky residue on it you have experienced this. Sooty mold can also grow on these sugary deposits found on the branches and leaves. This causes them to acquire a blackish discolouration.
THE PROBLEM BEGINS:
Winged aphids or "colonizers" will fly around searching for a suitable host tree or shrub. Once they have found such a plant, they will drop wingless young on the new growth and continue on their way. These nymphs feed voraciously on the sap and increase in size. In 7-10 days they will mature and are capable of producing live young, generally 40-60 young each. The interesting thing about this is that most are born female. This creates a population explosion in no time. An example of this is fewer than 12 colonizers can produce hundreds if not thousands of offspring in just a few weeks. This process continues until the plant can no longer support the population. At this point, new winged aphids are produced and the cycle is repeated.
By discovering the aphids early you can minimize the effects they will have on your trees and shrubs. Keep an eye on terminal growth. Examine underneath new leaf growth for groups of aphids. By being vigilant you can usually control theses colonies by hand, either crushing them or pruning. If however, you discover aphids on more than 10% of your plant, you may want to consider using a contact insecticide to control them. As this is an insecticide please be sure to follow the instructions on the label carefully before and when applying them to your plants. For a contact insecticide to work effectively you must actually hit the aphids before control will be successful. Ensure you use an even thorough spray pattern when applying. Concentrate on growing points and protected areas within the plant. If possible spray underneath leaves for effective control.
As stated earlier, you may be able to control small populations of aphids by crushing them by hand or by pruning out affected areas. A good rinse with your garden hose will also dislodge some of the colony. Ladybugs and lacewings are also very effective at controlling aphids. These predators will consume large quantities of aphids.