Aphids Make Trees Sticky
Marianne C. Ophardt
Washington State University Cooperative Extension
Area Extension Agent
“Why are the leaves of my trees shiny and sticky? My trees are weeping and get everything underneath so sticky, what’s wrong with them?” As summer progresses, we’re starting to see more insect pest problems bothering trees, shrubs, and other ornamental plants. One major insect offender is aphids. Aphids are often a perennial problem on some types of plants. With trees and shrubs, like maples, ash trees, and roses, it’s a continual battle to keep aphids at reasonable levels.
Knowledge of your adversary can give you a tactical advantage in your yard and garden confrontations. Aphids are soft-bodied insects which feed on plants by piercing leaves, stems, or twigs and then sucking out plant sap. They often suck out more sap than they can use and secrete the excess as a sticky waste... called “honeydew.” Honeydew is mostly sugar and water and isn’t harmful. This sticky goo, or honeydew, may be eaten by other insects, or a black mold may grow on it. This black mold, called sooty mold, is not harmful to the plants.
Along with the problem of “sticky trees,” aphid feeding can cause other problems. Excessive feeding can lead to a loss of plant vigor and retard growth. Aphids also secrete a saliva into plants while they feed. In many plants this saliva causes distorted plant growth in the form of curled leaves, swollen nodes, and distorted growth. Repeated severe annual infestations can lead to plant die-back.
Before trying to manage any insect population that has gone beyond tolerable levels, you need to understand the insect and it’s life cycle. There are many different types of aphids with varying life cycles. Entomologists say that there is probably at least one species of aphid that feeds on nearly every type of plant... trees, shrubs, flowers, vegetables, row crops, and weeds! With so many different species of aphids it’s hard to know the exact life cycles of all of them, especially since aphids seem to have such a complex and strange life cycle.
Aphids can overwinter as eggs or mature adults. In the spring, the eggs hatch into “Big Mommas” (actually, entomologists call them “stem mothers”) who produce live young without the “disadvantage” of mating. These daughters also mate asexually, producing even more daughters. You can see that without the need for mating that aphid populations can build up quite rapidly. In fact, it has been estimated that one cabbage aphid “Momma” can be responsible for a family of 1,560,000,000,000,000,000,000,000,000 aphids by the end of the season... under ideal conditions.
If you’re wondering where all these aphids might be hanging around, they’re probably not all still alive. Aphids take about one week to mature and then most live for only about three weeks. Many become the dinner for other insects. If the colony becomes too crowded or the host plant is stressed, winged aphids are produced so they can move out on their own and seek new sources of food. In the fall, some aphid species produce both male and female aphids for the purpose of mating. After mating the females lay eggs in protected locations, such as in buds or cracks in the tree bark, for overwintering.
Now we know the life cycle, but what about how aphids look? Identifying the right pest is important. As mentioned before, aphids are soft-bodied insects. They come in a variety of colors including green, white, yellow, brown, black, gray, pink, purple, or red. Some are even covered with a whitish “wooly” or cottony coating. Aphids are usually found in large groups on plants, often on tender new foliage or the under sides of leaves. Most aphids don’t move too fast... as you can’t budge them easily once they’ve found a good source of food. They know a good thing when they taste it!
Now that we know more about how aphids live and grow, let’s talk about when and how to “control” them. There are some fairly innocuous methods you can try... when and where they’re practical.
- First look for signs of natural control. Aphids have a number of natural enemies... ladybird beetles and their larvae, green lacewings, small parasitic wasps, syrphid fly larvae, predacious stink bugs, and assassin bugs. If you’re able to find evidence that these natural enemies are present and working, don’t do anything unless the aphid population appears to be beyond their control.
- Dislodge aphids with a fine but forceful spray of water from your hose. Be sure to move the spray back and forth as well as directing it towards the undersides of the leaves. This will kill some aphids and many will not make it back to the plant. Repeat this every several days. This method of control can’t be used effectively on small or tender plants.
- On some plants you can simply wipe aphids off plant stems or buds. However, you may want to wear a pair of garden gloves to do this. Snip off badly infested branch or stem tips. Avoid fertilizing your plants with too much nitrogen. Soft, lush growth encouraged by excess nitrogen is exactly the type of growth that aphids thrive upon.
- Avoid aphid buildup early in the season on woody trees and shrubs by applying dormant oils to plants that have had problems with aphids in the past. The dormant oil should be applied in early spring when temperatures are above freezing, but before the buds start to show green tissue. Use according to label directions... of course.
- If aphids are out of control and an insecticide is warranted, first consider the use of insecticidal soaps. These are good materials to use because they protect a number of the “good” insects and they are very low in toxicity. It’s important to get good coverage and to spray both the tops and bottoms of the leaves. Repeat as needed. Check product labels to avoid applying to plants sensitive to soaps. Don’t apply to stressed plants.
- When aphid infestations are so severe that no other method of control is practical, a chemical insecticide may be needed to prevent further plant injury. There are a number of pesticides labeled for control of aphids on plants. However, some of these can only be used on ornamental plants because of their systemic action... meaning that they’re absorbed into the plant sap and move within the plant’s vascular system. Systemics are particularly useful when aphids are protected by the leaves curled with their feeding damage. Curled leaves protect aphids against contact insecticides. Some materials are systemic sprays, such as Orthene, which is taken up into the plant sap of treated leaves. Others, such as Merit or Di-Syston, are applied to the soil and absorbed with water by the roots.
Root applied systemics are a practical way of treating a large shade tree with severe aphids populations. They’re also less detrimental to the beneficial insects in the area. Di-Syston and Merit (imidacloprid) are available to home gardeners. Other soil applied systemics, such as Cygon, are available only to licenced commercial pest control operators. Cygon can be used by home gardeners as a spray, but it’s illegal for them to use it as a soil applied drench on shade trees. Keep in mind that most systemic insecticides are quite toxic to humans and animals. Some plants may also be injured by the use of certain systemic insecticides. Carefully read and follow all the label directions when using a systemic insecticide or any other pesticide product.
Aphids can be pesky, but a little knowledge about how they behave and how to safely manage their populations can lead to truce in your yard and garden.