Aphids Cause Sticky Leaves & Black Patches

Aphids (sometimes called plant lice) are small insects that suck the sap from plant leaves, twigs and stems.

There are dozens of species of aphid here in Colorado affecting most shade trees and shrubs, as well as ornamental landscape plants. Some aphid species can even be found on conifers.

Identifying Aphid Damage

While aphids generally don’t cause significant damage to trees and shrubs, they can cause an unsightly mess. If you’ve ever noticed a sticky substance coating leaf surfaces, or even underneath trees on your deck, walls or walkway, then you’ve seen the result of an aphid infestation. You’ll often see this beneath aspen trees.

Sooty Mold

Sooty mold can leave black patches on walkways, decks and walls

The insects excrete honeydew, a sweet, sticky liquid that’s attractive to ants and even bees and wasps. Many homeowners think the tree is “bleeding” sap when it’s really just honeydew produced by aphids.

Honeydew is also prone to developing sooty mold, a fungus that covers surfaces with a grey, powdery coating. It doesn’t really harm the tree although it may reduce photosynthesis (and therefore decrease tree growth and vigor) if it coats most of the leaf surfaces on the tree.

In some cases, particularly if the infestation is widespread, aphids can affect plant health. If you notice leaves starting to curl on a plant with aphids, it’s generally a sign that treatment is needed.

Another telltale sign is the presence of large numbers of wasps and/or ants in your trees. They are collecting the honeydew for its sugar content. With these issues you want to treat the aphid, not the wasp or ant; those guys will leave once the aphids are gone.

Aphid Control Options

For smaller infestations, Mother Nature will often take care of the problem. Aphids have several natural predators, such as lady beetles (ladybugs), green lacewings, syrphid flies and some parasitic wasps. They’re generally attracted when the aphid population grows large enough, so if you plan to use an insecticide, check your plants before applying it to make sure you won’t be inadvertently killing beneficial insects.

If your tree or shrub is small enough, you can blast aphids off the plant with a strong spray of water from a garden hose. Once aphids have been knocked off, they’re unable to climb back onto the plant and will quickly die. Hosing plants down also helps remove the sticky honeydew and/or unattractive sooty mold.

Smaller trees and shrubs can also be sprayed with an insecticidal soap.

For larger trees, a systemic insecticide is needed (meaning that the insecticide is taken up by the tree and spread to all branches and leaves). This is the best option if leaves are curling.

When to Apply Aphid Treatments

There are two treatment timeframes here in Colorado, with two different types of treatment.

Early Season Treatment

green aphid

Aphids come in several colors, including green, orange and black.

Most aphids that cause problems early in the season will hatch from eggs in spring. Initially they stay on the plants where they hatched, but later in spring and summer you may see them moving to other plants (they’ll even infest conifers in late summer).

That means in early spring, before bud break (usually in April/early May), is a great time to treat plants with our soil injection method containing a systemic insecticide. This application will provide season-long control of aphids and other insects that can damage your plants. It also is a complete fertilizer, providing the nutrients needed throughout the season. We can make the mix for your site based on the plants and soil requirements we find.

If you had problems with aphids last year, call us in March/April to get on the treatment schedule before overwintering aphids hatch.

Summer Treatment

leaf curl from aphids

Aspen leaf with aphid damage (the black spots are aphids).

Aphid populations can increase explosively in summer, with females giving birth to between 1 and 20 live young each day! After only one or two weeks, those new aphids will each begin to produce more young, leading to huge growth in numbers in a very short period of time. At that point, we may need to employ a foliar spray to reduce the population and allow time for a systemic treatment to begin working.

If you notice sticky honeydew or blackish sooty mold, give us a call and we’ll see if an insecticide treatment is the right option for you.

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