Close up of leaf with wooly aphid

Have you ever noticed the leaves on your tree appear shiny as though someone sprayed them with shellac? Or maybe the hood of your car is covered with fine droplets like it was misted with a sticky material? Maybe there is a trail of ants marching up the trunk of your tree. Or maybe, those shiny leaves are turning black. If you have observed any of these changes to your tree then you have experienced life with aphids!

What is an aphid?
Aphids are small, soft-bodied insects with long slender mouthparts that they use to pierce stems, leaves, and other tender plant parts and suck out fluids—just like a vampire for plants! They are everywhere in your garden. Almost every plant from vegetables to flowering annuals to shrubs and every kind of tree whether it be a broad leaf or a conifer with needles has one or more aphid species that occasionally feed on it. They are easier to spot than other insects because they don’t move very fast. And they tend to hang out in dense groups. But boy, do they reproduce fast! In fact, because of California’s mild climate, they are able to reproduce asexually throughout most or all of the year with adult females giving birth to live offspring—often as many as 12 per day—without mating! (Where’s the fun in that??) 

Tulip tree leaf coated with honeydew


Are they bad for my tree?    
The good news is that low to moderate numbers of leaf-feeding aphids aren’t usually damaging in gardens or on trees. And they don’t always infest the whole tree. Sometimes they’ll just infest a section. Some aphid species inject a toxin, mostly on ash trees, which causes leaves to curl and further distorts growth, but is not lethal to the tree. Overall, aphids are more of an annoyance than a tree killer. 

What is it that is making my car sticky? 
Aphids are best known for the copious amounts of a sticky exudate known as honeydew. It is a great food source for both insects and fungi. Sooty mold, a fungus that thrives on the sugar-laden leaves of plants infested with aphids, loves the honeydew and will feast on it turning the leaves black with mold. The good news is that honeydew is fairly easy to clean up. Since it is mostly sugar it can be washed off with water, unlike sap which requires a lot more scrubbing to remove. 

How do I get rid of aphids?
The best way is to do nothing and allow the aphids’ natural predators to kill them. Read on, the next section will help explain the best way to approach eliminating aphids. 

Black sooty mold feeding on honeydew

It all starts with honeydew! 
Aphids, with their ability to produce valuable honeydew, provide us with an interesting relationship between a wide range of different bugs. Honeydew is a great food source and nobody knows this better than the ants. That’s one reason you may find a trail of ants on your tree. The ants so highly value this source of food that they will tend to the aphids like cattle, herding them around, milking them for honeydew, and will kill the aphids’ enemy, a tiny parasitic wasp that lays its eggs within the body of the aphid and kills it. Therefore, you don’t want to use a broad-spectrum pesticide in your yard, like Raid, because it will kill these beneficial parasitic wasps along with the ants and aphids. And since aphids can reproduce asexually, it doesn’t take much for them to start reproducing, and without any enemies, their populations can explode. 

Get rid of the ants!              
If you do have ants, and now that you know they are protecting the aphids, you can set out ant sticks that are specifically targeted to kill just the ants but not the wasps. 

Bring on the ladybugs!
Another strategy for controlling aphids is to increase the population of their natural enemies. Lady bugs, those “cute” black dotted red bugs love aphids as a food source. It is possible to buy a bunch of live ladybugs to release into your garden. They probably won’t stay for much longer than a day or two before they disperse to other gardens but if the aphid population is plentiful they might be tempted to stay longer and take advantage of an aphid feast. However, ladybugs aren’t the only bugs that feast on aphids. So that’s why you want to avoid spraying with chemicals that might kill these beneficial insects. 

Pruning can help
In large trees, some aphids thrive in the dense inner canopy, and pruning out these areas can make the habitat less suitable. Pruning out ant routes such as branches touching buildings, the ground, or other trees can help control aphids.

But I have a really bad problem with aphids in my trees.  The mess in my backyard is constant! Is there something I can spray them with?
When considering whether to apply insecticides for aphid control, remember that most larger plants can tolerate light to moderate levels of aphids with little damage. Also, aphids don’t like hot weather, so once we have a heat wave, the population of aphids rapidly declines. Often a forceful spray of water or water-soap solution, even on large street trees, can provide some control. 

The best way to attack the problem is to apply the insecticide through a soil drench. The infected tree takes up the insecticide through its roots and into its leaves where the aphids chew on them, digest the poison, and die. The product is not selective and will kill any chewing or sucking insect but does not kill the beneficial predators like lady bugs which eat the aphids. 

It is possible for you to purchase an over-the-counter soil drench for aphids you can apply yourself. They are very effective on small garden plants like roses and other shrubs. But when it comes to treating a large tree, leave it to the professionals. Professional applicators can use soil injectors, which provide better control with less runoff potential. Soil drenches on large trees need adequate rain or irrigation to move the product through the soil to the roots and up into large trees. In our area, it is recommended to apply the soil drench in December/January to allow for winter rains to irrigate the trees and move the insecticide into the emerging leaves in spring. Soil drenches can be very effective in controlling aphids and one application can last a whole year. 

So if it’s sticky it must be aphids, right?
Not necessarily. Unfortunately, aphids are just one of many bugs that can cause your tree to create a sticky mess. For example, if your oak tree is creating sticky droplets refer to our newsletter on “Drippy Nut Disease” published last year. It will tell you about another bug that infects acorns with a disease that causes them to drip a sticky residue. So if you are unsure if you have aphids or not, please call and arrange for one of our certified arborists to inspect your tree.