Dear Ray,  Thank you for contacting CSU Master Gardener Lawn & Garden Help Desk about your Linden Trees.

Myzocallis tiliae (Linden aphid) is probably the type of aphid on your Linden Tree.   

One thing you might want to consider, is the tree's stressed?  Look at your trees carefully and try to determine when it started to look stressed.  Are they getting enough water, have they been damaged by a weed trimmer or are they getting back drift from other insecticides for other plants?  If you determine one of these are causing your trees to be stressed, try to resolve the issue.  

A drench spray is systemic. That means the whole tree, leaves and flowers absorb the spray. This spray will kill the pollinators, especially bees. If you do decide to use a drench, please spray after the flowers have died off. Below is a link about the issues using a Drench Spray.

https://ncipmhort.cfans.umn.edu/sites/ncipmhort.cfans.umn.edu/files/2022-06/Protecting_Bees_from_Systemic_Insecticides_2022.pdf

But there is an option about using oils.  Below is a couple of links , that might resolve your problem.

https://cmg.extension.colostate.edu/wp-content/uploads/sites/59/2019/06/119-WS-PstMng.pdf

https://extension.colostate.edu/topic-areas/insects/insect-control-horticultural-oils-5-569/

I hope this helps, but if you have any additional questions please feel free to contact us again.

Good luck with your problem.

Stacy S

Master Gardener Apprentice

On Mon, Jul 1, 2024 at 10:09 AM, Ask Extension

<<personal data hidden>> wrote:

Good Morning Ray,

Thank you for contacting Ask Extension for additional information about how to manage the aphids on your Linden trees.  You would like to know what type of chemical spray you should use.  We cannot make specific pesticide recommendations but can send a fact sheet listing the choices.

Your first line of defense could be to spray your trees with a forceful jet of water to lethally injure or knock the aphids off of your trees.  This will not get rid of every aphid so you will need to monitor populations to determine when more spraying with water is needed,

I am sending an aphid fact sheet which discusses how to control aphids here in Colorado, suggestions for chemical controls are included.  If this is your choice, please read the label carefully and follow all of the instructions to ensure the safety of people, pets. beneficial insects such as bees, and the environment.  Personal protective equipment may be recommended.

https://extension.colostate.edu/topic-areas/insects/aphids-on-shade-trees-and-ornamentals-5-511/

If your trees are large and aphid management could be difficult, you may want an ISA certified arborist to come look at your trees.  Please visit the, Trees Are Good,  link included  below.    Instructions are provided about how to find an ISA certified arborist for your area.

  https://www.treesaregood.org/findanarborist/findanarborist

Regards,

Barbara S.

El Paso County Colorado Master Gardener

Hello Ray,

I am a different Colorado Master Gardener answering your question,  I have found the quote you are questioning,  "If you do decide to use a drench, please spray after the flowers have died off,"  Stacy made this recommendation to protect the bees who are visiting the flowers. A drench is applied to the soil and a spray applied to the leaves.   You would not need to spray another pesticide after using a drench. A systemic drench is applied to the soil to move through the tree to kill the aphids and unfortunately also the bees.  This is why you should wait until the flowers have died off to protect the bees before applying any pesticides, drench or spray.  

Again, we cannot recommend specific pesticides to treat aphids.  The fact sheet I am including lists many options.

https://extension.colostate.edu/topic-areas/insects/aphids-on-shade-trees-and-ornamentals-5-511/

"Soil-Applied Systemic Insecticides. The systemic insecticide imidacloprid can be applied to the soil where it may be picked up by plant roots then moved through the plant, concentrating in the newer leaves. This insecticide is widely available at retail and sold under several different trade names. In some formulations used on trees it is combined with a second systemic insecticide, chlothianidan."

"Non-persistent Contact Insecticide Sprays. Insecticidal soaps and pyrethrins are two commonly available and popular insecticides that can be used to control aphids. Neither of these persists for long (minutes to hours) on the foliage. Only insects that are covered with sprays during application can be killed with these types of products. Neither will move systemically in the plant and they cannot control aphids that have curled leaves and cannot be reached with sprays."

"Persistent Contact Insecticide Sprays. Many of the insecticides sold at retail stores will persist for awhile in their ability to kill aphids – and other insects. Some may be able to continue to kill insects for several hours, some for several days."

I hope this information can clarify the drench then spray misunderstanding.  

Barbara S.

Colorado Master Gardener