Thank you for reaching out to Ask Extension. As the owner of several crape myrtle trees, I can assure you that aphids on crape myrtles is a very common unsightly  problem, but thankfully not a fatal one. The aphids use piercing sucking mechanism to extract fluids from the tree and in turn, excrete a sweet clear substance called honeydew, which invites the fungus sooty mold to take hold on leaf surfaces. The life of the tree is not at stake, although a bad case of sooty mold can cut down on efficient photosynthesis.  Also periods of extended drought that we experience in early July and other episodes of heat and humidity this summer have stressed many area trees and crape myrtles are no exception. Early leaf drop in many tree species is a direct result of environmental stress.

Aphids can be controlled with spraying or using horticulture oils and soaps.  NC State Extension has a great fact sheet on control agents that do not contain pyrethroids - using them can damage beneficial insects.  The chart at the bottom shows what agents can be used that are compatible with beneficial insects.

Aphids are soft-bodied insects that can be controlled. Unfortunately a new pest is infecting our crape myrtles - crape myrtle bark scale or (CMBS). Scale insects are more challenging to control due to the protective covering of the scale.  You want to make sure you do not have CMBS - as a new emerging pest, the jury is still out on control, the best time to treat is when the crawlers are emerging from their scales in late spring.  I hope you don’t have them - here is an article recently published on our website to determine if they also are a source for tree damage. If so, the treatment, while evolving, is quite different than aphid control.  There are early suggestions and observations that naturally occurring biological controls can keep CMBS, and other pests for that matter, under control, therefore it is very important not to indiscriminately spray with pesticides that might kill off beneficial predators. Your inquiry here shows the wisdom of asking first before spraying!

Pruning is not going to help with this issue. I do advise raking up any fallen leaves and disposing of them (not composting) to reduce overwintering of the fungus that finds its way to the honeydew host. 

So my tree has aphids, why doesn't my neighbors trees have them when they are maybe 30 yards away..?

On Tue, Jul 30, 2024 at 10:28 AM, Ask Extension

<<personal data hidden>> wrote:

Insect populations are most often randomly distributed across the landscape.  Aphids will feed on a host and may not move to other suitable nearby hosts until their population density on the existing one reaches a critical threshold.  The aphids then will produce a generation that has wings and they will move to nearby hosts at that time.  It is difficult to determine why they choose one particular host over another; however, a number of factors such as plant stress and fertility levels could influence host suitability.  Aphids for example do well on plants that have been fertilized.  Your neighbor's crape myrtles most likely also have aphids but not populations as great as those found on your plants.  Diversity of plants in a yard can contribute to fewer insect issues.  More diverse groups of plants in a landscape increases the number of insects present which lowers the number of insect problems encountered.  This is due to the increased numbers of beneficial predators and parasitoids found in overall high insect populations.  Some insecticides used to manage perceived problematic insect populations on plants kill these beneficial arthropods and it takes awhile for beneficial arthropod populations to recover (longer then problematic insects).  

Basically, there are many factors that influence why insects are on some plants versus others and how large those populations are.  I suspect your neighbor's plants have aphids too, just not as many for whatever reason.