Field and Farm

Posted 8/5/18

Walker County Extension AgentAphids, or “plant lice” as I have often heard them referred to, belong to a very large group of insects that suck the sap and cause damage to a wide variety of …

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Field and Farm


Walker County Extension Agent

Aphids, or “plant lice” as I have often heard them referred to, belong to a very large group of insects that suck the sap and cause damage to a wide variety of plants. Aphids can be found in both winged and wingless forms and are often found in very large numbers once they get started. Although aphids are common prey for many other insects, their tremendous reproductive potential allows their numbers to increase very rapidly. 

Apparently, many of you have noticed this lately as well. I have received many phone calls and numerous samples, including several from local Master Gardeners of one aphid in particular, wooly alder aphid, that seems to be out in large numbers right now. Wooly alder aphid looks like a mat of white cottony material that, are actually waxy fibers, are found on the bottoms of leaves on many types of plants. I have seen these aphids on several tree species in our area including both red and Japanese maple as well as river birch and a few others.

Wooly alder aphid is not the only aphid around though. Aphids are small soft-bodied insects that come in a variety of colors including green, pink, yellow, red, or black. Once attacked by aphids, a plant will secrete sap or “honey dew." That is what makes your car anything else that you leave under a tree infested with aphids very sticky. The aphids themselves also excrete sugar loaded honeydew. The honey dew also contains lots of sugar and will mold over time giving the plant a black appearance. The black substance is called “sooty mold.” Remember last year if your crape myrtles or other plants suddenly turned black as if covered by soot?  The reason was sooty mold that resulted from an insect attack…. Probably aphids! The sooty mold that gives the plants infested with aphids the dirty, sooty appearance may further weaken the plant and compound the stress that it is under by interfering with the plant’s ability to conduct photosynthesis. I also had many calls over the years from homeowners who have noticed that their sidewalk or even shaded areas on their homes and roofs turned black during late summer. Again, this is caused by the same sooty mold after aphid feeding that causes your crape myrtle or rose leaves to turn black.

Most species of aphids overwinter as eggs. The eggs hatch in the spring into females that produce live young without mating.  Several generations of wingless aphids can be produced in this manner, then later in the season winged forms (the ones that are out in numbers right now) migrate to different host plants. Late in the season, aphids move back to the host and a generation of both males and females are produced. These aphids mate, and the eggs that are produced overwinter to ensure that we will have aphid problems again next year.

Aphids damage a wide variety of plants. Just about any living green plant is prone to aphid feeding damage. As they feed, they insert their mouthpart, called a stylus, that works like a hypodermic needle into the plant leaf or stem and suck out the plant’s sap. Leaf feeding aphids may cause leaves to become deformed when heavy infestations occur. Plants are generally weakened by aphid feeding.  Some plant diseases are spread by aphids as well.

Since aphid numbers build rapidly, frequent monitoring of your plants is essential to controlling the offending aphids. Natural predators, such as lady bugs, assassin bugs, and others help reduce aphid populations when they first begin to build up in the spring. Delay insecticide usage as long as possible to avoid killing off the beneficial insects. Also avoid insecticides such as carbaryl (including sevin dust) for aphid control. These products are much more effective on foliage feeding insects and not piercing sucking insects such as aphids. Using the wrong insecticide can actually make the aphid problem worse by killing off beneficial insects.

Use insecticides as a last resort – that is to say when they have built up very large numbers or when their feeding damage becomes very obvious. Re-treatment may be necessary late in the season when winged aphids move from plant to plant. Late summer to early fall treatment destroys egg-laying females of many aphid species thus reducing the number of overwintering eggs.

There are many products that are labeled to kill aphids. Just make sure that the product you select is labeled for the situation in which you are going to use it. For example, a product labeled for killing aphids on ornamental plants such as roses may or may not be labeled to kill aphids on tomato plants in your home garden. Also make sure that you carefully read and follow the labeled directions on whatever product you select in order for it to work optimally and for your personal safety as well as the safety of the environment.


The Walker County Extension Office will be offering our 2018 Master Gardener Course beginning on August 30 and running for 12 consecutive weeks through November 15, 2018. 

Each class will be taught at the Extension Office auditorium from 10 a.m. until 3 p.m. Home vegetable production, home orchards, soils and fertilizers, native plants, annuals and perennials, plant propagation, weed control in home gardens and landscapes, plant diseases and insects, home turfgrass, and backyard wildlife are just a few of the topics which will be covered.

The registration fee is $125 which includes all class supplies, instruction, and a copy of the newly updated Master Gardener Handbook. We will be accepting registrations through Friday, Aug. 24, 2018. 

If you are interested in participating in our Master Gardener Course, please contact us at the Walker County Extension Office at 205-221-3392 for more details and for assistance getting registered for the upcoming class.