Master Gardener: Aphids are a common plant pest. Here's what to do if you find them.

Lawanda Jungwirth
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Almost every gardener discovers aphids on their plants at one time or another.

Before I get into what they are and how to control them, here is the most interesting thing about them: Female aphids are born already pregnant. In fact, inside the newly born aphid girls are embryos that have more embryos inside them.

So a mother aphid is giving birth to her own great-granddaughters. Each succeeding generation can be born within a week and each female gives birth to 60-100 live nymphs, so you can see how the aphid population could explode. Fortunately, natural controls like insect predators and weather prevent aphids from overtaking the world.

You might wonder where the boys are. They aren’t really necessary, and aren’t even part of the aphid population until days get shorter and temperatures drop at the end of summer. At that time, some male aphids are born. They mate with females who lay eggs on host plants where they will overwinter.

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Aphids are small ⅛-inch pear-shaped, soft-bodied insects that are green, yellow, white, pink, brown, black or mottled. They are wingless until such time that the food supply is getting short or the area becomes over-crowded with aphids. Then some winged females are born. They fly off to another area and begin again laying wingless females. 

There are 1,350 different species of aphids in North America, and they feed on 25 percent of all plant species. The plant families most affected are in the rose, aster and conifer families. Many aphid species feed only on one specific plant or plant family, but there are so many species that just about any vegetable, fruit, flower, tree or shrub is susceptible.

You might first notice aphids because a plant is wilting, stunted or yellowing. You go to take a closer look and see tens or hundreds of lice-like insects on the stem or leaves.  You might also see a sticky honeydew or a sooty mold on the plant. 

An observant gardener will probably discover an aphid infestation before too much damage happens. Depending on the plant, it can take from 25 minutes to 24 hours for an aphid to insert its mouth parts into a plant’s stem, leaves or roots and begin sucking the sap. Unfortunately, some aphids transmit plant viruses. Generally, healthy trees and shrubs can withstand an aphid infestation, but large populations on herbaceous plants can cause significant problems.

What should you do if you discover an aphid infestation? Maybe nothing at first. If the plant isn’t looking sickly and aphid numbers aren’t excessive, natural enemies like lady bird beetles and lacewing larvae might soon get the aphid population under control for you. Otherwise a strong jet of water from the hose will dislodge the aphids. If you aren’t squeamish, you can crush them by hand. Insecticidal soaps works but kills by contact, so good coverage and repeated applications are necessary. Insecticidal soaps will kill some plants so read the label before using.


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